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The Role of Mangroves in Fisheries Enhancement

Authors: James Hutchison, Mark Spalding and Philine zu Ermgassen
James Hutchison, Conservation Science Group and The Nature
Conservancy, Department of Zoology, University of
Cambridge, UK.
Mark Spalding, The Nature Conservancy, Cambridge, UK and
Department of Zoology, University of Cambridge, UK.
Corresponding author:
Philine zu Ermgassen, Conservation Science Group and The
Nature Conservancy, Department of Zoology,
University of Cambridge, UK.
Produced by The Nature Conservancy and
Wetlands International in 2014.
The Nature Conservancy’s Mapping Ocean Wealth
project is a collaborative work to quantify the value
of coastal and marine ecosystem services at global
to local scales. On-going work under this project
includes efforts to quantify and map ecosystem
service values for mangrove coastal protection and
carbon storage and sequestration, and work on
other ecosystems such as coral reefs, seagrass,
oyster and saltmarsh communities. This work is
supported through a lead gift from the Lyda Hill
The Mangrove Capital project aims to bring the
values of mangroves to the fore and to provide the
knowledge and tools necessary for the improved
management of mangrove forests. The project
advances the improved management and
restoration of mangrove forests as an effective
strategy for ensuring resilience against natural
hazards and as a basis for economic prosperity in
coastal areas. The project is a partnership between
Wetlands International, The Nature Conservancy,
Deltares, Wageningen University and several
Indonesian partner organisations. This review was
made possible by the Waterloo Foundation.
Cover photo by Mark Spalding
About The Nature Conservancy
The mission of The Nature Conservancy is to
conserve the lands and waters upon which all life
depends. For general information, visit: For more information about the
Mapping Ocean Wealth project, visit:
About Wetlands International
The mission of Wetlands International is to
safeguard and restore wetlands for people and
nature. Wetlands International is an independent,
non-profit organisation, active in around 100
countries, which works through a network of many
partners and experts to achieve its goals. For more
information, visit
Suggested citation for this report
Hutchison, J; Spalding, M, and zu Ermgassen, P
(2014) The Role of Mangroves in Fisheries
Enhancement. The Nature Conservancy and
Wetlands International. 54 pages
Executive Summary 5
Introduction 9
Chapter 1: How mangrove characteristics enhance fisheries 10
1.1 Primary productivity in mangrove forests the foundation of the fishery food web 10
1.1.1. Primary production by mangrove trees 10
1.1.2. Primary production by periphyton 10
1.1.3. Primary productivity in the water column 11
1.1.4. Primary production from outside the mangrove 11
1.2 The detrital pathway 11
1.2.1. Leaching of soluble compounds 11
1.2.2. Colonisation by decomposers 12
1.2.3. Wood decomposition 12
1.3 Export of nutrients from the mangrove 13
1.3.1. Nutrient export mechanisms 13
1.4 Mangrove food chains 13
1.4.1. Detritivores and grazers 14
1.4.2. Deposit feeders 14
1.4.3. Filter and suspension feeders 15
1.4.4. Higher level consumers 15
1.5 Physical characteristics of the mangrove 15
1.5.1. Attachment points 15
1.5.2. Shelter from predators 16
1.5.3. Physical environment 16
1.6 Mangroves as nursery grounds 16
1.7 Linkages with adjacent ecosystems 17
1.7.1. Ecological linkages 17
1.7.2. Nutritional linkages import and export 17
1.7.3. Environmental modification 18
Chapter 2: Mangrove-associated fisheries 19
2.1 Valuing fisheries 19
2.1.1. Literature review 20
2.2 Mangrove-associated fisheries: summaries by fishery type 21
2.2.1. Inshore mixed species fisheries 21
2.2.2. Inshore mollusc and crustacean fisheries 24
2.2.3. Offshore commercial fisheries 25
2.2.4. Recreational fisheries 27
2.3 Drivers of mangrove fishery catch and value 27
2.3.1. Environmental factors and potential fishable biomass 28
2.3.2. Human impacts 29
2.3.3. Socio-economic factors and fished biomass 30
Chapter 3: Recommendations for management of mangroves and fisheries 31
3.1 Avoiding mangrove loss 31
3.2 Restoring mangroves 31
3.3 Managing fisheries 32
3.4 Communication and engagement 33
Chapter 4: Conclusions 35
Appendix 1: Fishery case studies 37
Inshore mixed species fisheries 37
Inshore crustacean fisheries 38
Inshore bivalve fisheries 40
Offshore fisheries 42
Recreational fisheries 43
Table 1: fishing gear commonly used in mangrove habitats 45
References 49
In 2011 humans caught and consumed 78.9 million
tonnes of fish, crustaceans, molluscs and other
species groups from the world’s oceans, accounting
for 16.6% of the world’s animal protein intake
(FAO 2012). This is projected to increase further, to
over 93 million tonnes by 2030 (World Bank 2013).
Global demand for fish products has increased
dramatically over recent decades. Fishing is also an
important livelihood, globally providing
employment to 38.4 million people of whom 90%
are employed in small-scale fisheries (FAO 2012).
The importance of fisheries continues to rise as
coastal populations are increasing, and rapidly
growing economies are driving up demand for fish.
While aquaculture is increasing to meet some of
this demand, wild capture fisheries continue to be
critically important.
This review of the scientific literature provides a
deep exploration of the importance of mangroves
for wild capture fisheries. While mangroves are
widely recognized for their role in enhancing both
small scale and commercial fisheries, they are
rapidly disappearing. A fuller understanding of this
ecosystem service and its value in both social and
economic terms will help enhance the sustainable
management of both mangroves and fisheries.
The report firstly discusses some of the ecological
processes which underpin the key role of
mangroves in fisheries enhancement, followed by
an exploration of the different mangrove-
associated fishery types. As the fisheries value of
mangroves is highly site specific, the report
explores the drivers and mechanisms which can
help to explain for different locations how many
fish a mangrove produces, how many are caught by
humans, and what the fisheries value is, both in
economic terms, as a food supply or through the
livelihoods that they support. Decision-makers can
use this information to determine where fish
productivity is highest, which allows them to make
adequate decisions relating to conservation and
restoration actions and sustainable fishing. We
conclude with management recommendations for
maintaining or enhancing the value of mangroves
for fisheries for the long-term.
Fishers at Ashtamudi Lake by Nisha D’Souza
Fish productivity from mangroves will be
highest where mangrove productivity is high,
where there is high freshwater input from rivers
and rainfall and where mangroves are in good
Fish productivity will increase with an increase
in total area of mangroves, but notably also with
the length of mangrove margin since generally
it is the fringes of mangroves where fish
populations are enhanced.
Mangroves with greater physical complexity
both in terms of patterns of channels, pools and
lagoons, as well as the structure of roots which
are important areas for shelter and for growth of
some bivalves will enhance fisheries to a
greater extent.
Fish catch will be highest close to areas of high
human population density that provide the
fishers and the markets for the catch. Of course
some of these mangroves close to populations
are also likely to be under greater threat than
those in more sparsely populated areas they
may be degraded, the waters may be polluted,
or they may be over-fished and hence less
productive. Where such mangroves are secured
through appropriate management regimes, and
where their fisheries are well managed they are
likely to give greatest value. Consequently,
conservation and restoration efforts in these
areas close to human populations will likely give
the greatest return on investment.
How mangroves enhance fisheries
Mangroves enhance fish production via two main
mechanisms the provision of food and of shelter.
Mangroves forests are highly productive, with
mean levels of primary productivity close to the
average for tropical terrestrial forests. Their leaves
and woody matter (detritus) form a key part of the
marine food chains that supports fisheries.
Decomposers of this detritus include micro-
organisms such as bacteria and oomycetes, as well
as some commercially important crab species.
These decomposers process the leaves and woody
matter into more palatable fragments for other
Mangrove productivity is further enhanced by
productivity of periphyton and phytoplankton
occurring on mangrove trees, in their soils and in
the water column, which typically have lower rates
of productivity than the trees themselves, but are
nutritionally more accessible to consumers.
Moreover, mangroves often benefit from incoming
nutrients from rivers and other adjacent habitats.
They may also export nutrients, in the form of
dissolved and particulate organic carbon, and living
biomass, such as planktonic larvae and maturing
fish and invertebrates.
Species of interest to the fisheries sector are found
at all levels of the food chain, with detritivores such
as mangrove crabs, prawns and mullet; filter
feeding bivalves, planktivorous fish such as herring
and anchovy species, and higher consumers such as
some mud crabs and many other fish including
snappers and groupers.
It is not only the high productivity of the
mangroves that creates value for fisheries, but also
their physical characteristics. Mangrove roots and
trunks provide a structure that species such as
oysters can grow on. Their roots also trap fine
particles, creating soft soils ideal for molluscs and
crustaceans to burrow in. Mangroves also provide
shelter for many species, enabling them to avoid
predation and also invest more time in feeding.
Finally, driven by the nutritional and physical
benefits, many species use mangroves as nursery
grounds. These include species that spend time in
mangroves as juveniles before moving to offshore
habitats such as coral reefs. Thus fisheries in these
offshore habitats benefit from stock replacement
from mangroves.
Values of mangrove-associated fisheries
Some 210 million people live in low elevation
areas within 10km of mangroves and many of these
benefit from mangrove-associated fisheries. The
economic values of mangrove-associated fisheries
vary widely, reflecting the wide range of different
fisheries, economic markets, and levels of
utilisation. Besides economic values, mangrove-
associated fisheries provide jobs and food supplies
for millions of people. In turn this may provide
multiplier benefits such as political or social
stability. We summarise the different types of
mangrove-associated fisheries into four broad
Inshore mixed species fisheries
These are mostly low-income fisheries undertaken
in mangroves close to settlements. They include a
broad range of fishing techniques, but many are
opportunistic and fishers often return with a highly
mixed catch of finfish, molluscs and crustaceans. A
large proportion of the catch is for domestic
consumption, but some is sold, usually in small
local markets. The median value for mixed fisheries
from our review was US $106/ha of mangrove/year,
but variation either side of this was high.
Inshore mollusc and crustacean fisheries
Certain mollusc and crustaceans caught in
mangroves generate quite high market values, and
although they may still be harvested at local and
small-scales, in many cases fishers are operating a
targeted fishery and generating income through
market sales. The most important of these are a
number of crab species, oysters and other molluscs,
and some harvesting of juvenile prawns for
stocking of aquaculture ponds. Economic
valuations are rare in the literature, but the one
value we found, for a crab fishery in Micronesia,
was US $423/ha of mangrove/year
Offshore commercial fisheries
These fisheries may operate many kilometres from
the mangroves, but benefit from the mangroves’
nursery habitat function. This distance makes it
challenging to quantify the extent of this benefit.
The importance of mangroves is best documented
for offshore prawn fisheries, although it is rarely
possible to attribute catches to specific mangrove
areas. These fisheries can generate high value
returns, but much of this value lies in the
industrialisation of the fishery, with high volumes
of catch for small numbers of fishers. We found two
studies giving economic valuations, with values of
24.3 and 1394 US $/ha/year for fisheries in
Indonesia and Mexico respectively.
Recreational fisheries
Mangroves are a critical habitat for a number of
species that are considered prize game fish, and
locations where there are healthy stocks of such
fish have become favoured fishing grounds for a
fishery that can be both high value and low impact.
Calculations of value for these fisheries are
challenging, but most efforts have included
estimates of financial flows to associated
beneficiaries including accommodation, transport,
food and fishing guides. For example, Fishing for
bonefish, permit and tarpon was worth US $56.5
million to Belize in 2007 and US $141 million to
the Bahamas in 2008.
Modelling the drivers of mangrove-associated
The review work has helped to inform an
understanding of the processes which drive value.
The value of mangrove-associated fisheries varies
greatly between different locations. To understand
this variation in value we break down the benefits
that mangroves provide to fisheries into three
1. Potential fishable biomass
This is the biomass that would be present in a
location in entirely natural condition. The
productivity and availability of fish will be strongly
linked to the area of mangroves. Further influence
comes from the length of mangrove margin as it is
primarily at the fringes of the mangrove where fish
populations are enhanced. The physical complexity
of the mangrove forest may also play a role, both in
terms of patterns of channels, pools and lagoons,
and also the structure of roots which are important
areas for shelter and for growth of some bivalves.
Climate, freshwater and nutrients also influence
primary productivity in mangrove areas which
affects fish productivity.
2. Actual fishable biomass
Most mangrove fish stocks have been influenced by
humans, directly through the harvest of fish, and
indirectly through changes to the environment.
Even low levels of fishing will have some impact on
the remaining fishable biomass, while overfishing
can greatly reduce fish productivity and potential
yields. Mangrove areas in many places are also
compromised by pollution, while impacts to the
mangroves from harvesting and clearance directly
impact primary productivity and thus influence fish
3. Fished biomass
The amount of fish actually being caught is
demand-driven, but that demand can be
understood and modelled in relation to coastal
population sizes, the influence of markets, of
economic drivers, cultural traditions and so on.
Fished biomass represents one core measure of
value, but it is also a key component of other values
measured in terms of money, jobs, food security
and other metrics.
Recommendations for proper mangrove
The tremendous value of mangroves for fisheries,
in both social and economic terms, provides a
strong incentive to secure mangroves for the long-
term through proper management of both
mangroves and mangrove-associated fisheries. We
outline three broad classes of management which
need to be considered:
Avoiding mangrove loss
Maintaining mangrove areas is almost always the
most cost-efficient way of ensuring value flows
over time. Critical to success are both the
establishment of clear and effective regulatory
frameworks, and the establishment and recognition
of tenure and use rights, ideally at local or
community levels. Protected areas, established for
conservation purposes, already include over 25%
of the world’s remaining mangrove forests.
However, other mechanisms ranging from nation-
wide regulations on mangrove clearance, to
controls ensuring sustainability of mangrove
silviculture can also be effective. In all cases such
regulations are most effective when mangrove
ownership is clearly established and where
communities are fully aware of the benefits they
derive from adjacent mangroves.
Restoring natural mangroves
Where mangroves have been degraded or lost they
can still be restored, enabling the return of
ecosystem services relatively quickly. Critical to
successful restoration are understanding the
causes of loss in order to ensure these can be
prevented in the future, and ensuring that the
communities or owners of mangroves are
supportive of restoration. Where these conditions
are met, the main focus of restoration should be
restoring growing conditions tidal flows,
freshwater inflow and sediments. These alone may
be enough to allow natural mangrove recovery, but
in some cases mangroves may need to be planted
to commence or enhance recovery.
Managing fisheries
Fish stock management is a core principle of
ensuring continued supplies and in attempting to
maximise yield and/or profits. A large body of
management interventions have been developed,
alongside the science to inform such management.
Despite this, understanding of management of
mangrove-associated fisheries per se remains
limited. Key fisheries management tools include
regulating access to the fishery, through ownership
or licence; regulating fishing methods, for example
to prevent wasteful bycatch or damage to the
seabed; spatial controls such as the closure of
certain areas permanently or seasonally to allow
survival of breeding populations or key nursery
areas. Market mechanisms, such as sustainable
fisheries certification can provide important
incentives in some fisheries to encourage and
ensure implementation of such measures. Lastly,
since aquaculture is a major driver of mangrove
conversion, it is imperative to simultaneously work
towards more sustainable aquaculture so that the
fisheries enhancement function of mangroves is
not jeopardized.
Education and communication are key tools in all
these management interventions, to build public
and political support. Many fishers are unaware of
the key role mangroves may play in supporting
fisheries, even those far offshore. Raising
awareness can simply involve disseminating the
key facts, but new ideas, including the more
accurate quantification of value and explanation of
key underlying drivers may greatly increase
openness and enthusiasm for improvements in
mangrove management.
Natural mangrove forest, Senegal. Photo by Wetlands International
In 2011 humans caught and consumed 78.9 million
tonnes of fish, crustaceans, molluscs and other
species groups from the world’s oceans, accounting
for 16.6% of the world’s animal protein intake
(FAO 2012). Global demand for fish products has
increased dramatically over recent decades. For
example, annual per capita fish consumption
doubled from 9.9 kg in the 1960s to 18.8 kg in
2011. Fishing is also an important livelihood,
globally providing employment to 38.4 million
people of whom 90% are employed in small-scale
fisheries. The importance of fisheries continues to
rise; coastal populations are increasing, and rapidly
growing economies are driving up demand for fish.
While aquaculture is increasing to meet some of
this demand, wild capture fisheries continue to be
critically important
Mangroves are forests that grow in the inter-tidal
zone, at the interface of land and sea. They cover
around 150,000 km² of coastline in the tropics and
warm temperate regions, and are widely held to be
important to both small scale and commercial
fisheries. For example, 80% of all commercial or
recreational species in Florida are mangrove-
dependent (Hamilton and Snedaker 1984), and
mangroves are crucial for 72% of the commercial
fish catch in the Philippines (Paw and Chua 1991).
This ecosystem service that mangroves provide has
considerable economic value, in excess of US
$18,000 per ha in the most productive locations
(de Groot et al. 2012).
Mangroves support fisheries through two main
ecological functions: their primary productivity,
which forms the foundation for marine food chains
that support fisheries, and their three-dimensional
structure, which provides a physical environment
suitable for many fishery target species. Relatively
few fishery species are mangrove residents. Rather
most of them are transient visitors, using mangrove
forests for part of their life-cycle. Often this is
during their juvenile development stage, meaning
that mangroves are a nursery ground for many
commercially important species.
The fisheries that mangroves support vary in scale,
fishing methods and target species. They include
fisheries within the mangroves themselves for
mangrove-resident species such as crabs and
molluscs, fisheries in mangrove channels and
Fish Market in Java, Indonesia. Photo by Alexander van
lagoons, and offshore fisheries for species such as
penaeid prawns that use the mangroves as
juveniles but move out to the continental shelf as
adults. Mangrove-associated fisheries range in
scale from subsistence fishing, with catches of a
few hundred grams to highly commercial
mechanised trawling taking hundreds of tonnes of
fish or shrimp.
