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https://academic.oup.com/bioscience February 2022 / Vol. 72 No. 2 •BioScience 123
Climate-Friendly Seafood: The
Potential for Emissions Reduction
and Carbon Capture in Marine
Aquaculture
ALICE R. JONES , HEIDI K. ALLEWAY, DOMINIC MCAFEE, PATRICK REIS-SANTOS , SETH J. THEUERKAUF, AND
ROBERT C. JONES
Aquaculture is a critical food source for the world’s growing population, producing 52% of the aquatic animal products consumed. Marine
aquaculture (mariculture) generates 37.5% of this production and 97% of the world’s seaweed harvest. Mariculture products may offer a
climate-friendly, high-protein food source, because they often have lower greenhouse gas (GHG) emission footprints than do the equivalent
products farmed on land. However, sustainable intensification of low-emissions mariculture is key to maintaining a low GHG footprint as
production scales up to meet future demand. We examine the major GHG sources and carbon sinks associated with fed finfish, macroalgae
and bivalve mariculture, and the factors influencing variability across sectors. We highlight knowledge gaps and provide recommendations for
GHG emissions reductions and carbon storage, including accounting for interactions between mariculture operations and surrounding marine
ecosystems. By linking the provision of maricultured products to GHG abatement opportunities, we can advance climate-friendly practices that
generate sustainable environmental, social, and economic outcomes.
Keywords: climate change, carbon sequestration, food production, sustainability, blue carbon
Food production systems operate within the resource-
constrained biosphere and are often dependent on non-
renewable energy sources and increasingly compromised
ecosystem services (Rasmussen et al. 2018). Because of
this reliance on natural resources, climate change poses a
tremendous challenge to the continued provision of nutri-
tious, secure, and affordable food for the world’s growing
population (Porter etal. 2014, EAT-Lancet Commission
2019). However, food production also contributes signifi-
cantly to climate change through both direct and indirect
emissions of greenhouse gases (GHG; Springmann et al.
2018), estimated to account for 20% to 37% of anthro-
pogenic GHG emissions annually (Poore and Nemecek
2018, Rosenzweig etal. 2020). There is a need to embed
consistent GHG accounting practices into food produc-
tion systems to effectively measure and move toward
reducing emissions and climate change impacts (Godfray
et al. 2010, IPCC 2019a). This will need to be a process
of continual improvement within each food production
sector, regardless of how its GHG footprint compares with
other sectors.
Not all food is created equal in terms of climate impacts.
Large variability exists in the GHGs emitted per portion of
protein produced, both within and between food production
sectors (Hilborn etal. 2018). The GHG emissions per unit of
protein produced by aquaculture generally compare favor-
ably with most livestock production and some wild-caught
fisheries (Tilman and Clark 2014, MacLeod etal. 2020), but
considerable variability exists within each food type (Poore
and Nemecek 2018). Different GHGs have different global
warming potential; therefore, they are often expressed as
carbon dioxide equivalents (CO2e) so they can be compared.
The best estimate of total annual GHG emissions from aqua-
culture (marine and freshwater) was 385 million metric tons
(Mt) CO2e in 2008 (Hall etal. 2011). An updated estimate
of 245 Mt of CO2e in 2017 was equivalent to approximately
0.49% of that year’s total global anthropogenic GHG emis-
sions, but this estimate is limited to emissions from shell-
fish, crustaceans and finfish (which, combined, account
for approximately 93% of global aquaculture production;
MacLeod et al. 2020). This is substantially lower than the
GHG emissions footprint of terrestrial farming, estimated
BioScience 72: 123–143. © The Author(s) 2022. Published by Oxford University Press on behalf of the American Institute of Biological Sciences. This is
an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which
permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
https://doi.org/10.1093/biosci/biab126 Advance Access publication 25 January 2022
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124 BioScience •February 2022 / Vol. 72 No. 2 https://academic.oup.com/bioscience
at 4 billion–6.6 billion metric tons (Gt) of CO2e per year
(from agriculture and livestock combined; Smith etal. 2014).
The lower emissions intensity of aquaculture is mostly
attributable to a lack of direct GHG emissions from land-
use change and more favorable feed conversion ratios (Fry
etal. 2018, Hasan and Halwart 2009, MacLeod etal. 2019,
MacLeod etal. 2020).
Although aquaculture’s current contribution to GHG
emissions from food production is small, there is high like-
lihood of aquaculture expanding, given the human health
benefits and increasing social preferences for seafood (Clark
etal. 2019). Therefore, it is critical to identify pathways to
advance the growth of climate-friendly practices. Doing
so provides an opportunity to avoid further environmental
degradation associated with the expansion of food produc-
tion (Ellis 2011). Ultimately, responsible development of
aquaculture is a key strategy to meet growing food demand
and nutritional needs and to achieve food security within
planetary boundaries (EAT-Lancet Commission 2019, FOLU
2019, Stuchtey etal. 2020).
Mariculture
In the present article, we examine the major sources of GHG
emissions and assess both the opportunities for emissions
reduction and the potential for carbon sequestration from
three key marine aquaculture (mariculture) sectors: seaweed,
bivalve, and fed finfish. We synthesize the available evidence
on these sectors’ GHG footprint and explore the influence
of farming practices on local marine carbon dynamics
(figure 1, box 1). The result is an improved understanding
of the factors driving variability in GHG emissions footprint
within and between these sectors. This enables us to provide
guidance on climate-friendly mariculture practices that can
reduce emissions or enhance marine carbon storage and to
identify key knowledge gaps for future research.
Cultivation of aquatic algae is dominated by the produc-
tion of seaweeds in shallow to moderately deep coastal waters
and, rarely, in offshore marine environments. Seaweeds are
produced for