In this report, we first discuss the underlying
ecological factors of primary productivity and
three-dimensional structure that make mangroves
so crucial to fisheries in Chapter 1. Chapter 2 then
discusses some of the main fishery types that are
supported by mangroves. Finally, we discuss the
factors that drive the variation in the importance of
mangroves to fisheries in Chapter 3, and use these
to highlight priority topics for mangrove
conservation and restoration.
Benefits to fisheries from mangroves come via two
main mechanisms. The first is the high level of
primary productivity from the mangrove trees and
from other producers in the mangrove
environment. This forms the basis of food chains
that support a range of commercially important
species. The second is the physical structure that
they provide, which provides attachment points for
species that need a hard substrate to grow on, as
well as shelter from predation and a benign
physical environment. These two mechanisms
combine to make mangroves particularly effective
as nursery grounds for juveniles of species that
later move offshore or to adjacent habitats such as
coral reefs.
In this section, we discuss these mechanisms in
more depth, beginning with mangrove primary
productivity and the food chains that depend on it
in 1.1-1.4. In 1.5 we discuss the role that the
physical structure of the mangrove plays in
supporting fisheries. We then discuss the nursery
function of mangroves in 1.6, and the linkages
between mangroves and adjacent coastal habitats
in 1.7.
1.1 Primary productivity in mangrove
forests the foundation of the fishery
food web
Mangroves are highly productive ecosystems, with
rates of primary productivity often rivalling those
of tropical terrestrial forests. This primary
productivity comes from three main sources: the
mangrove trees themselves, algae growing on tree
roots and on the forest floor and phytoplankton in
the water column. Additionally, mangroves may
receive nutrients from external sources. Each of
these sources contributes to the enhanced fisheries
(secondary) productivity mangroves are known to
1.1.1. Primary production by mangrove trees
Mangroves are highly productive ecosystems.
Current estimates suggest an average above
ground net primary production of 11.1 t dry weight
(DW) ha¹yr¹ (Alongi 2009), which is very similar
to the value of 11.9 t DW ha¹yr¹ for tropical
terrestrial forests. Such average numbers mask
enormous spatial variation, and actual mangrove
primary productivity is determined by a range of
factors, including climate, fresh water input and
nutrient availability. As a general rule, productivity
decreases with distance from the equator (Twilley
et al. 1992), but there is high variability between
Carbon captured from the atmosphere by
mangrove trees is built into leaves, trunks,
branches and below-ground and aerial roots. A
suite of primary consumers, ranging in size from
insects to monkeys and even deer and cattle, feed
directly on the trees. However, with the exception
of a few crab species, these are mostly terrestrial or
arboreal, rather than marine. The main way in which
primary production from trees supports marine
secondary production is through the decomposition
of fallen leaves. A recent review of mangrove
litterfall found that an average of 9.6 t DW ha¹yr¹
(range 1.75 25.2 t DW ha¹yr¹) of leaves,
propagules, twigs and branches fall to the forest
floor, where it is processed by a range of
consumers and decomposers (Hutchison et al.
High biomass mangroves in the Berau Delta, Kalimantan,
Indonesia, a location with high rainfall and rich nutrient
and freshwater supplies. Photo by Mark Spalding.
1.1.2. Primary production by periphyton
Periphyton macroalgae and photosynthetic
microorganisms growing on roots, fallen tree parts
and the sediment surface can contribute
significantly to primary productivity in mangrove
systems. Periphyton productivity can reach 9.9 t C
ha¹yr¹ for epiphytic algae growing on
pneumatophores in Florida (Dawes 1999), but is
typically much lower, for example 0.1 t C ha¹yr¹
on the sediment beneath an Australian mangrove
(Alongi 1994). Periphyton productivity depends on
the availability of light, which in turn depends on
the degree of canopy closure in the mangrove
forest, and on water turbidity, which in some
mangrove can reduce light intensity by 99% within
1 m (Harrison et al. 1997). Highest productivity is
therefore found along creek banks and forest edges
where there is more direct sunlight. Periphyton
productivity will also be influenced by tidal regime,
with lower productivity in areas subject to frequent
desiccation, and by nutrient availability, so it may
be very high in nutrient rich estuaries and may
even be enhanced by nutrient pollution.
Although periphyton is likely to generate lower
levels of primary productivity than the mangrove
trees themselves, it is still an important component
in food chains because it is easier for consumers to
digest than tree detritus. Thus a number of studies
have shown that algae- and phytoplankton-derived
carbon may play a more important role in the diet
of key commercial species than mangrove detritus
(Dittel et al. 1997).
1.1.3. Primary productivity in the water column
Phytoplankton in the water column beneath
mangroves forests provides a third source of
primary production. Like the periphyton, planktonic
production will be impacted by light availability,
nutrients and tidal regime. As phytoplankton are
found in the water column itself, the tidal regime
and hydrology of the mangrove determines the
relative importance of phytoplankton in the
mangrove food web; they are more important in
mangroves that are frequently or continuously
flooded than in dryer mangroves where standing
water is confined to channels on all but the highest
tides. The tidal and hydrological regime also
determines the degree to which phytoplankton is
retained within the mangrove or flushed out to sea.
A study in the Indus delta in Pakistan found
planktonic productivity in a mangrove creek ranged
from 0.5 3.7 t C ha¹yr¹ (Harrison et al. 1997),
and similar values have been found in an Australian
estuary (O’Donohue and Dennison 1997). Higher
values can occur with high nutrient input, reaching
5 g C m²d¹ downstream of a shrimp farm effluent
outflow (McKinnon et al. 2002). Like benthic and
epiphytic algae, phytoplankton are more palatable
than mangrove detritus and are therefore
important in mangrove food webs. In particular,
they are grazed by zooplankton including
crustacean larvae and filter feeders such as
1.1.4. Primary production from outside the
In addition to the significant primary production by
trees, periphyton and phytoplankton, mangroves
also receive external organic material that is
imported by the tide. The physical structure of the
mangrove stems and roots slows water flow,
leading to material being deposited. Tides import
phytoplankton and zooplankton, including large
numbers of crab larvae which are an important
food source for many of the fish species found in
mangroves (Rönnbäck 1999). Estuarine mangroves
also receive plant material from terrestrial sources
that is carried by the river and trapped in the
mangroves, where it is incorporated into the
detrital pathway described below.
This interconnection with other ecosystems is most
clearly seen in the many places where mangroves
are physically adjacent to, or even intermixed with,
other coastal ecosystems, including tidal forests,
salt marshes, seagrass meadows or macroalgal
beds. These interconnections can be important
components of a wider ecosystem, and important
contributors to coastal fisheries which need to be
taken into account in developing coastal
management and evaluating fisheries.
1.2 The detrital pathway
While mangrove trees provide the vast majority of
the primary productivity in the mangrove forest,
this productivity is largely inaccessible to marine
fauna. It is only when the leaves fall from the trees
and enter the detrital pathway that they
significantly contribute to marine food webs that
ultimately support fisheries. Even having fallen
from the tree, mangrove leaves are a poor food
source for most animals. They have thick waxy
cuticles and large quantities of lignin which is
difficult to digest. Furthermore, although they are
rich in sugars and other carbon compounds, they
are poor sources of nitrogen and phosphorous. The
leaves therefore only really begin to contribute to
fisheries productivity once they have been
processed by a range of decomposers. This
processing begins with leaching of soluble
compounds, followed by colonisation by
decomposing microorganisms. The whole process
takes months to years, but can be rapidly
accelerated by crabs and other animals that feed on
leaf litter directly, making it more accessible to
other consumers (see Box 1).
1.2.1. Leaching of soluble compounds
The first step of the decomposition process is the
leaching of soluble materials into the water. Within
two weeks of immersion in water, 20-40% of the
total carbon content of the leaf will be lost as
dissolved organic carbon (DOC). The compounds
lost in this way include sugars, but also tannins and
other phenolic compounds. The loss of these latter
two groups is important in allowing colonisation of
the leaf by decomposers, as they inhibit microbial
growth when present. The DOC produced by this
leaching process is used as a food source by
microbes in the water column, supporting their
secondary productivity. The material left after this
process consists of largely insoluble structural
compounds within the leaf, such as cellulose and
1.2.2. Colonisation by decomposers
Once microbe-inhibiting compounds have been
leached away, the remaining material is colonised
by a variety of bacteria, fungi and oomycetes (water
moulds). Unlike in terrestrial systems, fungi appear
to play a relatively minor role in the decomposition
process, with oomycetes, particularly the genus
Halophytophthora, being the key players (Newell
1996). These microbes break down polysaccharides
such as cellulose that make up the leaf structure.
Lignin, found in cell walls in leaf veins and
especially in woody parts of the tree, is the slowest
compound to be broken down and thus contributes
the least to secondary productivity and makes up a
large proportion of the material that is
incorporated in the soil. The microbes that colonise
the leaf litter are themselves a more digestible and
available source of nitrogen and phosphorus, thus
making the leaf litter a more useful source of
nutrition for detritivorous consumers such as small
1.2.3. Wood decomposition
In addition to leaf litter, woody material falls to the
mangrove floor when branches fall from trees or
when trees die or are blown down by storms. Like
leaves, woody debris is broken down by a range of
microbial decomposers, but more important are the
teredinid wood-boring molluscs. Many mangrove
species contain chemicals that are toxic to these
wood-borers, so, as with leaves, there is an initial
leaching phase before colonisation can occur. After
this, colonisation occurs rapidly. Teredinids are
bivalves in which the shell is specially adapted to
bore into wood and which have symbiotic bacteria
that break down cellulose. They turn branches and
logs into a network of tunnels, consuming
significant amounts of biomass directly as well as
increasing the surface area for attack by microbial
decomposers. Despite these specialised
consumers, fallen wood remains slow to break
In many mangroves a large proportion of the leaf litter is directly consumed by crabs, particularly those in the
family Sesarmidae. This dramatically accelerates the incorporation of mangrove biomass into the food chain.
The acceleration happens in three main ways:
1. Shredding as crabs feed on leaf litter they shred it into fine particles, increasing the surface area for
leaching and microbial colonisation. An Australian study found that 20% of the material processed by crabs
is dropped without being ingested (Camilleri 1989), but even this is shredded into fine particles.
2. Accelerated leaching 85% of the leaf litter ingested by crabs ends up in faeces. Processing of this material
in the crab gut reduces content of unpalatable tannins in faeces to < 3%, compared to 13% in freshly fallen
leaves (Lee 1998). This process means that decomposing microbes can colonise the processed leaves in
hours or days, rather than the weeks required without crabs.
3. Assimilation around 12% of the leaf litter processed by crabs is assimilated as crab biomass. A range of
predators then feed on these crabs, including a number of fish species that are of high importance to
fisheries (Sheaves and Molony 2000). Additionally, crabs will invest a proportion of the energy assimilated
in reproduction, producing large numbers of crab larvae which are an important food source to smaller
predators. This “short circuits” the mangrove food chain, allowing production from mangrove trees to reach
commercially important species without passing through the detrital pathway.
Estimates for the amount of litter consumed by crabs vary. In many mangroves, crabs play a major role: one
Australian study found that 70% of Bruguiera leaves and 88% of Ceriops leaves were taken down crab burrows
in an Australian mangrove, and those left on the surface were eaten by crabs where they fell (Robertson and
Daniel 1989b). Similar results have been found in Brazil, where the crab Ucides cordatus was found to consume
84.2% of the total daily litterfall (Nordhaus et al. 2006).
Where crabs are less abundant, snails may play a similar role, consuming up to 42% of the total litterfall in a
Kenyan mangrove (Slim et al. 1997). Similarly, other small invertebrates such as amphipods and isopods may
contribute to leaf shredding. The role of macro consumers may, however, be minimal in some locations: a
Florida study found no leaf consumption by crabs and only very minor grazing by snails (McIvor and Smith
down: in a study in an Australian mangrove, only
50% of the total carbon was lost from wood over
6.5 years (Robertson and Daniel 1989a).
1.3 Export of nutrients from the
Water flows from mangroves may also export
nutrients, thus supporting food chains in other
ecosystems. The extent and importance of this
exported material varies greatly from one
mangrove forest to another. Forest productivity and
litterfall is a key driver determining the total
amount of material available for export. The
proportion of this available litter that is exported
depends on geomorphological characteristics such
as mangrove area and shape, rainfall and river
flows, and on tidal regime including tidal range,
periodicity and volume (Alongi 2009), as well as
the activity of crabs and other herbivores. This
combination of factors means that some mangrove
sites are large net exporters whilst other may be
net importers of organic material.
1.3.1. Nutrient export mechanisms
Organic matter is exported from mangroves in at
least three ways. Firstly, solid material can be
picked up by the flow of water and carried out of
the mangrove as particulate organic carbon (POC).
Secondly, soluble compounds such as sugars and
tannins are leached out of leaves and carried out to
sea as dissolved organic carbon (DOC). Both of
these forms of transport will be most effective
where tides are large and ebb-dominant (where the
outgoing ebb tides has a stronger flow than the
incoming flood tide), or where there is a net flow of
water out to sea such as in an estuary or delta. By
contrast, there will be less export of material in
locations such as oceanic islands where tides are
smaller and freshwater input is limited.
Nevertheless, the quantity of carbon exported is
often substantial, with global estimates ranging
from 29 to 46 Tg C yr-1 (Jennerjahn and Ittekkot
2002, Alongi 2009). The higher estimate represents
11% of the total terrestrial carbon input to the
oceans. It is less clear what role this exported
material plays in offshore food webs. Studies in
Kenya (Hemminga et al. 1994) and Florida (Lin et al.
1991) found that mangroves contribute to food
webs in adjacent sea grass beds, but this influence
declines rapidly with distance from the mangrove
and is generally undetectable a few kilometres
offshore (Kristensen et al. 2008).
The third method by which material is exported is
in the form of living biomass. This living biomass
may be exported at a range of trophic levels,
including as phytoplankton, zooplankton, fish and
crab larvae, or as biomass consumed by adult fish
that move into the mangrove to hunt before
moving back out to other habitats. (Sheaves and
Molony 2000, Oliveira-Neto et al. 2007)
1.4 Mangrove food chains
The high level of mangrove derived primary
production forms the base of a complex food web
that supports a diverse mangrove fauna and
numerous commercially valuable species (Figure
1). This primary production is accessed by fauna in
a variety of ways some primary production is
directly grazed, but much is also accessed via
decomposition and detrital pathways. Mangrove
detritivores may be relatively generalist and often
graze algae as well as consuming detritus; deposit
feeders feed on detritus that has been incorporated
into the sediment; and filter feeders and
suspension feeders (including both sessile
organisms and free living zooplankton) directly
consume phytoplankton. Most of these groups
contain species of commercial fisheries
importance, but most of the high value fish species,
and some important crustaceans are at the next
trophic level, preying on these primary consumers.
Creek flowing through mangrove. Photo by Mark
Figure 1: Simplified mangrove food web showing the broad trophic groups
1.4.1. Detritivores and grazers
Key groups: Crustaceans, gastropods, fish
Important fishery species: Penaeid prawns, and
other larger species from detritivore groups such as
mangrove crabs Ucides cordatus, and mullet
Detrivores and grazers directly consume mangrove
detritus as well as periphyton and algae from the
sediment surface. Some species such as mullet are
relatively non-selective in what they eat; they feed
by scooping up a mix of detritus, algae and
sediment, which they then grind up in their thick
walled, muscular stomach, drawing out the
available nutrients, with the rest passing through.
Crabs and prawns, by contrast, graze on individual
leaf fragments and algae from the sediment surface
or periphyton. Mangrove leaves are originally rich
in carbon but poor in other nutrients, but become
colonised by microbial decomposers rich in
nitrogen and other essential nutrients. These
microbes are a crucial part of the diet of
detritivores, and selective detritivores therefore
preferentially eat older leaf litter rather than
freshly fallen leaves (e.g. Micheli 1993).
1.4.2. Deposit feeders
Key groups: Polychaete worms, gastropods, crabs
Important fishery species: N.A.
Deposit feeders consume organic material in the
sediment, typically by ingesting large quantities of
sediment and extracting the edible material from it
in the gut. The most obvious examples are
polychaete worms which consume sediment as
they burrow. Many crab species also process
sediment to extract nutrients, including some of
the species that primarily feed on leaves. Mangrove
detritus, bacteria, benthic diatoms and algae are all
consumed in this way.
Deposit-feeding fiddler crab. Photo by Mark Spalding.
1.4.3. Filter and suspension feeders
Key groups: Bivalves, planktonic crustaceans, fish,
sponges, polychaete worms
Important fishery species: Oysters (Crassostrea
spp., Saccostrea spp.), mangrove cockles (Anadara
spp.), Fish in the Clupeiformes, including herrings
(Clupeidae) and anchovies (Engraulidae)
Filter feeders feed on organic matter in the water
column, using various methods to capture their
food. Crustaceans generally use hairs on their legs
or claws as their filter, while filter feeding fish use
their gill rakers, swimming forward with their
mouth open to pass food across them. Bivalves use
cilia, microscopic beating hairs, to draw water
across their gills, where suspended food is trapped
in a layer of mucus and then transported to the
mouth. Small planktonic filter feeders feed on
phytoplankton and bacteria, whereas larger filter
feeders such as fish and bivalves also consume
zooplankton and mangrove detritus suspended in
the water column.
1.4.4. Higher level consumers
Key groups: Fish, crustaceans, gastropods
Important fishery species: Portunid crabs e.g.
Scylla spp., Callinectes spp., Fish e.g. snappers
(Lutjanidae), groupers (Serranidae)
Most of the primary consumers described above
are preyed upon by a broad suite of predators.
Whilst many animals at lower trophic levels are
mangrove residents, most predators are transient
visitors. Some species use mangroves only at
certain life history stages, for example snapper may
live in the mangrove as juveniles before moving to
coral reefs as adults. Other species live outside the
mangrove but enter it at high tide to feed. The
larger portunid crabs are an exception to this.
Several species, notably those in the genus Scylla,
are mangrove residents and only leave the
mangrove to spawn offshore.
Mud crab Scylla sp. with newly caught fish, Queensland,
Australia. Photo by Mark Spalding.
1.5 Physical characteristics of the
Alongside the high productivity of mangroves,
another key feature that makes mangroves
attractive to fishery target species is the physical
structure they provide. Mangroves provide a solid,
three-dimensional structure in the form of trunks,
branches, complex aerial roots and fallen debris.
The geomorphology of the mangrove sediments
can also be complex, with dense networks of
winding and branching drainage channels, smaller
pools and larger open areas and lagoons. This
structural complexity is beneficial to fish and
invertebrates through the provision of attachment
points, by providing shelter from predation and by
reducing physical disturbances and stress.
1.5.1. Attachment points
The three-dimensional structure provided by
mangrove trunks and roots provides attachment
points for organisms that need a hard substrate to
attach to and are therefore unable to live in the soft
sediments typical of unstructured estuarine
habitats. These include macroalgae which provide
additional primary productivity and a food source
for animals. Of more direct interest to fisheries
though, are several species of mangrove oyster. In
the Atlantic, these include Crassostrea rhizophorae
from the Americas, Crassostrea tulipa from West
Africa (Vakily et al. 2012) and Crassostrea gasar,
found on both sides of the Atlantic (Lapègue et al.
2002). In the Indo-Pacific these are replaced
primarily by species of the genus Saccostrea,
including Saccostrea cucullata (Jana et al. 2013),
and Saccostrea echinata. Oysters are harvested
directly from mangrove roots, and in many
locations are also cultured by collecting spat
(oyster larvae) on suitable surfaces and growing
these on wooden structures or ropes suspended
from rafts. Other species that are harvested directly
from mangrove roots include wood-boring bivalves
of the genus Teredo, which are a delicacy called
“tamilok” in the Philippines and are also eaten in
Brazil, as well as many species of snail that are
harvested by local subsistence fisheries.
Oysters on Rhizophora roots in Fiji. Photo by Mark Spalding.
1.5.2. Shelter from predators
The structural complexity of the mangrove
environment also provides shelter from predators.
Roots and trunks reduce prey visibility and impede
access of large predators into shallow areas. Shade
from the mangrove canopy and turbid water, a
characteristic of many mangroves, also make prey
harder for predators to detect. This reduces
predation pressure on juvenile fish and prawns,
increasing the number that survive and can be
recruited into the fishery. This is supported by
studies which find higher numbers of fish near to
mangrove shelter than in clear habitats (Sheaves
1996), or higher fish abundance deeper into the
mangrove where larger carnivorous fish were not
found (Rönnbäck et al. 1999). The latter study also
found more fish amongst Avicennia
pneumatophores than amongst Rhizophora prop
roots, suggesting that the type of structure may
also be important. Experimental studies also show
that fish and prawns spend more time near shelter
in the presence of predators, and that tethered fish
are more likely to be eaten on a mudflat than in
mangrove or seagrass habitats (Laegdsgaard and
Johnson 2001, Meager et al. 2005). The presence of
structure may also increase growth rate, as the
reduced predation pressure means that potential
prey are able to spend more time feeding and less
time avoiding predators.
1.5.3. Physical environment
Finally, the structure provided by the mangrove
generates a relatively benign physical environment
for juvenile fish and prawns, with low current
speeds, soft sediment, shallow water and reduced
wave action. The mangrove vegetation creates
friction with the water, slowing the rate at which it
flows. This means that water is retained after the
tide has fallen. This phenomenon is known as
lateral trapping, and it increases the retention of
planktonic larvae of fish and invertebrates that are
imported from outside the mangrove. This gives
these larvae an opportunity to settle in the
mangrove environment, which increases fish and
invertebrate populations, as well as providing a
food source for small predators already living in
the mangroves. Additionally, the mangrove trees
provide shade. This regulates water temperatures,
reducing stress on juvenile organisms, and also
further reduces the risk of predation by making
prey less visible due to lower light availability.
The slowing of the water flow by this lateral
trapping effect also causes the deposition of fine
sediment, creating a soft muddy floor beneath the
mangrove trees. This substrate makes it easy for
prawns to bury themselves and for crabs and other
invertebrates to excavate burrows, providing an
additional means of shelter from predators
(Rönnbäck et al. 1999). The deposition of sediment
creates areas of shallow water around the
mangroves trees, interspersed with a network of
deeper channels, providing further protection from
predators for juvenile fish and prawns. The
vegetation also absorbs wave energy, rapidly
reducing wave heights as they pass through the
mangroves (Mazda et al. 2006), which may also be
an important factor for some juvenile fish and
invertebrate species.
1.6 Mangroves as nursery grounds
One of the most important and widely recognized
ways in which mangroves support fisheries is by
providing a “nursery ground” where juvenile
fishery species can grow to a size where they are
less prone to predation and therefore have higher
survival. Species that use mangroves as nursery
grounds often move out of the mangrove as adults,
perhaps to coral reefs, other offshore habitats or
even freshwater rivers. Nursery grounds can be
defined as habitats that produce more recruits to
the adult population per unit area than other
habitats in which juveniles are found (Beck et al.
2001). This is dependent on a number of factors:
Accessibility: The nursery ground must be
accessible to settling larvae or juveniles. While
many species are likely to be able to actively
select and settle amongst mangroves, such
settlement may also be enhanced by the lateral
trapping effect of mangroves which leads to
their holding back and slowing water flows
(Alongi, 2009).
Survival: The complex structure of mangrove
ecosystems provides shelter from predation
meaning that juvenile survival rates are likely to
be higher than those in unstructured estuarine
Growth: Growth rates, and therefore the length
of time to maturity, are strongly influenced by
food supply. Mangroves are highly productive,
providing an abundant food supply for juveniles
and facilitating high growth rates. The shelter
from predation may also allow juveniles to
spend less time seeking shelter and more time
feeding, further increasing growth rate.
Among the many species of fishery importance
which use mangroves in this nursery function are
many species of penaeid prawns, but also finfish
species including snappers, grunts, drums,
emperors, and whiting.
1.7 Linkages with adjacent ecosystems
Most marine ecosystems are open systems with
considerable fluxes of both organisms and non-
living components with adjacent coastal or riverine
waters. Mangroves are no exception as
demonstrated by their widespread utilisation as a
nursery habitat for many species (Manson et al.
2005) that remain outside of mangroves as adults,
and the noted import or export of primary
productivity from mangrove systems (Jennerjahn
and Ittekkot 2002). The magnitude of these fluxes
is challenging to quantify both at the species and
the ecosystem level and is likely highly variable
between sites.
1.7.1. Ecological linkages
Many offshore species are found in mangroves
during part of their life cycle, most commonly as
juveniles. Indeed, juveniles of some species of
penaeid prawn are found almost exclusively in
mangroves. Many fish species are also found in
mangroves as juveniles, and studies have
demonstrated the movement of juveniles from
mangroves to coral reefs and other offshore
habitats (Kimirei et al. 2013).
Commercially valuable penaeid prawns are found
in high abundance in mangrove systems as
juveniles (Rönnbäck et al. 1999, Vance et al. 2002).
Penaeid prawns spawn offshore, but their
planktonic larvae drift and ultimately settle in
estuarine waters where they spend a few months to
a year before once again migrating off shore (Potter
et al. 1986). Mangroves are believed to be a
particularly important part of this estuarine stage,
providing them with both abundant detrital food
resources and a refuge from predation. The degree
to which these services result in increased off
shore abundances, and in particular in offshore
catches, is still debated. There are numerous
studies that show a positive correlation between
catches and mangrove forest area (e.g.
Martosubroto and Naamin 1977, Paw and Chua
1991), but it is difficult to untangle the influence of
mangroves from that of other habitats within the
broader estuarine system (Manson et al. 2005), or
other factors such as freshwater input. Estimation is
further complicated as prawns may be caught many
kilometres from their nurseries.
Mangroves also have high abundances of juveniles
of many fish species, and their presence has been
documented to increase the abundance of fish in
adjacent habitats such as coral reefs and seagrasses
( e.g. Mumby et al. 2004, Jelbart et al. 2007). In
some cases fish that commonly utilise mangroves
as juveniles have been found to be absent from
stretches of coast with little or no mangrove
(Mumby et al. 2004). The linkage between
mangroves and adjacent habitats can be locally
strong. In Tanzania, for example, 99% of Dory
snapper, Lutjanus fulviflamma, caught on coral
reefs had lived in mangroves as juveniles (Kimirei
et al. 2013).
Aside from mangroves, structured marine habitats
such as coral reefs and seagrass beds are
themselves believed to be important nursery
grounds for some fish species (e.g. Nagelkerken et
al. 2000, Verweij et al. 2008). The linkage between
these habitats is therefore not always
straightforward. The benefits of having habitats
adjacent to one another may be additive (e.g.
Nagelkerken et al. 2001), or the habitats may
provide redundancy, with one being used if the
other is lost (e.g. Bologna 2014). There are
numerous studies that find movement of fish
between coastal habitat types at different life
stages (e.g. Cocheret de la Morinière et al. 2003,
Lugendo et al. 2005). This highlights the potential
importance of habitat linkages in enhancing fish
productivity, while also making it challenging to
isolate the role of mangroves in supporting
fisheries in such mixed habitat systems.
1.7.2. Nutritional linkages import and export
Mangroves export considerable volumes of organic
carbon into adjacent waters (see 3.1). As much as
11% of the total terrestrial carbon exported to
marine ecosystems may be mangrove-derived
(Jennerjahn and Ittekkot 2002). This exported
material plays a role in food chains in adjacent
seagrass and coral reef habitats, although this
effect appears to be very localised (Nagelkerken et
al. 2008; also see 3.1). Additionally, not all
mangroves are net nutrient exporters. Many benefit
from allochthonous nutrient and mineral inputs
either from terrestrial and riverine sources, or
indeed from marine transport.
1.7.3. Environmental modification
Mangroves benefit adjacent habitats such as coral
reefs and seagrasses which are also crucial habitats
for many fish species and therefore of great
importance to fisheries. Both habitats are
vulnerable to the effects of sediment carried out to
sea by rivers. Seagrasses are benthic plants, which
require clear water through which light can
penetrate for photosynthesis. Corals also gain
much of their energy from the photosynthesis of
symbiotic algae called zooxanthellae, and are
vulnerable to physical smothering by sediment.
Mangroves can act like a sieve for this sediment,
with the network of pneumatophores, aerial roots
and trunks slowing water flow and causing the
deposition of much of this sediment, preventing it
from reaching other habitats. Estimates for the
proportion of sediment trapped by mangroves
range from 15-40% (Golbuu et al. 2003, Victor et
al. 2006). In some settings, mangroves also remove
nutrients from the water, thus reducing algal
growth which can compete with corals for light on
Mangroves and seagrass in close proximity, Cuba. Photo by Mark Spalding.
Much has been written about the value of
mangroves for fisheries, and a key part of the
present work has been to understand and to
attempt to quantify this value. It is particularly
important that such values are not simply seen as
one-dimensional economic statistics. Mangrove
values for fisheries need to be viewed in a host of
different contexts. For example, inshore fisheries
are more valuable as a protein source in coastal
communities where there is no agriculture, or
where poverty prevents the purchase of other
protein. In terms of livelihoods, low-value fisheries
may provide much higher employment than high-
input, high value shrimp aquaculture which
employs few or no people from local communities.
Even monetary values need to be seen as multi-
dimensional, depending on which sectors of society
catch, prepare or market the catch, or how the
spending from recreational fishing is distributed
within local communities.
Coastal areas have high population densities and
some of the fastest population growth rates on the
planet. Some 210 million people currently live in
low elevation areas within 10 km of mangrove
habitats (TNC statistics). All but the most
inaccessible or strictly protected mangroves will
host fisheries of some sort, ranging from
individuals collecting crabs and bivalves to large
scale mechanized trawling or high value
recreational fisheries.
Fisheries use different parts of the mangrove
habitat. Molluscs and crabs can be collected
amongst the trees themselves, whilst finfish can be
caught in mangrove channels, estuaries, mangrove-
fringed lagoons and “flats”. Species such as
penaeid prawns that only use mangroves for a part
of their lifecycle may be caught many miles
offshore from the mangrove itself.
Numerous fishing methods are employed
depending on target species, cultural traditions,
and on the resources available to individual fishers.
These range from hand-harvesting by individuals
operating on foot, to large fixed traps in mangrove
channels, to complex gears operated from large
vessels. Appendix 1 provides a detailed listing of
the most widely used mangrove fishing methods.
In this section we firstly consider the valuation of
mangrove-associated fisheries, describing previous
work and summarising the methods used in this
project to expand and enhance this work. We then
go on to describe a number of mangrove-
associated fisheries following four broad
1. Inshore mixed species fisheries
2. Inshore mollusc and crustacean fisheries
3. Offshore commercial fisheries
4. Recreational fisheries
While these categories are clearly overlapping they
also have distinct characteristics based on the scale
of the operations, the target species and the fishing
gears that are typically used.
2.1 Valuing fisheries
That mangrove-associated fisheries are of
considerable value is widely accepted, and
numerous studies have detailed particular values in
locations around the world (see Tables 1-5).
Despite this, valuing mangrove-associated fisheries
is challenging, particularly at regional or global
Field assessments provide the most reliable
estimates of mangrove-associated fishery values
(in a given location), but the quality and usefulness
of such assessments is variable. Many are based on
catch-at-port, which is difficult to relate back to
specific mangrove areas. Studies also use highly
variable approaches: while some cover all harvest
from entire coastal communities, others focus on
sub-sections of the fisher communities. Likewise
values are reported over variable time-frames, from
single rapid assessments though seasonal, to multi-
year measures. The complexity and variability of
the fisheries themselves is a further challenge and
many studies focus on individual target species or
specific fishing methods, therefore only capturing
part of the total fisheries value. Smaller-scale
fisheries are often overlooked as being of little
economic value, but financial metrics may provide
a poor measure of value in rural settings. Also,
there are many species which rely on mangroves
during parts of their life cycle but which are caught
far from the mangroves and are therefore not
counted as “mangrove species”. One further
challenge which frequently arises is incomplete
description of studies, preventing any clear
discernment of the breadth of the study in relation
to the overall fisheries activities in the study
Despite these challenges a number of authors have
attempted to summarise the value of mangroves to
fisheries from local to global scales (Table 1). As
might be expected their numbers range over
several orders of magnitude. While some authors
have reported global values (e.g. Rönnbäck 1999,
Table 1: Existing estimates of the value of mangroves to fisheries on global or local scales.
de Groot et al. 2012), the substantial variability in
estimated value across studies suggests that such
extrapolations, especially when expressed as
simple averages, are highly uncertain and
potentially misleading. Such global extrapolations
also miss the spatial variability in mangrove-
associated fishery values due to both local
ecological factors, and a host of social, cultural and
economic influences.
In preparing this report we have compiled the most
detailed synthesis of the literature to date on
valuation of mangrove-associated fisheries. From
this we have begun to build an understanding of
the main drivers of spatial variance in value. Here
we briefly describe the work involved in that
literature review. We go on to summarise our
understanding of the major groups of fisheries,
with tables of values from a range of field studies.
In the final section, we describe our findings on the
main drivers of fisheries value, and propose a
simple conceptual model of how these drivers
2.1. 1. Literature review
Data on mangrove-associated fisheries around the
world was collected using a systematic literature
search. This used three sets of search terms to
represent mangroves, fishery target groups and the
action of fishing or harvesting (see Table 2). For a
result to be returned it had to feature at least one
term from each group in its title or abstract. The
terms were used to search three scientific
databases: ISI Web of Knowledge, Science Direct
and Scopus, returning just over 4000 papers.
Asterisks are used to denote wild-cards, so fisher*”
would search for “fisher”, “fishery” and “fisheries”.
(US $/ha/yr)
Value range
(US $/ha/yr)
De Groot et al
Median: 234
Mean: 1,111
Figure is for food provision
by all coastal wetlands, not
just mangroves.
Oropeza et al
Applies only to 5-10 m
seaward fringe of mangrove
Sathirathai &
Barbier (2001)
Based on a production
function linking mangrove
loss to changes in catches.
Naylor & Drew
Fish and crabs
Samonte-Tan et
al (2007)
Fish, molluscs and
Gunawardena &
Rowan (2005)
Mugilidae and
Walton et al
Within mangrove
Walton et al
contribution to
coastal and
offshore fisheries
Rosa et al
Crabs and
molluscs within
the mangrove
This review
Median: 77.3
Mean: 3114.8
0.2 12,305
See Table 3
This review
Mixed species
Median: 213
Mean: 623.7
17.5 3,412
See Table 3
Mangrove Terms
Fish Terms
Action Terms
cockle*, crab*, finfish, fish*,
oyster*, prawn*, shrimp*
aquaculture, artisanal, capture, catch, farming,
fisher*, fishing, gather*, growth, harvest,
landing*, nursery, refuge, survival, trap*, valu*,
Table 2: The final set of search terms used in the literature review.
The results were sorted by title and abstract, and
around two thirds were discarded as irrelevant. The
remaining 1579 were sorted into categories and
those most likely to contain useful data on fishery
catches and values were processed. This work is
ongoing and further categories may be processed
in future iterations.
Summary data from the identified papers were
stored in a purpose-built database, from which the
following data tables were extracted. Unless given
in the paper, the catch/unit area and economic
value/unit area were calculated by measuring the
area of mangrove within the study area, using data
from the USGS Global Distribution of Mangroves
layer (Giri et al. 2011). In some cases this may
produce very high or very low values/unit area if
the mangrove area is very large or very small.
Summary information, including estimates of the
catch and value, from some of these studies is
provided in Tables 3-5 below.
2.2 Mangrove-associated fisheries:
summaries by fishery type
In this section, we describe and summarise the
main characteristics of each fishery class. To
illustrate the diversity and the potential value of
mangrove-associated fisheries around the world,
we also present a series of case studies for each of
the broad mangrove fishery categories in appendix
2.2.1. Inshore mixed species fisheries
It has been estimated that over 90% of the world’s
fishers are employed in small-scale subsistence
and artisanal fisheries (FAO and World Fish Centre
2008), and small-scale marine fisheries land some
30 million tonnes of fish each year. Given the large
populations adjacent to many mangrove areas and
the occurrence of small-scale fisheries in most of
the world’s mangrove forests, mangrove or
mangrove-associated fisheries are likely to be an
important part of that catch. Mangrove-associated
fisheries are particularly important in developing
countries, where they provide a critical source of
food and income for many who have few livelihood
alternatives. Typically such fisheries use a broad
range of fishing methods, and exploit a wide range
of species.
Inshore mixed species fisheries include artisanal
fisheries conducted with limited equipment, on
foot or from open boats, for small-scale commercial
purposes and subsistence fisheries where the
catch is primarily used to feed the fisher, family
members and close community, with limited
market transactions. These categories have a large
overlap, with artisanal fishers using part of their
catch for subsistence, and many subsistence
fisheries making occasional or opportunistic use of
markets. In this report we have distinguished these
fisheries from inshore mollusc and crustacean
fisheries which, although also often artisanal,
usually target specific species or groups to supply
commercial markets.
Catches from inshore mixed-species fisheries are
rarely recorded or reported, making it difficult to
assess the volume or value of fish caught.
Nevertheless, several estimates that clearly
illustrate the importance of mangroves to inshore
mixed fisheries are available in the literature. A
study in a coastal province in Madagascar found
that 87% of the adult population were employed in
fisheries, with the majority of the men fishing and
the women gleaning along the shoreline (Barnes-
Mauthe et al. 2013). The 2756 fishers caught 5500
tonnes of fish and invertebrates in 2010, of which
almost 2000 tonnes was caught in and around
mangroves. Eighty-three percent of the catch was
sold earning an average of about US $2200 per
fisher. The rest was eaten by fishers’ families and
friends. Other studies show similar patterns in
Thailand (Islam and Ikejima 2010) and Mozambique
(de Boer et al. 2002). A summary of literature
derived values for mixed species fisheries can be
found in Table 3.
Mangrove fishers, Papua, Indonesia. Photo by Wetlands
Fishing methods used in these small-scale fisheries
are highly variable spatially, culturally and by
sector of society. Various types of net are common,
including seine nets, gillnets, cast nets and lift nets,
as well as traps such as fyke nets, pound nets and
crab pots. Hooks and lines are also common for
catching finfish. In Thailand, Islam and Ikejima
(2010) described six distinct techniques observed:
crab traps, channel traps, gill nets, catfish hooks, lift
nets and hand capture. Hand-collecting is common,
especially in the context of opportunistic gleaning
along the shoreline or within the mangroves, a
practice often dominated by women and children.
Small-scale fisheries often have lower
environmental impacts than larger scale
operations. Discards are generally low as species
with low market value may still be eaten by the
fisher and his family, while the high value species
are sold. Environmental impact is also relatively
low compared to techniques such as trawling.
Overfishing can still be a problem, however,
because small-scale fisheries are often open access
and may require little or no equipment making
them an attractive option for those with no other
source of income. This means that economic
pressures and growing populations can increase
fishing pressure leading to overfishing. This is
demonstrated by declining catches in both
Mozambique and Thailand (de Boer et al. 2001,
Islam and Ikejima 2010).
Table 3: Catches and economic values of small-scale mixed fisheries from studies found in our literature
review. Where economic values were given in local currency, they have been converted to US $ using
present-day exchange rates.
area (ha)
catch value
Bennett &
US $
-Henao et al
San Ignacio
- Navachiste
- Macapule
401.7 t/yr
et al 2008
Dam Doi
US $
Location: Mostly within the mangrove and close to
settlements. Fishers travel on foot or by small boat,
often human-powered, limiting the area that they
can fish. Subsistence and artisanal fisheries are
mostly found in developing countries, although
indigenous populations in developed countries
such as Australia may also use traditional fishing
Target species: Subsistence and artisanal fisheries
tend to be mixed-species. Higher value species are
targeted, but they may also include species such as
gastropods and small crabs that have no
commercial value but are still edible.
Consumption: In some cases, the catch may be
mostly consumed by the fisher and their family,
but most fishers will also trade or sell part of their
catch when they have a surplus. This trade will
often be local, within the fisher’s village, but may
be to middle men or in larger markets if the fisher
lives in or near a large population centre.
Methods: Hand-collecting is universal for molluscs
and crabs. Crabs may also be trapped in pots. For
fish, cast nets and gill nets are commonly used in
channels and at the edges of the forest. In some
regions, fishers build brush parks to attract fish and
invertebrates (see case study in Appendix).
area (ha)
catch value
Conde 1996
Laguna de
de Boer et
al 2001
26.2 t/yr
de Graaf &
Xuan 1998
Ca Mau
1,379 t/yr
na & Rowan
Sri Lanka
25.2 t/yr
na & Rowan
Sri Lanka
Lagoon -
36 t/yr
Sri Lankan
na & Rowan
Sri Lanka
Lagoon -
92 t/yr
Sri Lankan
Islam &
496.4 t/yr
551,050 US
Janssen &
Kairo et al
Gazi Bay
et al 2000
Naylor &
Drew 1998
States of
170,000 US
790266 US
Qin et al
Hong Kong
Mai Po
US $
Tan et al
3,236 US
Tan et al
49 US $/ha
1,369 t/yr
Singh et al
2,061 t/yr
Singh et al
539.7 t/yr
28.8 million
Singh et al
744.6 t/yr
38.1 million
Walton et al
213 US $/ha
2.2.2. Inshore mollusc and crustacean fisheries
In addition to the mixed-species fisheries described
in the previous section there is a substantial
targeted fishing effort within mangrove areas that
focuses on a few key species and is typically
commercial, though often small-scale. These
fisheries target a small number of invertebrates
with relatively high market values, notably crabs
and bivalve molluscs. Such fisheries will typically
employ distinct harvesting techniques for the
desired species, rather than more indiscriminate
methods. These fisheries require markets for their
catch, so where coastal populations are low and
there is no easy access to markets they may be less
common. Many of the target crab species spawn
offshore and spend part of their life cycle as
pelagic zooplankton before migrating back to the
mangroves where they remain resident for the
remainder of their lives and are therefore
extremely mangrove-associated (e.g. Hill 1994,
Oliveira-Neto et al. 2007). Mangrove crustacean fisheries
Numerous crab species are found in mangroves,
ranging from herbivores feeding on mangrove
leaves to key predators. Many species are exploited
by small-scale subsistence fisheries, but a few also
have significant commercial value. Two examples
are discussed in the case studies (Appendix 2). The
other major crustacean groups linked to mangroves
are the prawns, and while a large part of
commercial prawn harvests are linked to offshore
commercial sectors (see 11) there is also an
important inshore fishery for juvenile prawns that
are used to supply aquaculture. This fishery is
gradually being replaced by larvae from hatcheries,
but wild larvae are considered to be of better
quality and have higher survival than hatchery-
produced larvae so remain in high demand in many
Relatively few examples of specific crab fisheries
were found in our literature search, although the
majority of the small-scale mixed catches include
crabs. The examples found show that catches can
reach thousands of tonnes per year, but are often
much smaller than this (Table 4). Mangrove bivalve fisheries
Bivalves are abundant in mangroves, where they
benefit from the high primary productivity, and
from the soft sediment for burrowing species and
the solid structure that roots provide for species
living on hard substrates. Many bivalve species are
collected as part of mixed-species fisheries, but
targeted fisheries also exist for a few commercially
valuable species groups, notably oysters and
mangrove cockles (see Appendix 2: Case studies).
Location: Mostly within the mangroves, but
occasionally in estuaries or just offshore. Fishers
mostly travel on foot or by small motor boat, often
a long-tail boat using an adapted car or motorbike
engine for power. They are limited by distance
from settlements and to the larger population
centres that provide markets for their catch.
Widespread in many developing countries, but also
economically viable in many developed countries.
Target species: Large crabs, oysters, cockles and
sometimes juvenile penaeid prawns to stock
adjacent aquaculture ponds.
Consumption: A key component of the catch is sold
to traders or direct to markets. Some part of the
catch may also be consumed by the fisher and their
family, who may also consume some of the bycatch
if the method used produces any. Highest value
species may be transported internationally.
Methods: Catch methods are diverse and often
highly specialised to each target species. Hand
collecting is still common, particularly for sessile
bivalves. Traps and nets are also used for some
Table 4: Catches and economic values of mangrove crab fisheries from studies found in our literature review.
Where economic values were given in local currency, they have been converted to US $ using present-day
exchange rates.
area (ha)
(US $)
Henao et al
San Ignacio -
lagoon system
Dumas et al
Temala mangrove
18.2 t/yr
Dumas et al
Voh mangroves
36 t/yr
Dumas et al
Xujo mangrove
35.4 t/yr
Lebata et al
Naylor & Drew
States of
Bivalve collectors are often amongst the poorest
members of society, as the open access conditions
and little equipment required makes it an attractive
option for those with no other sources of income
The literature review found a number of studies of
mangrove bivalve fisheries, but these were largely
focussed on the socio-economic aspects of the
fishery so had little catch data (e.g. Mendonca and
Machado 2010, Beitl 2011).
2.2.3. Offshore commercial fisheries
Most reports of mangrove-associated fisheries
focus on fishing activities that take place within the
mangroves and associated channels and lagoons, or
in adjacent waters. In reality many species derive
benefits from mangroves for only part of their life
history, often migrating out from the mangroves as
they mature. A number of publications have looked
at the relationship between mangroves and coral
reef fish (e.g. Kimirei et al. 2013, see also 7 above).
The quantification of this relationship is very
Location: Offshore and across the continental shelf, sometimes 10s or 100s of kilometres from the mangroves.
Fishers use larger boats capable of travelling long distances to the best fishing grounds. Much of the fishing
process may be mechanised, for example through the use of winches to haul nets, reducing the number of crew
needed to man the boat. Large boats may also have on board freezers to freeze the catch, meaning that fish
caught do not have to be immediately brought to markets. This enables boats to stay at sea for longer and to
fish further from port.
Target species: Highly targeted towards high value species, with lower value bycatch often discarded. The
primary commercial target species of interest here are a number of penaeid prawns, many of which spend post-
larval stages in estuarine and mangrove habitats before moving offshore as adults. A number of finfish species
also show varying degrees of mangrove association in juvenile life-history stages, and are of considerable
commercial importance, including barramundi Lates calcarifer, various species of snapper (Lutjanidae), mullet
(Mugilidae) and sea catfish (Ariidae). In all cases the degree of dependence of these species on mangroves is
less clearly defined and hence these fisheries are not considered further in this report.
Consumption: Catches are usually landed at larger ports with factory-scale processing facilities. Much of the
catch may be frozen for export to other countries.
Methods: Penaeidae prawns are almost entirely fished with benthic trawls. Some high value finfish species such
as barramundi are also targeted with gill nets and hook and line.
challenging and in the present work these fisheries
enhancement benefits are not considered further,
with the single exception of benthic shrimp
species. Offshore prawn fisheries
Commercially valuable prawns, mostly in the family
Penaeidae, spend a few months as juveniles in
inshore, especially mangrove, areas before
migrating off shore for the remainder of their lives
(see Appendix 1). The magnitude of the role that
mangroves play as prawn nursery grounds is
difficult to quantify, as prawns may be caught tens
of kilometres from the mangroves they benefited
from as juveniles. Nevertheless, numerous
attempts have been made to determine the value
of mangrove areas to offshore prawn fisheries. The
reported values cover a range of spatial scales
(Table 6). Reported values per unit are of mangrove
vary widely. This is because the catch associated
with a given area of mangrove varies not only with
the extent of mangrove, but also by rainfall and
temperature (Vance et al. 1985), and hence estuary
location. Prawns are also highly dependent on
estuaries in general, making it difficult to tease out
the relative enhancement due to the presence of
mangroves specifically (Lee 2004). Nevertheless,
juvenile prawns are often found in greater
abundance in mangroves than in other estuarine
sites (e.g. Robertson and Duke 1987, Primavera
1998), and therefore it is likely that offshore
fisheries are enhanced by mangroves.
Table 6: Catches and economic values of mangrove prawn fisheries from studies found in our literature
review. Where economic values were given in local currency, they have been converted to US $ using
present-day exchange rates.
area (ha)
t area
catch value
value (US$)
Barbier & Strand
$139,352 US
Henao et al 2013
San Ignacio
Navachiste -
lagoon system
1960 t/yr
Chavez-Rosales et
al 2008
Magdalena Bay
- Bay
55.4 t/yr
Chavez-Rosales et
al 2008
Magdalena Bay
- Mangrove
92 t/yr
Chavez-Rosales et
al 2008
Magdalena Bay
- Offshore
130.5 t/yr
Chavez-Rosales et
al 2008
Magdalena Bay
- Total
277.9 t/yr
Fouda & Ali-
Muharrami 1995
370 t/yr
Grasso 1998
97 t/yr
Gunawardena &
Rowan 2005
Sri Lanka
10.8 t/yr
Ruitenbeek 1994
Bintuni Bay
5500 t/yr
68.75 billion
Semesi 1998
188.1 t/yr
2100 t/yr
2.2.4. Recreational fisheries
Recreational fishing is carried out for pleasure. In
some cases it generates a small harvest, usually for
personal consumption, but in other cases the catch
may not even be kept. As a hobby or sport it is at
least 350 years old, and is popular with millions of
people worldwide. Mangrove fishing includes
simple shore-based efforts, but also boat based
fishing, and most popularly using rod-and line
Among the highest value recreational targets are a
range of fish species valued for their “fight”; the
challenge of catching them, as opposed to their
nutritional value. These fish species, such as tarpon
(Megalops spp.) and bonefish (Albula spp.), attract
recreational fishermen on holidays or daytrips. The
transport, accommodation, food and guiding
associated with these trips usually requires a high
economic input and hence the value of recreational
fisheries can be very high, and often higher than
other mangrove-associated fisheries. For example,
catch-and-release fishing for bonefish contributes
around US$1 billion per year to Florida’s economy
(Ault et al. 2010). In developing countries
recreational fishing can be a major part of the
income from tourism, which may be the main
source of income for many coastal communities.
Fishing for bonefish, permit and tarpon was worth
US $56.5 million to Belize in 2007 (Fedler and
Hayes 2008), and US $141 million to the Bahamas
in 2008 (Fedler 2010).
2.3 Drivers of mangrove fishery catch
and value
In order to estimate the extent to which a given
area of mangrove will benefit fisheries within and
around it, it is necessary to understand the drivers
of fish productivity and fishery value. As a habitat
type, mangroves are highly variable. They are found
across a broad range of climate types from wet
tropical to desert and temperate regions. Individual
mangrove areas may have anything between one
and 50 of the roughly 65 mangrove species, and
the trees may be anything from small shrubs to
40m tall forests. Environmental settings are also
important: estuarine mangroves with abundant
nutrients and fresh water input will be taller and
more productive than mangroves on oceanic coral
islands. Each individual mangrove forest is
therefore unique, and this extensive variability
makes predicting fish production a challenge.
Nonetheless, there are some common factors that
influence production and fishery value in all
We have decided to focus valuation around three
key sets of drivers (Figure 2). Firstly we consider
the environmental drivers which can predict
potential fishable biomass of a place the likely
amount of fish available for fishers under natural
conditions. Secondly we consider the measures of
condition which can helps us to predict actual
fishable biomass in a place mangroves, the
surrounding waters and their fish-stocks are
already impacted in almost all areas by human
impacts and these typically reduce the amount of
fish biomass available to fishers. Thirdly, a host of
socio-economic factors determine the fished
biomass in any place this is a critical metric for
ascertaining value, which can then be measured in
many different ways
Location: Recreational fisheries are mostly
undertaken by boat and mostly based in the flats
between and adjacent to mangroves.
Target species: Target species closely linked to
mangroves include: bonefish (Albula spp.), tarpon
(Megalops spp.) barramundi (Lates calcarifer),
snook (Centropomus spp.), and snapper (especially
mangrove snapper Lutjanus argentimaculatus).
Consumption: Obtaining fish as a source of food is
often only a secondary goal in recreational
fisheries. Indeed, many recreational fishers
practice “catch and release” fishing, especially
with species that are of conservation concern.
When catches are retained they are generally small
compared to commercial fisheries, although they
can be a significant part of the catch of the most
popular target species.
By far the most important fishing method
used in recreational fisheries is hook and line.
There is a wide range of different techniques
within this, with different baits and lures based on
target species, location and the preferences and
experience of individual fishers. Some of the main
techniques used include trolling, where a lure is
towed behind a boat, lure fishing where a plastic or
metal lure is retrieved through the water to mimic
a small prey fish, and fly fishing where the lure is
made of feathers or synthetic equivalents and is
cast with the aid of a weighted line. Other less
common forms of recreational fishing include bow
fishing and spear fishing, which use arrows or
spears to impale the fish. These may be used from
the shore or a boat, or in the water whilst
snorkelling or scuba diving. In some countries such
as Australia the recreational fishery extends to
cover other fishery targets, including crustaceans.
Figure 2: A conceptual model of the drivers of mangrove fishery catch and value. Environmental drivers
determine the potential fishable biomass that might be present in natural conditions. The actual fishable
biomass is derived by modifying the potential biomass based on human impacts on the mangrove ecosystem
and fish stocks, which may be mitigated by conservation and fishery management. The catch depends on the
actual fishable biomass, and the socio-economic drivers that determine fishing effort.
2.3.1. Environmental factors and potential
fishable biomass
Productivity: Primary productivity, both of the
mangroves themselves and of other producers on
solid surfaces and in the water column, is one of
the key reasons why mangroves are important to
fisheries. This primary productivity provides the
basis of a food web that ultimately supports
species that are valuable to fisheries. High primary
productivity of both mangroves and other
producers will lead to increased fish production.
Nutrient supply: High levels of nutrient input will
enhance mangrove growth, but perhaps as
importantly the primary productivity of periphyton
and phytoplankton. While it might be expected to
see increased fisheries productivity associated with
high nutrient levels, such a relationship would
unlikely be simple and it might be expected, in
extreme high nutrient settings that de-oxygenation
and dead zones could lead to a dramatic fall in fish
Freshwater input: This has been positively
correlated with both mangrove productivity and
fish and prawn production (e.g. Vance et al. 1985,
Meynecke et al. 2006). Once again this relationship
will be complex: while mangroves may benefit
from brackish water settings not all fishery species
will survive in very low salinities. Some of the most
extensive high-productivity mangroves are found
in estuarine and deltaic settings, particularly in
areas of year-round high flow rates. Climate can be
important, with high rainfall areas also benefitting
from lower salinities, while the most arid areas can
suffer hyper-salinisation.
Mangrove area/length of mangrove margin: If
mangroves enhance fish production and stocks,
then the total area of mangrove is clearly important
in determining the total numbers of fish. The shape
of the mangrove block is also likely to be
important. Fishery target species only penetrate a
certain distance into the mangrove from the sea or
from rivers. This distance is dependent on tidal
height, channels in the mangrove and relief of the
coastline. In larger mangrove blocks, only those
fringes accessible to areas of more permanent
inundation (seaward fringes, channels, lagoons, and
pools) will play a significant role in enhancing
fisheries, so mangrove area within a set distance of
the sea or estuary may be a better indicator than
total area (Aburto-Oropeza et al., 2008) as well as
the length of the mangrove margin.
Climate: Climate will directly influence mangrove
productivity. Temperature, rainfall and seasonality
are all likely to be important. It may also influence
productivity of fishery target species. In particular,
the growth rate of invertebrates such as prawns
and crabs are directly affected by water
temperature (Staples and Heales 1991).
Biogeographic and ecological setting: Diversity of
both mangroves and marine animals is not evenly
distributed around the world. South-East Asia has
almost 50 species of mangrove, whilst Africa and
the Americas only have around 10. Even within a
region there will be variation, with small islands
tending to have lower diversity than large mainland
forests. Physical structure of the mangrove trees
varies by species, and is important in providing a
large surface area for primary production;
attachment points for sessile invertebrates;
protection from predation; and a benign physical
environment. In particular, the different aerial root
structures may benefit fisheries to different
extents. For example, in the Philippines, more fish
were found amongst Avicennia pneumatophores
than amongst Rhizophora prop roots. The same was
true of juvenile prawns in the interior of the forest,
but not at the seaward margins (Rönnbäck et al.
1999). Prop roots, on the other hand, are likely to
provide better attachment points for bivalves such
as oysters. Patterns of mangrove diversity are
paralleled in a number of other marine and coastal
taxa, including seagrasses, corals and fish, and it
seems likely that the areas of highest mangrove
diversity (from the Bay of Bengal to Northern
Australia and the Solomon Islands) will also be
home to the highest diversity of species of fishery
interest. It remains unclear whether this will
influence total productivity and biomass.
It seems likely that some combination of these
drivers might be used to develop an understanding
of fish productivity or standing biomass, or indeed
a subset of these numbers, focusing on key targets
for different fisheries. These could be represented
in a conceptual model of potential fishable
biomass, as shown in Figure 2.
2.3.2. Human impacts
In reality, many of the world’s mangroves, and their
fish-stocks, are degraded to some degree by human
impacts. For the purpose of conceptualising such
conditions we have distinguished three main
drivers, all of which can be mitigated by
conservation efforts and by management of fish
stocks (included in 2.3.3).
Mangrove condition: Human influences on
mangroves may also impact the extent to which
they benefit fisheries. Degradation through
logging, wood cutting for charcoal and pollution
damage may reduce the productivity of the forest
and the amount of physical structure it provides.
There may also be differences between natural
mangroves and mangroves that are replanted,
either as forestry plantations or for restoration
Water condition: Pollution will directly influence
fish and invertebrates living in the water. In some
cases nutrient pollution may actually enhance
productivity, but other impacts which can have
negative impacts include oil pollution, anoxia
driven by extremely high nutrient levels, and the
occurrence of harmful aquatic blooms which can
threaten marine life, but also greatly impact
fisheries through human health risk impacts.
Fish-stock condition: Fisheries around the world
have suffered from poor management and over-
harvest, or indeed from damaging fishing practises.
Mangrove-associated fisheries are similarly
impacted and there are many cases where over-
harvest of target stocks have been recorded and
where fishers are travelling further or otherwise
suffering from reduced yields. In looking at simple
studies of harvest it is typically difficult or
impossible to ascertain where yields are with
respect to sustainable maxima.
Mangrove conservation: Around a quarter of all the
world’s mangrove forests are found in areas
designated for the conservation of biodiversity
(Spalding et al. 2010). In a few cases such protected
areas may be closed to fishing, but the majority
have fewer restrictions, more equivalent to
fisheries management that may enhance yields and
stability. In addition, such protected areas in many
cases will reduce the likelihood of degradation and
pollution, and so have the potential to enhance
fisheries yields or value. As with fisheries
management efforts, protected areas declared in
policy statements or even legal documents are not
always effective.
By building these condition measures onto the
potential fishable biomass we can envisage a
measure of actual fishable biomass, as shown in
Figure 2.
2.3.3. Socio-economic factors and fished biomass
The actual, or realised, value of any resource
depends not only on its availability, but on its
utilisation and on the many factors which influence
demand. In some cases those demands may also be
the drivers of declines in condition, but this is not
always the case, demand can also be regulated to
prevent such declines.
Proximity to people: The presence of people within
the vicinity of mangroves is a key determinant of
the demand for fish. Demand might be expected to
show some correlation overall coastal populations
numbers, but this will also influence the type of
fishing, with small-scale mixed species fishing
being overtaken by more targeted and higher value
fisheries as populations increase. Large markets
can have a disproportionate influence on fishing
effort, provided those markets can be connected
(often a function of infrastructure as much as
physical distance).
Economic conditions: available wealth, as
measured through GDP or other metrics, can have a
dramatic influence on the simple market values of
fisheries. Other metrics, including the distribution
of wealth, and social measures such as employment
may also be important in understanding value
independent of currency and national wealth
Cultural traditions: alongside current economic
drivers, societal traditions can be of considerable
importance in driving levels of fishing effort and in
creating market demand. Perhaps the most obvious
of these is a strong tradition of fishing and of fish
consumption. Economic development may increase
the availability of other protein sources, and other
forms of employment. However, fish consumption
may still rise, and with it the values of fish. This is
demonstrated by China, where average fish
consumption has increased from 4.3 kg per person
in 1961 to 31.9 kg in 2009.
Alternative livelihoods: in some places,
irrespective of cultural traditions, fishing may be
encouraged by a lack of alternative livelihoods,
while fishing pressures are likely to be low where
manufacturing, agriculture or service sectors are
strong, and where the relative profits from fishing
are low.
Fisheries management: Fisheries management
aims to maximise long-term catch from fisheries.
Where effective management is in place, it can lead
both to stability, and to increased yields compared
to unmanaged and over-fished locations. One
common management tool in fisheries
management can be closure of some areas to
secure breeding populations for a wider area, and it
is important in such places for values to be
calculated over the entire management area.
Similarly, unmanaged areas can produce very high
value benefits in the short term prior to fisheries
collapse. The lower, but more stable returns from
well managed fisheries cannot be directly
compared to such benefits.
One key metric for assessing final value then, is the
fished biomass (Figure 2), ideally broken down by
key target species. This value provides a base
metric which underpins many other value metrics
which might be measured in terms of hard
currency, jobs, or food security. Once fishing
pressure is high, the management of fishing effort
may determine the longer term value and stability
of those values.
Selling fish in India. Photo by Adriaan Backer.
Fish is a common source of protein. Photo by Mark
In order to maintain, or indeed to enhance, the
value of mangroves for fisheries it is critical that
every effort is made to manage them appropriately.
The need for active management increases with the
number of people living close to the mangroves.
Active management has two broad components:
maintaining or restoring the mangroves and
managing the fisheries. Critical to the uptake of
these activities is the establishment of a desire to
instigate such interventions, which may require
active engagement in science, outreach and
education as well as changes to management
3.1 Avoiding mangrove loss
Mangrove loss has been driven in large part by
conversion to other uses. Perhaps the biggest
single driver has been conversion to aquaculture
ponds, but large areas have also been converted to
agriculture and to more intensive uses such as
urban and industrial expansion. In a few places
such conversion may bring genuine benefits, but all
too often such conversion is justified on
incomplete economic arguments and short time
horizons. Many aquaculture ventures are only
highly profitable for a few years before
productivity declines dramatically; meanwhile
many aquaculture benefits are typically
concentrated in the hands of a small number, while
losses may be felt by a much wider community.
Many areas of agriculture conversion have been
unsuccessful as soils have become saline or acidic.
Meanwhile any developments in intertidal areas
are an increasingly risky proposition particularly in
the face of accelerating sea level rise.
Avoidance of mangrove loss is most effectively
achieved through regulation protection and/or the
development of strong local or community level
Some countries have established blanket
protection at national or regional levels, thus in
Tanzania, and Malaysia, mangrove areas are state
owned, by law, and there are strict regulations to
control or prevent loss. In Australia and Florida,
permitting requirements for mangrove loss are
often granted only under some provision of “no net
loss”, requiring that any mangrove loss is
compensated by some restoration in other
Protected areas are another widely used tool, and
over a quarter of the world’s mangroves are located
in areas designated for nature conservation. The
regulations around protected areas are highly
variable, and in some cases active management and
harvesting may be permitted, but full-scale
clearance would unlikely be permitted in any
protected area.
Local or community level ownership has proved a
powerful tool for protection in many countries.
Such ownership contrasts markedly with both
private ownership and full open access. Individual
private ownership has often led to damage and
loss, while full open access can lead to the “tragedy
of the commons” with over-utilisation leading to
degradation or loss of the resource. Limited
ownership of resources, following Orstrom’s model
of “common pool resources” (Ostrom 1990) has
often promoted wise and sustainable use in
communities in communities in the Philippines, for
Controlling ex situ threats can be another critical
activity, which may require engagement with a
much wider group of stakeholders depending on
the nature of the threats. Such controls may include
inland modifications of dams and irrigation,
alteration of coastal development and engineering
works, regulation of extractive industry such as oil
and gas.
3.2 Restoring mangroves
Mangroves are robust opportunistic species, which
means that they can re-colonise or recover in many
areas where they have been lost, and will also
establish themselves in new areas where soil and
hydrological conditions are good. Such recovery
can be enhanced with a variety of interventions,
but it is also important to note that it is almost
always more cost-effective to prevent mangrove
loss than to allow loss and to have to invest in
rehabilitation or restoration.
Around the world hundreds of thousands of
hectares of mangroves have been actively planted
in mangrove restoration projects from Bangladesh
to Cuba, and from Florida to Australia. Not all
restoration projects have been successful, but
likewise many areas of mangroves have re-
established themselves without active planting or
management (Lewis III and Brown 2014).
There is not a large literature on the fisheries
production value of restored or recovering
mangroves. However it seems likely that fishery
benefits do recover following mangrove
restoration, although this may take time (Bosire et
al. 2008). Younger and less diverse forests will
have lower productivity, and the simplistic
structure of a young plantation may be less
effective in producing fish than a more physically
and ecologically diverse mangrove system.
Before any efforts are made towards restoration it
is critical to understand both the cause of original
loss and the current ownership and regulatory
regime. There is simply no point in restoring
mangroves if the threat to their existence remains,
or if the present owners do not want, or do not
stand to gain, from mangrove restoration. Assuming
that enabling conditions are good, however, there
are some valuable interventions.
Restore hydrological conditions. Widespread
mangrove loss has been caused by deliberate or
accidental physical disruption to the mangrove
sediments in the case of aquaculture this typically
involves the building of dykes and basins, often
leaving only narrow areas of intertidal sediments
where mangroves can colonise. Restoring a more
natural and even surface can be achieved at small-
scales by local communities, but can be greatly
facilitated by heavy earth-moving machinery,
taking down dykes and even creating more natural
drainage channels to encourage tidal flows. In
many places tidal flows have been interrupted by
the building of roads which have then cut off
natural water movements. Such flows can often be
restored without complete removal of the
infrastructure, but with the building of channels or
tunnels to restore semi-natural tidal flow regimes.
Restore sediment supply. A loss of the supply of
sediments may be an important driver of erosion
and mangrove loss in many areas. This can include
riverine sediments failing to reach estuarine and
deltaic settings as a result of upstream water
abstraction and the building of dams as observed in
the Indus Delta in Pakistan (Gupta et al. 2012). It
can also include the natural movements of
sediments along coasts: erosion along muddy
coasts in Thailand, China, Java, Suriname and
Guiana has been linked to the conversion of wide
parts of the mangrove to aquaculture or agriculture.
These changes have greatly decreased onshore
sediment fluxes across the remaining, narrow
mangrove strip and led to net erosion along what
had been stable or accreting shores (Winterwerp et
al. 2013). Restoring such settings may require
highly active interventions, and a number of
communities are experimenting with approaches
such as the building of semi-permeable barriers to
trap sediment, combined with mud nourishment or
agitation dredging to enhance sediment in the
water column (Winterwerp et al. 2013)
Restore freshwater flows. Many of the most
abundant, diverse, and productive mangroves are
found in estuarine and deltaic settings where they
benefit from more brackish waters and there is
evidence that decreases in freshwater inputs into
these settings has led to decreased diversity among
mangroves and to loss of primary productivity, with
a knock-on impact on fisheries.
Allow natural recovery. In many places mangroves
will recover naturally simply following the
restoration of conditions of hydrology, sediments
and/or freshwater inputs. In some places such
natural recovery can be helped by the removal of
dense ground-cover from salt-tolerant grasses and
ferns which otherwise prevent mangrove
colonisation. Natural regeneration is a widespread
practise in many mangrove “plantations”, and in
some places this is facilitated where some larger
trees have been left un-harvested to provide new
Full plantation. Where recovery is not occurring, or
appears too slow, it can be enhanced through
active plantation. If this is attempted it is critical to
follow natural requirements of mangroves.
Expensive and catastrophic failures in mangrove
restoration have taken place because people
attempted to plant the wrong species, often in the
wrong places. Mangroves are intertidal species
which grow best above mid-tide levels. Planting
them in deeper water areas may seem an easier
proposition, particularly where higher elevations
are privately owned, but mangroves cannot grow
out of their depth. Likewise the easiest species to
plant, such as the large Rhizophora propagules, will
not always grow on exposed outer margins. Ideally
plantations should mimic nature, replicating
density, structural complexity, and natural
restocking, using multiple species with natural
3.3 Managing fisheries
Maintaining or restoring mangrove coverage and
health may be critical to securing the natural fish
production properties of mangroves. However, in
many heavily fished locations, fish production is
primarily constrained by the impact of fishing
itself, with overfishing or destructive fishing
practises reducing the productive capacity of the
stocks. Few stocks are sufficiently well understood
or modelled to be able to know exactly what
natural stocks should look like, or to be able to
model or predict Maximum Sustainable Yields. Even
so, precautionary management can lead to
improved yields and increased profits and many of
the standard fisheries management methods can
be applied in mangroves:
Local resource ownership and use. Strict local
ownership, through systems such as Locally
Managed Marine Areas (LMMAs) or Territorial Use-
Right Fisheries (TURFs) essentially devolve all
ownership of natural resources to a community
who will greatly benefit from its ongoing
sustainable management, and will often actively
police this area, preventing use by outsiders.
Gear or harvest regulations. Certain approaches
such as the placing of gillnets across major
channels, the use of non-discriminatory gears with
excessive bycatch, or the cutting of aerial roots in
order to harvest mangrove oysters are
unsustainable in most settings and direct
regulation to restrict or ban such practises may be a
critical component of any management. Further
regulation may restrict the size of traps or mesh-
sizes to reduce catch of juveniles, or set size-limits
for the harvest of key species, typically also to
protect juveniles, but in some cases to maintain a
stock of some of the largest, most fecund, adults in
a population.
No-take zones. Many small-scale coastal fisheries
have been shown to benefit from the complete
closure of all fishing activities in certain “no-take”
zones. These zones appear to be most effective in
areas where fish are closely linked to a fixed
substrate. In those areas no-take zones become
refuges which export larvae to enhance
recruitment in surrounding fished areas, as well as
generating some spill over of adult fish.
Access agreements. Restrictions on who can fish
can have a similar effect to more formally agreed
spatial use agreements. This may be achieved
through government level licensing or more local
controls, such as the brush park fisheries in Sri
Certification. There is a growing demand in certain
export markets for a guarantee of sustainability. For
example, the prawn fishery in Northern Australia
has achieved certification from the Marine
Stewardship Council which may be an important
driver of high value sales. This requires that
fisheries demonstrate that their fish stocks are
sustainable, their environmental impact is
minimised and that management measure are
effective (Marine Stewardship Council 2013).
Similarly, standards for sustainable aquaculture like
the Aquaculture Stewardship Council exclude
development of aquaculture in mangrove areas and
demands rehabilitation of degraded mangroves.
3.4 Communication and engagement
Both mangroves themselves and mangrove-
associated fisheries are highly amenable to
management interventions which can generate
considerable social and economic benefits. For
such management to be undertaken there needs to
be a clear understanding and ideally an accurate
quantification of these benefits. Such values must
also be communicated to a broad audience. And for
any management to be implemented a viable
enabling environment must be present in terms of
policy and legal frameworks.
Quantifying values. Some of the most effective
changes in management can be driven by a real
understanding of underlying values, ideally with
models of sufficient capacity to enable scenario-
building. Such quantification needs to be site
specific and more reliable than simplistic
extrapolation from studies conducted in other
places, or generic statements of global value based
on averaged studies. It needs to report in relevant
numbers for managers, which may be fishable
biomass, jobs, food security, or direct economic
As with other fisheries, reliable models will be
valuable not only for making a case for mangrove-
associated fisheries management, but in driving the
ongoing management of the fishery through time
as conditions and stocks fluctuate.
Communicating value. The value of mangrove-
associated fisheries, and of mangroves to offshore
fisheries, is often underestimated or overlooked, in
some cases even by the fishers themselves. In
order to drive a change in the way mangroves are
managed it may be necessary to challenge such
views, reaching out to multiple sectors from coastal
populations to consumers of mangrove produce to
coastal developers and senior policy-makers,
including economists. Such values need to be
communicated simply in relevant metrics. Ideally
they should be placed alongside alternatives, with
trade-off analyses enabling all sectors to
understand all relevant costs and benefits.
One key additional component in understanding
mangrove value is to consider fisheries values
alongside other benefits. Mangroves provide a
whole bundle of ecosystem services of which
fisheries may only be a small fraction. This may be
in stark contrast to alternative uses such as
aquaculture or built sea defences. Aquaculture
may, in some places, or in the short term, generate
higher economic returns, but will not provide
coastal defence, carbon storage or water
purification. Sea walls may provide defence against
storms with a smaller spatial footprint, but
replacing mangroves with sea walls leads to loss of
associated fisheries enhancement, timber provision
and other benefits.
Creating an enabling environment. Even when
there is understanding of the value of mangroves,
and a willingness to make management changes
there may be a need for reform of legal, policy and
tenure frameworks. Key among these may be
reform of subsidies that encourage mangrove
conversion to aquaculture or coastal agriculture.
The treatment of mangroves as “common goods”
can drives over-harvest and loss, but fully private
ownership may also encourage conversion to the
detriment of the wider community. Building a more
robust tenure system can go a long way towards
ensuring continued or enhanced productivity.
Another key feature of mangroves is their ability to
provide key goods and services over indefinite
timescales. Financial and development planning
needs to avoid the trap of focussing on narrow
timescales, such as electoral cycles, when
considering the value of mangroves, but to build in
the ongoing provision of benefits in perpetuity as a
highly valuable feature of these ecosystems.
Coastal Fishery in Java, Indonesia. Photo by Alexander van Oudenhoven
Mangroves enhance fish production via two key
mechanisms the provision of food and of shelter.
High levels of primary productivity within the
mangrove forest provide the basis of food chains
that enhance the growth of many fishery species.
At the same time, the three dimensional structure
provided both by the complex of channels and
pools, and by the complexity of roots and branches,
gives shelter from predation and beneficial
physical conditions such as shading and reduction
of water flows. Some commercially important
species of crabs and some finfish live year-round in
mangroves, but for many other species, including
many finfish and prawns, mangroves are of
particular importance as a nursery habitat.
The science underpinning our understanding of the
role of mangroves is rapidly growing, and there is
an increasingly strong body of evidence supporting
their effects in enhancing coastal and cross-shelf
fisheries. This includes correlations between
catches of fish and mangrove area (e.g. Paw and
Chua 1991, Aburto-Oropeza et al. 2008), higher
abundances of fish (particularly juveniles) in
mangroves than in other habitats (e.g. Laegdsgaard
and Johnson 2001, Nagelkerken et al. 2001) and
stable isotope studies showing that fish move from
mangroves to coral reefs and other habitats as they
grow (e.g. McMahon et al. 2011, Kimirei et al.
A number of authors have sought to summarise
mangrove-associated fishery values (Table 1) with
estimates suggesting mean values often in excess
of US$1000 per hectare per year, but with very
broad ranges and median values typically in the
order of $10s to $100s of dollars. Our own
extensive review suggested a global median value
of US $77/ha/yr for fish, and US $213/ha/yr for
mixed species fisheries. However, understanding
the enormous variation in value, illustrated in the
tables through this report, is far more important
than establishing ever more accurate global totals
or average numbers. Values, which can be stated in
simple catch statistics, in monetary terms or other
metrics, are site specific and it is at these sites or
places that management decisions, conservation
actions and fishing activities take place. In order to
be able to make decisions we need to know or
predict actual or potential value in these precise
locations. In turn this requires an understanding on
what external factors may drive value.
Ideally, a detailed numerical model of mangrove-
associated fisheries should be built up from field
and experimental data, which could then form a
basis for predicting value in other locations, and for
modelling potential values in future planning. As
part of the current work we began a review of over
1500 papers on mangrove-associated fisheries.
Despite this impressive base, we did not find the
data that would enable us to determine how these
drivers interact to produce the range of observed
values. The complexity of the different fishery
types, scales, and fishing methods, coupled with
the range of different study methods and reporting
units, meant that we were unable to develop a
model linking the drivers to the observed catches.
This represents a data gap in the current literature.
Such a gap could be addressed by further studies,
particularly if they report their findings using
standardised measures of fishing effort and catch.
Such measures are available in the literature (e.g.
Salthaug and Godø 2001, Maunder and Punt 2004),
but they mostly compare different fishing vessel
sizes so would not cover the varied techniques
used in small-scale mangrove-associated fisheries.
The review work has helped to inform a detailed
understanding of many of the fisheries and a
generalised understanding of the processes which
drive value, and from this work we have developed
a simple conceptual model of the key drivers of
fisheries value, including the biophysical factors
that determine how many fish a mangrove
produces, and the socio economic factors that
determine how many of these fish are caught by
humans, and what they are worth in economic
terms, as a food supply or through the livelihoods
that they support. From these we can predict where
mangroves are likely to be particularly valuable to
Mangrove dependent fishers in Latin America. Photo by Wetlands International.
Fish productivity from mangroves will be
highest where mangrove productivity is high,
where there is high freshwater input from
rivers and rainfall and where mangroves are in
good condition.
Fish productivity will increase with an increase
in total area of mangroves, but notably also
with the length of mangrove margin since
generally it is the fringes of mangroves where
fish populations are enhanced. This will also be
influenced by geomorphology, with the
network of channels, pools and lagoons all
contributing to the margin length.
Moreover, mangroves with greater structural
complexity will enhance fisheries to a greater
extent. The structure of roots varies between
different mangrove tree groups, and is
important for shelter that the roots provide to
juvenile fish and prawns, and attachment
points for bivalves.
Fish catch will be highest close to areas of high
human population density that provide the
fishers and the markets for the catch. Of course
some of these mangroves close to populations
are also likely to be under greater threat than
those in more sparsely populated areas they
may be degraded, the waters may be polluted,
or they may be over-fished and hence less
productive. Where such mangroves are secured
through management regimes, and where their
fisheries are well managed they are likely to
give greatest value. Conversely, conservation
and restoration efforts in these areas close to
human populations will likely give the greatest
return on investment.
These case studies give representative examples to
support the discussion of mangrove fishery types in
section 2. They also provide insights into the range
of fisheries management alternatives currently in
practice and their perceived efficacy. Numbers
used in discussion of catches come from the FAO
via the FishstatJ software package (FAO 2011),
unless other sources are cited.
Inshore mixed species fisheries
Case study: Brush park fisheries
Description: Brush parks are clusters of twigs and
branches stuck into the muddy bottom of shallow
lagoons. They are built and left in place for a period
of a week to a month or more, after which the fish
are harvested by surrounding the brush park with a
net and removing the branches. These twigs and
branches often come from mangroves, and the
lagoons where the fisheries operate are often
fringed by mangroves. Many of the species caught
are therefore also likely to be linked to mangroves,
and it seems likely that the structure of submerged
branches mimics mangrove habitats. They are
found in West Africa, Madagascar and Asia,
including Bangladesh, China, India, Sri Lanka and
Cambodia (Welcomme 2002). The technique has
also been introduced to some lagoons in Mexico
(Baluyut 1989).
This case study focusses on the brush park fishery
in the Negombo lagoon on the west coast of Sri
Lanka. The data comes from two papers, one by
Costa and Wijeyaratne (1994) and the other by
Amarasinghe et al. (2002).
Site: The Negombo lagoon is a shallow lagoon with
a water surface area of 3502 ha. Around the edge
of the lagoon are roughly 350 ha of mangroves, the
branches of which are used in brush park
construction. The lagoon has a large brush park
fishery, with 2-3000 brush parks accounting for just
over a third of the total fish catch in the lagoon.
Brush park construction: Most brush parks are 6-12
m in diameter, and use 300-600 branches inserted
into the bottom vertically or at a slight angle. Brush
parks are constructed in areas of the lagoon with a
muddy bottom and moderate water currents. Most
brush parks are left for 2-3 weeks before
harvesting, which is a trade-off between total catch
and waiting time. Amarasinghe et al. (2002) found
that yield of finfish and catch value rise at a
decreasing rate until plateauing after around 40
days, but the need for immediate income means
that they are rarely left this long. Yield of
crustaceans appears to rise steadily even up to 60
days, but they are a small part of the total catch.
The brush park is harvested by surrounding it with a
net, removing the branches and using a scoop net
to capture the trapped fish. After harvesting the
brush park is reconstructed using the same twigs.
The branches are generally replaced annually.
Catch statistics: In the 1998-99 fishing season, the
total catch was 12.46 t ha¹ brush park yr¹. 84% of
this yield was finfish, with the rest made up by
crustaceans. The fish species caught vary with
season and location in the estuary, with the green
chromide chichlid Etroplus suratensis often making
up the majority of the catch. Other main food fish
included seabream (Sparidae), eel catfish
(Plotosidae), gobies (Gobiidae), silver-biddy
(Gerreidae), barramundi Lates calcarifer, mullet
(Mugilidae) and rabbitfish (Siganidae). Grouper
(Serranidae) species are sold live for aquaculture
and rarer chichlids are caught in small numbers and
sold live for the aquarium trade. Crustaceans
include mud crab Scylla serrata, and sub-adults of a
number of species of penaeid prawns. Mean catch
value was 228 Sri Lanka Rupees, equivalent to US
$2.9, per brush park harvested. This gives an annual
income of around US $35 per brush park, assuming
each park is harvested 12 times annually, which
equates to a total value of US $75,000 $105,000
annually for the lagoon as a whole.
Sustainability: Like most fishing methods, brush
parks have the potential to over-exploit fish
populations. This did not appear to be a problem in
the Negombo lagoon at the time of the studies
used as sources for this case study. Although the
brush park fishery is not managed through any
legal framework, access to the fishery is limited by
social factors. The total number of brush parks in
the lagoon is limited by the availability of space in
the most profitable areas, determined both by fish
abundance and by suitable depth and bottom
composition. Brush parks in these areas are
generally controlled by a single village. These
“owners” are territorial towards outsiders trying to
construct brush parks in their area, through direct
aggression or the destruction of their brush park.
Fishing rights to particular areas are passed down
in families. This limited access helps to prevent
overfishing. In other parts of the world, brush parks
may be used as a form of aquaculture to enhance
fish populations. In Benin, for example, large,
permanent brush parks are used to provide a
breeding habitat for fish. Smaller brush parks
around the large central one are then used to catch
fish emigrating from the central park. This system
enhances fish populations, making it inherently
sustainable (Welcomme 2002).
Brush parks also have an environmental impact on
mangroves and other forests through the wood that
is cut and used in their construction. In the
Negombo lagoon, this had led to the development
of mangrove cultivation, where mangroves are
grown for construction of brush parks and for poles
for terrestrial building. This relieves pressure on
the natural mangrove forests, also helping to
protect the fish stocks that depend on them.
Inshore crustacean fisheries
Case study: Scylla serrata (mud crab) fishery
Description: The mud crab Scylla serrata (called the
Indo-Pacific swamp crab by the FAO) is amongst the
most important commercial crab species in the
world. It is found in estuarine habitats, particularly
mangroves, throughout the Indo-Pacific, occurring
from the east coast of Africa through to Polynesia
in the Pacific, south as far as New Zealand and
north to Okinawa, Japan. It has also been
introduced to Hawaii and to the Gulf of Mexico. It is
a large crab, reaching up to 3 kg in weight, with
flattened back legs for swimming and strong, heavy
pincers, which it uses to feed on molluscs, other
crabs and occasionally fish.
Catch statistics: In 2010, the global reported catch
was 37,000 tonnes (FAO 2011, Grubert et al. 2012),
although this is likely to include other species from
the genus Scylla which are misidentified. 30,000
tonnes of this came from Indonesia, with the
Philippines, Thailand and Australia accounting for
the majority of the rest. Many countries within the
mud crab’s range do not report catches of mud crab
to the FAO, and these figures also do not account
for the unreported catch in subsistence and
recreational fisheries. The total global catch is
therefore likely to be much higher than the FAO
figures suggest. Mud crabs are highly prized for
their sweet tasting flesh and can command high
prices with websites in Australia selling them for
US $35/kg as of January 2014.
Catching methods: Mud crabs are often caught
using pot traps set in mangrove channels or just
offshore. Hand collecting and use of a wire loop to
hook crabs out of their burrows is also common,
particularly in developing countries.
Sustainability: In many countries, mud crab
fisheries are unregulated, particularly where
fishing is primarily artisanal. Mud crabs produce
large numbers of offspring which have high
dispersal ability as pelagic plankton, meaning that
they are able to withstand moderate fishing
pressure. Nonetheless, catches and average sizes
are decreasing in parts of the mud crab’s range,
indicating overfishing (e.g. Ewel 2008, Kosuge
2001). In countries where mud crab fisheries are
regulated, minimum size limits appear to be very
effective at maintaining stocks. Some areas also
ban taking of female crabs, which have higher
natural mortality due to their migration to and from
deep water to spawn.
Case study: Ucides cordatus (mangrove crab) fishery
Description: Ucides cordatus is known as the
mangrove crab (although that name can also be
applied to numerous other species), or by its
Portuguese translation “caranguejo-uçá”. It is found
on the tropical Atlantic coast of the Americas, from
the south of Brazil through the Caribbean and the
Gulf of Mexico to Florida in the north. It is a semi-
terrestrial species, sheltering in burrows at high
tide and emerging to feed when the tide recedes. It
is almost invariably associated with mangroves and
plays an important ecological role by recycling
large amounts of mangrove leaf litter on which it
feeds. It has a tall, egg shaped carapace and
reaches a maximum size of around 280 g (Ivo et al.
1999). Females are smaller than males, which
makes them less commercially valuable.
Catch statistics: The FAO does not record catches of
this species, so there is little data on the total catch.
However, catch data does exist in the scientific
literature for a number of individual estuaries. In
the state of Piauí, average annual production from
1994-1999 was 1,093 tonnes, with the majority of
this coming from the Parnaíba River delta. North-
east Brazil as a whole produced 7,452 tonnes per
year over the same period (Ivo et al. 1999). In the
Caeté Estuary in Pará, Diele et al. (2005) report
average annual catches of 1200 tonnes between
1997-2003, equivalent to a productivity of 85 kg
ha-1. Extrapolating this figure to the whole of Pará
and the neighbouring state of Maranhão, they
estimate a potential catch of 76,000 tonnes per
year assuming that 60% of mangrove stands are
accessible to fishers. Thus it is clear that the
species is of significant importance to fisheries
throughout the region.
Catching methods: Catching methods are often
highly specialised. They can be divided into two
broad classes: hand collecting and traps. Hand
collecting methods include “tapamento” and
“braceamento”. The “tapamento” technique
involves a complex set of actions, including
widening the opening of the burrow, then blocking
it off with a ball of mud and tree-roots inserted into
the widened opening (Nascimento et al. 2012). This
apparently causes the crab to become
disorientated, making it easy to capture when the
stopper is removed from the burrow (Nordi et al.
2009). Collectors leave stoppers in the burrows for
a period of time, following a circular route so that
they can return to stopped-up burrows to capture
the crabs. “Braceamento” is a simpler technique, in
which the crab collector inserts their arm into the
burrow of the crab and catches the crab by its shell.
“Braceamento” is more productive, as crab
collectors do not have to follow a pre-defined route
or visit burrows more than once. However,
“tapamento” is more selective, and crabs captured
using this technique were 12% larger on average in
a study in Paraíba, Brazil (Nordi et al. 2009).
Two types of traps area commonly used. A
“redinha” is a snare made from a shredded
polypropylene bag. They are placed over the
entrance to crab burrows and secured in place with
stakes cut from mangrove prop roots (Nascimento
et al. 2012). When crabs attempt to emerge from
their burrows, they become entangled in the snare
and are caught. This technique is illegal in Brazil,
due to its lack of selectivity and the damage that
root-cutting causes to the mangrove. However, a
study in Rio Grande do Norte found that 74% of
collectors were using this technique, although all
reported that they captured crabs manually
(Capistrano and Lopes 2012). The second trap type
is called a “forjo” and involves an oil can or plastic
bottle with a door. The door is sprung using rubber
from a bicycle inner tube, and is triggered by a
system of levers when a crab touches the bait
inside the trap.
Sustainability: In Brazil, mangrove crab fisheries
are regulated to some extent, including a closed
season during the crabs mating period when the
crabs leave their burrows and are easily caught, and
a ban on certain capture methods. However, as
discussed above, enforcement of these regulations
is often lacking. Like mud crabs, mangrove crabs
have high fecundity and are relatively resistant to
overfishing. Market forces also regulate harvesting,
with strong demand for large crabs meaning that
females and immature males have little commercial
value and are rarely harvested (Diele et al. 2005).
However, pressure on populations has increased
recently due to an influx of migrants into coastal
areas coupled with high unemployment. Crab
fishing is open access and has low entry costs,
making it an attractive option for people who are
unable to find other work. Use of non-selective
trap-based methods has also increased, possibly
because these techniques provide more success to
inexperience collectors. As a result, signs of
overfishing are appearing in some areas, including
diminishing catches and reduced average size of
crabs (Nordi et al. 2009, Capistrano and Lopes
Case study: Inshore prawn fisheries for aquaculture
Prawns are an extremely popular food globally and
form the basis of a high-value fishery. Some of the
highest values species, mostly in the family
Penaeidae, appear to have a strong degree of
dependence on mangroves as nursery grounds. The
adults migrate offshore after several months in the
mangroves, where they are targeted by a large
offshore prawn fishery (see the following section
on mangrove offshore fisheries). The juveniles
within the mangroves also support local fisheries
for larvae, which are sold on to aquaculture
Description: For many of the most important
commercial prawn species, aquaculture production
dwarfs production from wild-capture fisheries. For
example, aquaculture production of the giant tiger
prawn Penaeus monodon was 3.5 million tonnes in
2011, compared to just 222,000 tonnes from
fisheries. Similarly, aquaculture production of the
giant river prawn Macrobrachium rosenbergii was
1.1 million tonnes whilst only 11,000 tonnes were
caught in the wild. Aquaculture ponds are stocked
with post-larvae, the stage in prawn development
following metamorphosis from a nauplius to a
miniature version of the adult. Although hatcheries
are increasingly being used to supply juveniles, in
many parts of the world prawn aquaculture is still
heavily dependent on wild-caught post-larvae.
Wild post-larvae are considered to be of better
quality and have higher survival than hatchery-
produced post-larvae. Both freshwater and marine
prawn species use mangroves as nursery grounds,
so fisheries for prawn larvae tend to be undertaken
within the mangroves themselves.
Catch statistics: Prawn post-larva fisheries are
often small-scale, carried out by the poorest
members of society. As such, they tend to go
unreported and do not appear in FAO data.
Bangladesh does not report any wild prawn
capture, despite producing around 300,000 tonnes
of M. rosenbergii and 390,000 tonnes of P. monodon
in 2011. However, Nuruzzaman (2002) estimates
that 1.5-2 billion prawn post-larvae are collected
from the wild each year in Bangladesh, with a value
of around US $30 million. Aquaculture in
Bangladesh in 2002 when this estimate was made
produced just over 1% of the total global
production in 2011, implying that current global
prawn larva catch could be 100 times this estimate,
although this will be partially offset by
improvements in efficiency and greater use of
hatchery-produced post-larvae.
Harvesting methods: Fisheries for prawn post-
larvae are often carried out on small scales by the
poorest members of society, who have no income
from other sources. Methods used tend to reflect
this. In Bangladesh, two main methods are used to
fish for post-larvae of M. rosenbergii (Ahmed and
Troell 2010). Pull nets are nets mounted on frames
2-2.5m wide, which are dragged behind the fisher
at the surface as they wade against the current in
shallow water. Set bag nets (see “Stow nets” in
table 1) are fixed in position at the surface in areas
with deeper water and strong currents. They are
larger than pull nets, up to 5 m wide, and catch
prawn post-larvae as they are swept into them by
the current. In other regions, seine nets may also be
used in shallow areas such as mangrove channels.
All nets used must have a fine mesh, as prawn post-
larvae are small.
In regions where aquaculture is less intensive,
prawn post-larvae may be collected through the
tidal inflow of water into ponds. This system is used
in the Mekong delta in Vietnam, where ponds are
built within a network of tidal channels in former
mangrove. Sluice gates connecting the pond to the
channel are opened to allow water and the prawn
post-larvae in it to enter the pond. The pond is then
closed for a period of time to allow growth of the
prawns, then harvested by draining the water back
into the channel through a net.
Sustainability: Studies in various regions, including
Bangladesh and Vietnam, report declining catches
of prawn post-larvae. The huge numbers of post-
larvae being caught in Bangladesh suggest that
overfishing might be one of the causes for this
decline, coupled with wild-capture and broodstock
fisheries for adults. Destruction of mangrove
nursery grounds is also a factor, and this is likely to
be particularly important in the Mekong delta
where brackish water aquaculture increased by
almost 400% between 1985 and 1994 (Johnston et
al. 2000). This is a sustainability issue for pond
aquaculture in general, rather than the prawn
larvae fishery alone.
Beyond the potential for overfishing, the biggest
sustainability issue with prawn larvae fisheries is
the high bycatch. This is the result of the fine-mesh
nets used, which catch everything that enters them,
along with the comparative rarity of the target
prawn larvae compared to other more common fish
and crustacean species. In both pull nets and set
bag nets in Bangladesh, over 90% of the catch was
non-target species (Ahmed and Troell 2010), while
in the Indian Sundabans prawn larvae make up only
0.25% of the catch (Sarkar and Bhattacharya 2003).
Pull net catches are sorted on the bank, so bycatch
is unlikely to survive. In set bag net fisheries the
catch is sorted in a boat, so some of the bycatch
may be released alive, but mortality will still be
high for animals such as sea turtles, which drown in
the net, or for fish that require forward movement
through the water for respiration.
Concerns about overharvesting and bycatch have
led to attempts by governments to regulate prawn
larvae fisheries, but these regulations are
frequently not enforced. In Bangladesh, fishing for
prawn larvae was banned in September 2000, but
this ban has had little effect due to a lack of
motivation and resources for enforcement, as well
as a lack of alternative livelihoods for those
involved in prawn larvae fishing (Ahmed and Troell
2010). Similar situations exist in India and other
countries where prawn larvae are harvested to
supply the aquaculture industry.
Inshore bivalve fisheries
Case study: Mangrove oysters
Description: Oysters are bivalve molluscs in the
family Ostreidae. They have irregular, ridged shells,
which mould themselves to the shape of the
substrate on which the oyster grows. Many oyster
species can live on mangrove roots and a few are
mangrove specialists. Inside the shell, the oyster
has a fleshy body. The amount of meat varies
significantly with the spawning cycle, with shells
being fullest immediately before the oysters
spawn. In many countries they are considered a
delicacy eaten raw, but they are also often cooked
and may be dried or salted as a preservation
method. In the Atlantic, the main species are
Crassostrea rhizophorae from the Americas,
Crassostrea tulipa from West Africa (Vakily et al.
2012) and Crassostrea gasar, found on both sides of
the Atlantic (Lapègue et al. 2002). In the Indo-
Pacific these are replaced primarily by species of
the genus Saccostrea, including Saccostrea
cucullata (Jana et al. 2013), and Saccostrea
echinata. All species use mangroves as a solid
substrate to grow on in an otherwise muddy
environment. Like most bivalves, oysters feed by
filtering algae from the surrounding water.
Catch statistics: The wide diversity of species and
the challenges in identifying oysters to species
level makes it difficult to estimate the global catch
of mangrove oysters. Some Caribbean countries
report catches of mangrove cupped oyster
(probably Crassostrea rhizophorae) to the FAO.
Catches have declined in recent years, but peaked
at 4705 tonnes in Venezuela in 1990 and 2316
tonnes in 1989 in Cuba. Other countries with
significant mangrove areas are likely to have
similar harvests, although in many areas stocks and
harvests have declined due to overharvesting (e.g.
Appeldoorn 1997, Mendonca and Machado 2010).
Harvesting methods: Mangrove oysters are
collected by hand at low tide, either on foot or from
a small boat. They may be cut from the mangrove
roots with a small knife, or the whole root may be
cut and the oysters removed later. In many areas,
oysters are cultured on artificial ‘mangroves’, made
by suspending tree branches in the water from a
man-made pier or jetty. These can be placed below
the low-tide mark, which enables the oysters to
feed continuously giving faster growth rates.
Sustainability: Like crabs, oysters have high
fecundity. In addition, they are fast growing,
attaining harvesting size at 4-5 months and can
spawn as little as three months after settling
(Mackenzie 2005). Mangrove oysters are also less
vulnerable to over-harvesting than reef-forming
oyster species, as the mangrove trees provide the
hard substrate they require to settle on, as opposed
to relying on their own shells as a substrate for
recruitment. Their reliance on mangroves for
substrate, however, makes them very sensitive to
clearance of mangrove forests. Fishing techniques
that involve removing sections of mangrove root
along with the attached oysters, rather than
removing the oysters from the root can contribute
to mangrove loss.
Most of the fisheries that have been studied have
some level of management, including minimum
sizes and closed seasons. However, these are often
weakly enforced. In particular, the minimum size
limit may be circumvented by selling shucked
oysters without their shells (Mendonca and
Machado 2010). Oyster recruitment varies
naturally, which can lead to overharvesting in years
when recruitment is low but fishing effort remains
high from previous years. Like crab fisheries,
mangrove oyster collecting also has a low barrier to
entry, meaning that socio-political conditions can
lead to an increase in the numbers of oyster
collectors. These factors have led to declines in
oyster stocks in some fisheries, but the oysters’
fecundity means that stocks can recover rapidly if
fishing pressure is reduced. Stocks can also be
enhanced by culturing, which ranges from simply
placing mangrove branches in the water to
collecting spat on specifically designed collectors
and transplanting these onto growing surfaces (e.g.
Buitrago and Alvarado 2005).
Case study: Mangrove cockles
Description: Mangrove cockles are bivalves in the
genus Anadara. They are in the ark shell family
Arcidae, but bear a strong resemblance to the true
cockles (Cardiidae) with radial ridges on the shell
extending from the hinge. Not all Anadara species
are mangrove dependent. This case study will focus
on the small-scale commercial fisheries on the
Pacific coast of Mexico and Central and South
Catch statistics: Mexico, Costa Rica and Ecuador all
report catches of mangrove cockles to the FAO, and
harvested 654, 45 and 599 tonnes respectively in
2011. Other countries in the region do not report
catches to the FAO, but probably take similar
catches. Columbia in particular has large sections
of mangrove coastline and in a 2001 study had
similar estimated numbers of cockle collectors to
Ecuador (Mackenzie 2001). The blood cockle
(Anadara granosa) is found throughout the Indo-
Pacific from Africa to Polynesia (FAO 2014), and is
harvested in large quantities, with Indonesia alone
producing 39,000 tonnes in 2011. This species is,
however, less mangrove dependent, therefore not
all of this catch can be attributed to mangrove
Harvesting method: Mangrove cockles are
harvested by hand, by probing in the mud for
cockles buried beneath the surface. In some
locations, fishers also dive for cockles growing in
the bed of rivers. Different species are found at
different depths, with Anadara grandis often found
protruding slightly above the surface, A.
tuberculosa at wrist depth and A. similis requiring
inserting the arm into the mud up to the elbow.
Harvesting conditions are harsh, with deep mud
and potential for injury from sharp mangrove roots,
shells and fish species with sharp spines. Many
collectors therefore wear gloves or fabric tubes on
each finger to protect their hands. Cockle collecting
communities are generally amongst the poorest in
their countries, and cockle collecting generally
includes a subsistence element, with collectors
keeping some of the catch to feed their own
families. The rest is sold to provide a small income.
In small villages this may be directly to consumers
at stalls by the roadside, whilst in larger
settlements it tends to be to dealers, who then sell
them on to restaurants.
Sustainability: As with oysters and mangrove crabs,
harvesting pressure on mangrove cockles is heavy
due to the low barrier to entry and the lack of
alternative livelihoods and food sources for many
cockle collectors. Most countries have minimum
size limits for cockle collecting, but enforcement is
variable. Catches and average sizes have fallen in
Ecuador (Beitl 2011) and one species is listed as
vulnerable in Colombia (Lucero et al. 2012). In
2000, local collector associations in two regions of
Ecuador were given the right to manage their own
fisheries. The managed areas, known as custodias,
have greater catch per unit effort and larger
average cockle sizes thanks to the strict
management, but are controversial due to the
exclusion of non-members from the managed
areas. In general, whilst cockle populations are
vulnerable to depletion by overharvesting, they are
safeguarded by cockles living in impenetrable
mangrove stands which can replenish the
population of the surrounding areas. A more
serious threat is the loss of mangrove area through
conversion to shrimp farming, which accelerated
rapidly in the region in the 1980s and 1990s.
Offshore fisheries
Case study: Fisheries for adult prawns
Description Of all the offshore fisheries that have
been linked to mangroves, the fishery for prawns is
the one with the most robust evidence. Numerous
studies exist showing positive correlations
between mangrove area and catch of various prawn
species (Manson et al. 2005). Juvenile prawns of
many commercially important species are
widespread in mangroves (Rönnbäck et al. 1999,
Vance et al. 2002), and at least some of these
juveniles are much less common in other estuarine
habitats (Robertson and Duke 1987, Chong et al.
1990). Commercially important species for which
mangroves appear to be particularly crucial as
nursery habitats include the banana prawn,
Penaeus meguiensis, the Indian white prawn,
Fenneropenaeus indicus, and the greasyback
shrimp, Metapenaeus ensis, as well as freshwater
river prawns in the genus Macrobrachium, which
migrate to brackish water to spawn. Some of the
prawn fishery catch will be consumed domestically,
but much will also be exported along with prawns
produced by aquaculture. The top importers of
penaeid prawns are European countries, importing
over 300,000 tonnes between them in 2009. In
some countries, egg bearing females are separated
from the rest of the catch as broodstock, which are
used to supply larvae to hatcheries. They are
usually caught in prawn trawls, but are kept alive
due to their high value for producing juveniles,
which may range to hundreds or even thousands of
dollars per egg-bearing female (Rönnbäck et al.
Catch statistics: Prawn fisheries are better reported
and monitored than many smaller scale mangrove-
associated fisheries. 11 species found in mangrove
regions had catches of over 10,000 tonnes in 2011,
and five had catches over 100,000 tonnes. These
species were: Southern rough shrimp
Trachypenaeus curvirostris (293,000 tonnes), giant
tiger prawn Penaeus monodon (222,000 tonnes),
oriental river prawn Macrobrachium nipponense
(138,000 tonnes), fleshy prawn Fenneropenaeus
chinensis (126,000 tonnes) and banana prawn F.
merguiensis (102,000 tonnes). A number of other
species are also important but less well recorded,
due to the level of monitoring in the countries in
which they are caught. The total global catch of
mangrove-related prawns is thus likely to be
somewhere over 1 million tonnes, and is highly
valuable; the prawn fishery in Bintuni Bay,
Indonesia is worth over US $6 million annually
(Ruitenbeek 1994).
Catches of egg-bearing females for hatcheries
largely go unreported. However, one study in
Andhra Pradesh on the east coast of India found
that an average hatchery used around 900 egg-
bearing females annually, and produced around 60
million larvae, each individual thus producing
around 70,000 larvae (Rönnbäck et al. 2003).
Mortality is significant at all stages of the process,
with up to 10% of females failing to spawn in
hatchery conditions, 10-25% off eggs failing to
hatch and 50-70% of the hatched nauplii not
surviving to become post-larvae. However,
mortality in the wild is likely to be equally high due
to predation. Extrapolating from these figures, it
might require 20 million female spawners to supply
the entire global aquaculture industry. Assuming a
weight of 100g per individual, this would be a catch
of around 2000 tonnes. This is a very small part of
the 972,000 tonnes of mangrove-related prawns
harvested in 2011 and in reality many shrimp farms
use wild-caught post-larvae rather than those from
hatcheries. Thus whilst the fishery for prawn
broodstock is locally important, especially because
of their high value, it is small in comparison to wild-
capture prawn fisheries.
Catching method: Commercial fisheries for adult
penaeid prawns primarily use various forms of
trawling. Most penaeid prawn species live on or
close to the bottom, so trawling is the only feasible
method to catch them. Common trawling methods
include otter trawling, where large rectangular
boards are used to open the net, and beam trawling
in which the net is held open by a rigid beam.
Large, powerful boats may drag three or four nets
simultaneously, enabling them to cover a strip of
the sea bed 50 m wide or more in a single pass.
Fisheries for Macrobrachium river prawns are
generally smaller in scale as they are based in
rivers and estuaries, and tend to use a mix of traps
and seine nets.
Sustainability: Environmental concerns associated
with prawn trawling include high levels of bycatch,
damage to sensitive communities on the sea bed
and overfishing of the prawns themselves. Bycatch
is a particular problem, with 62% of the total catch
being discarded, and prawn trawl fisheries
accounting for 27% of the total discards across all
capture fisheries (Kelleher 2005). This is the result
of the small mesh sizes required to retain prawns,
as well as the relatively low biomass of prawns
compared to other species on prawn fishing
grounds. Fishing grounds are also often long
distances from markets where bycatch could be
sold, meaning that much of it is discarded.
The extent to which these negative impacts affect a
given fishery depends on the management of the
fishery. Due to its commercial scale, prawn fishing
often has greater levels of regulation than small
scale fisheries. However, the extent to which these
regulations are enforced varies widely from
country to country. In Australia, the northern prawn
fishery targets a range of penaeid prawn species
and is tightly managed. The management includes a
licensing system, and uses catches as a way of
monitoring prawn stocks. If catches fall below a set
threshold, that area of the fishery is closed for a
period of time or even the rest of the season. There
are also laws on bycatch; all nets have to be fitted
with a turtle excluder device and any interaction
with turtles, sea snakes, sea horses and certain
species of shark and ray have to be reported. As a
result of these measures the fishery is certified
sustainable by the Marine Stewardship Council. By
contrast, in Bangladesh, turtle excluder devices
were made a legal requirement but this
requirement was removed following a high court
injunction after trawler owners complained that it
would prevent larger commercially valuable fish
being caught. Bycatch in this fishery is over 80% of
the total, and catches of the target prawn species
are also declining indicating overfishing (Rahman
Recreational fisheries
Case study: Catch and release bonefish in the
Description: Bonefish are wary and powerful fish, a
combination that makes them challenging and
hence appealing to catch for sport. Bonefish are
often found close to mangroves and enter them at
high tide to feed. The fishery is generally
undertaken in the “flats”, shallow calm areas,
generally just offshore from mangroves This
provides a beautiful setting which increases the
appeal of bonefishing as a recreation activity.
Tourists are the main participants in the catch-and-
release bonefish industry in the Bahamas.
Catch statistics: As this is a catch-and-release
fishery, it is difficult to report catch numbers, but
the importance of bonefishing to tourism can be
quantified. Of the 1.5 million tourists who visited in
2004, 0.3% of them primarily visited to undertake
bonefishing or fly-fishing (Danylchuk et al. 2007a).
Bonefishing contributed US $141 million to the
Bahamas in 2008 (Fedler 2010), with some villages
almost entirely dependent on this recreational
fishing industry (Danylchuk et al. 2007a).
Catch method: Fish are caught from small boats
using light fly-fishing and hook and line gears.
Sustainability: As a catch-and-release fishery,
bonefish fishing should be sustainable. Catch-and-
release can, however, have impacts. Bonefish which
are poorly handled following being caught can
suffer a high (17%) mortality rate from predation
within one hour of release (Danylchuk et al.
2007b). Training in handling and releasing of fish
can therefore ensure this fishery remains
sustainable in the face of high fishing pressure.
Case study: Barramundi Lates calcarifer recreational
fishery in Australia
Description: Barramundi are large, predatory fish
found in the sea perch family (Centropomidae).
They are found throughout the Indo-West Pacific.
They are prized for their fighting ability when
hooked, and for the eating quality of their flesh
unlike the Caribbean bonefish fishery, barramundi
are often kept for eating, subject to the size and
bag limits described below. Barramundi are
generally catadromous, meaning they spend much
of their lifecycle in fresh water but move to
estuaries to breed. Juveniles remain in estuarine
habitats, especially mangroves, for 6-9 months
before migrating upstream to fresh water. Some
may also remain in coastal habitats for life if access
to rivers is limited.
Catch statistics: Landings by recreational fishing
are usually dwarfed by commercial landings. In
Australia in 2010, the commercial catch of
barramundi Lates calcarifer was 1676 tonnes, whilst
the catch in recreational fisheries was 303 tonnes.
However, as with the Caribbean bonefish fishery,
the scale of the catch does not reflect its value. The
Queensland recreational barramundi fishery alone
is estimated to be worth AU $8-15 million (US $7.5-
14 million) each year.
Catch method: Barramundi are commonly caught
by lure fishing, moving a metal or plastic lure
through the water to imitate a small prey fish. They
are also caught by fly fishing or using natural baits.
Sustainability: In Western Australia the bag limit
for barramundi is two per day, and there is also a
possession limit meaning that no individual can
have more than two barramundi in their possession
at any moment in time. There is also a minimum
size limit of 55 cm and a maximum size limit of 80
cm (Department of Fisheries Western Australia 2014).
Fish outside these limits must be returned to the
water. This means that juvenile fish are given a
chance to breed, and the largest fish with the
highest reproductive output are not removed from
the population. Other Australian states have similar
regulations. Like other recreational fishery target
species, barramundi are large, top-level predators
and are longer lived and slower to reproduce than
species lower in the food chain, making them
vulnerable to overexploitation. In Australia,
populations are monitored and bag limits are
enforced. However, as recreational fishing becomes
more popular as a tourist activity in countries with
less strict regulations, it could contribute to the
threat to this and other similar species.
Fish in Rhizophora roots. Photo by Mark Spalding.
Gear type
Scale of
Seine nets
Seine nets are simple nets that hang vertically in the water column, suspended using floats at the top and weights at the base, forming a
barrier through which fish cannot escape. Globally, seine nets catch around 25 million tonnes of fish each year, mainly through the use of
purse seines in large scale pelagic fisheries. In a mangrove setting they are commonly used from a beach or in estuaries and channels, and
tend to be much smaller scale.
Beach seines are seine nets operated from the
shore. They are generally hauled by hand,
requiring at least two people. Often larger
numbers of people work together to haul larger
nets, and a small boat may be used for net
Small scale
to or
Seines, like most net-based techniques,
have the potential to cause bycatch of
unwanted species. Depending on mesh
size, they may also catch undersized
Similar to a beach seine, this technique involves
hand-hauling a seine net. However, the net is
pulled along a channel rather than in to a beach.
Small scale
prawn larvae
As for beach seines
Trawling is an active fishing method involving dragging a net through the water behind a moving boat. It includes bottom trawling, where
the net is dragged along the sea bed, and midwater trawling. Each type of trawling catches about 15 million tonnes per year annually,
making trawling the most important commercial fishing method in coastal waters. Trawls are used offshore from mangroves to catch
adults of species that use the mangroves as juveniles, particularly penaeid prawns.
Bottom trawling uses a net towed behind a boat
that drags along the sea bed, catching benthic
species. Typically, the net is held open by two
timber or metal boards called trawl doors. The
front of the net often has a heavy chain which
stirs up the bottom, encouraging benthic species
to swim up so that they will be caught in the net.
Large scale
The main sustainability concern related
to bottom trawling is the damage it
causes to benthic communities,
destroying slow-growing, structure
forming organisms such as corals.
Bycatch is also a concern, although this
can be partially remedied through the use
of bycatch exclusion devices and square-
mesh nets
Lift nets
Lift nets are submerged, then raised vertically upwards catching any fish or other species that are in the water column above them. Fish
may be attracted using bait or lights. In some places they are used in commercial fisheries, notably the Caspian Sea. In mangroves,
however, lift nets are used in small scale fisheries, usually by a single fisherman hauling the net by hand.
lift nets
These usually include a net on a square frame,
with ropes attached at the corners so that it can
be lowered into the water with the net remaining
horizontal. The ropes are generally attached to a
long stick, which is used as lever to lower and
raise the net.
Small scale
prawn larvae
Lift net fisheries are generally small-scale
with few sustainability concerns. Nets
used may have fine meshes leading to
some bycatch, but much of this can be
quickly released back into the water.
This includes any gear which falls onto fish from above. The only example applicable to mangrove systems is cast nets.
Cast nets
Cast nets are circular nets with a weighted line
around the perimeter, which enables the net to
be thrown over a shoal of fish. Lines attached to
the weighted edge of the net are then retrieved
through a hole in the centre, closing the net
around the fish.
Small scale
pools etc.
Cast nets are targeted at a specific shoal
of fish, meaning the potential for bycatch
is relatively low. The method is used to
catch small fish, which may not yet have
reproduced. This could deplete stocks if
fishing pressure is high.
This category includes all nets that entangle fish or other fishery species in their mesh. This is one of the most widely used fishing gears,
catching between five and 10 million tonnes annually. Much of this comes from large scale pelagic drift net fisheries, but entangling nets
are also widely used in small scale fisheries. As a passive gear type, there are concerns over "ghost fishing", where lost or discarded nets
continue to catch and kill marine life, sometimes for many years.
Gillnets catch fish by allowing them to pass
partially through the net. They are unable to
escape backwards due to the mesh catching
behind the gill covers, hence the name. Gill nets
are suspended vertically in the water column
using floats at the top and weights at the base.
Set gillnets are anchored in position.
Gill nets are size-selective, allowing a
specific size class of fish to be targeted.
Targeting of immature fish is still
possible, but means that larger fish will
not be caught. In mangroves, gillnets can
be set across channels, giving high
catches but with a significant risk of
depleting stocks, at least on a local scale.
Drift nets
Drift nets are gillnets that are not anchored. In
pelagic fisheries they may be many kilometers
long, but in a mangrove context they are usually
10s to 100s of meters long, and are used offshore
from the mangroves from a small boat.
As for set gillnets. Additionally, drift nets
have a higher chance of being lost,
leading to ghost fishing. They may also
have a greater risk of bycatch of
cetaceans, sharks and other non-target
species which are not present in the
channels where set gillnets are used.
Trammel nets have three layers of netting, one
fine mesh layer sandwiched between two large
meshed nets. The fine middle mesh is slack, so
that a fish hitting it will push a section of it
through the large mesh on the other side, forming
a pocket in which the fish is caught. Like gillnets,
they are set vertically in the water column, and
are often anchored on or near the bottom.
Trammel nets are less size selective than
gillnets, increasing the risk of bycatch of
undersized fish, in addition to the
potential bycatch of larger non-target
species. Like gillnets, they are also
responsible for ghost fishing if gear is
lost, although their use as anchored nets
makes this less likely.
Traps are gear that fish or crustaceans are able to enter, but unable or unlikely to exit. They often use funnel-like entrances with a large
external aperture and a small internal one. They may be baited, or they may use the flow of water to encourage target species to enter.
They have limited importance in terms of tonnage of global catch, but in the small to medium scale fisheries where they are commonly
used they are one of the main catch methods, particularly for use within the mangroves.
Pound nets are nets are fish traps with net floors
and walls which run from the bottom to the
surface. Fish are directed to a gap in the wall by
long lines of netting which form a V shape
leading to the trap. The top of the net is open, and
fish are harvested either by hauling in the net or
using a scoop net. They are set in shallow areas
with fast-flowing water which push fish into the
net, and may be a semi-permanent installation.
species that
migrate with
the tide.
Catch of undersized individuals or
undesirable species is potentially a
problem, depending on the mesh size
used. Unlike with nets that are hauled
onto a boat, however, it is often possible
to release this bycatch alive and
Fyke nest
Fyke nets are similar to pound nets, but instead of
running from the bottom to the surface they have
a cylindrical net, into which fish are guided by
long wings. They are fixed in position with
anchors or stakes, and catch fish swimming on the
prawn larvae
As for pound nets. As fyke nets are set
underwater, they have to be hauled out
for fish to be harvested, potentially
increase the risk of bycatch being injured.
Stow nets
Stow nets are simple bag or cone-shaped nets
that are fixed in position in areas of strong
current, such as river estuaries. They may have a
frame to hold the mouth of the net open. They act
as filters, catching any organism that is swept into
them. To harvest the catch, the end of the net is
brought aboard a boat and opened, leaving the
rest of the net in place.
prawn larvae
Stow nets often use fine meshes, leading
to catches of undersized individuals,
potentially damaging populations. Stow
nets are one of the main ways of catching
prawn larvae for aquaculture, and up to
90% of the catch may be discarded.
corrals etc
These methods use permanent or semi-
permanent installations. They may work similarly
to pound and fyke nets with walls directing fish
into an enclosure, or they may rely on the falling
tide leaving fish stranded inside a wall or fence.
Like pound nets, there is a risk of bycatch
but often these may be returned to the
water alive, or allowed to remain in the
enclosure until the tide rises again.
Pots are one of the main methods use to catch
crustaceans, but may also be used to catch fish
and cephalopods. They are essentially a cage or
basket with one or more funnel-shaped entrances
or internal baffles. These provide an easy entry
into the pot, but only a small hole for the exit.
Pots are usually baited to entice target species to
but also
Bycatch of undersized individuals is a
potential issue, as is ghost fishing by lost
pots. These can be mitigated by escape
panels that allow small individuals to
leave the pot, and by biodegradable
materials that limit ghost-fishing time.
Hooks and
Hooks and lines are another main fishing method, catching over 5 million tonnes of fish per year globally. Much of this catch comes from
commercial fisheries for pelagic species such as tuna, but hooks and lines are common in fisheries of all scales around the world. They are
also the main method employed by recreational fishers.
and pole-
This includes various different forms of hook and
line fishing. Lines may be handheld, or attached
to a pole or rod. The bait may be a natural or
artificial food of the target species, presented on
the bottom or suspended from a float, or it may
be a synthetic lure, moved through the water to
mimic a prey item.
Used at all
scales, and
Used in all
Hook and line fishing is one of the less
environmentally damaging techniques.
Hook and bait size can be adjusted to
target specific species and size classes.
There is some potential for bycatch of
unwanted species, and lost hooks can
remain lodged in fish.
Set lines
These are lines, usually with multiple baited
hooks, that are fixed in place for a period of time,
then retrieved with any fish caught on the hooks.
Within or
close to
As with hand and pole-lines, there is
potential for bycatch. In addition, the
unattended nature of set lines means that
bycatch is more likely to be dead before
it can be released.
Harpoons, spears and bows and arrows are all used as fishing techniques. They are often used by indigenous people as a traditional form
of fishing, and are also used in recreational fisheries. They may be used from land, for example at channel edges within the mangroves, or
in the water by snorkelling or scuba diving. As they allow single individuals to be targeted, they have little environmental impact beyond
the taking of those individuals.
Hand collecting is ubiquitous in small-scale fisheries around the world. It is particularly common for animals that live in the intertidal
zone and can be collected on land at low tide. Examples include bivalve molluscs and some crab species. Techniques vary and can be
highly specialised - see the section on the mangrove crab Ucides cordatus fishery for examples.
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... Mangrove forests of the tropics and subtropics are important for coastal protection, conservation of biological diversity, and protection of coral reefs and seagrass beds [1]. They provide habitat, spawning grounds, and nutrients for three-quarters of all commercially fished species in the tropics [2]. Mangroves are also among the most carbon-rich forests in the tropics, [3] providing potentially low-cost options for carbon sequestration and storage [4,5]. ...
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Numerous studies have been done using remotely sensed data to produce global mangrove height and biomass maps; however, little is known about the worldwide pattern of mangroves in the Northern and Southern Hemispheres that corresponds to their height and biomass. The objective of this study was to investigate whether there is a specific pattern that can be seen between northern and southern mangroves according to height and biomass. Based on an empirical model, we processed Shuttle Radar Topographic Mission (SRTM) elevation data in combination with 450 field data points to produce a global mangrove height map and its corresponding above-ground biomass (AGB) per hectare at 30 m spatial resolution. We also refined the global mangrove area maps and provided a set of equations to determine the maximum mangrove height at any given latitude. Results showed that 10,639,916 ha of mangroves existed globally in the year 2000, with a total AGB of 1.696 Gt. Even though the areal coverage of mangroves was higher in the Northern Hemisphere, the total mangrove AGB was higher in the Southern Hemisphere. The majority of mangroves in both hemispheres were found to be between 6 and 8 m tall, although height distribution differed in each hemisphere. The global mangrove height equation for northern and southern mangroves produced from this study can be used by relevant stakeholders as an important reference for developing an appropriate management plan for the sustainability of the global mangrove ecosystem.
... For example, the economic value of coastal protection from storms is greater when mangroves, seagrasses, and coral reefs co-occur (Guannel et al., 2016;Barbier, 2018;Carlson et al., 2021). Research indicates that in some places offshore fisheries catch is ecologically connected to the structure and function of nearshore vegetated seascapes (Hutchison et al., 2014;zu Ermgassen et al., 2021). To maintain and enhance benefits from synergistic interactions across seascapes requires that NbS strategies ensure seascape connectivity is recognized, protected and restored for sustainable and regenerative outcomes (Olds et al., 2016;Hilty et al., 2019), yet rarely is seascape composition (patch types and variety) and structural connectivity (spatial arrangement of patches) integrated into site selection frameworks (Pittman et al., 2018;Daigle et al., 2020). ...
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Coastal seascapes are productive and diverse land-sea systems that provide many valuable benefits yet are increasingly threatened by human activity. Scaling up of nature-based solutions (NbS) to effectively protect, sustainably manage, and restore coastal seascapes is urgently required for mitigation of climate change and biodiversity loss while also providing socioeconomic benefits. Evidence-based site selection is an important first step to improve the outcomes and avoid negative impacts when prioritizing NbS investments at national level. We developed a spatially explicit, integrative and culturally relevant ecosystem-based site selection process to identify a portfolio of seascapes for NbS consideration in the United Arab Emirates (UAE). The primary goal was to rank planning units based on potential for climate change mitigation action, positive impact to biodiversity and socioeconomic benefits to people. The multi-criteria site-selection framework provided a rapid, transparent, repeatable and scalable tool. The highest weightings were assigned to blue carbon storage value, biodiversity conservation features, and local stakeholder preferred areas. Spatial proxies for benefits to people were represented by population density and accessibility to coastal seascapes, relative tourism and recreation potential, and importance of fish habitat and fishing grounds for food security. Participatory mapping of local knowledge and review of existing data ensured that both qualitative and quantitative criteria were reliable, up-to-date and locally relevant. Two distinct clusters of high suitability planning units were identified in the Abu Dhabi region and four along the northwestern coast of the UAE. Most high suitability sites were located outside of existing marine protected areas. Alternative spatial scenarios without stakeholder bias underscored the suitability of sites identified through participatory mapping and highlighted additional priority sites for future scaling-up of NbS. A spatial corridor of medium and high suitability planning units across the region offers potential for designing well-connected NbS investments to accelerate and boost synergistic outcomes and increase resilience. The site selection framework provides a rapid tool that integrates local and global open access data at a range of scales with great potential for transferability to other regions worldwide.
... Indeed, mangroves' high primary production is important for local food webs and those of adjacent or connected habitats, thus contributing to nearshore fisheries sustainability (Abrantes et al., 2015;Kelleway et al., 2018). Moreover, mangroves habitats enhance coastal fisheries yield by serving as a nursery for commercial species carrying ontogenetic migrations from the mangroves to the open sea (Hutchison et al., 2014;Seary et al., 2020). As these migratory species need to use several components of the coastal continuum throughout their life cycle, they rely on the complementarity and proper functioning of the different coastal ecosystems (mangroves, coral reefs, seagrass beds) (Berkström et al., 2013;Olds et al., 2016Olds et al., , 2018. ...
For centuries, intertidal fishing in mangroves using block nets has been of socio-economic and cultural importance to many tropical fishing communities. However, data characterising these fisheries are scarce, and no study on their selectivity has been conducted. Additionally, assessing the impact of block net fisheries is complex because they target migrant fish species with different life cycles and morphologies in highly dynamic environments. Here, we investigated the catch composition and selectivity of intertidal fishing in northeastern Brazil to discuss how this should be considered for management purposes. We described the spatiotemporal variability of catch composition in four es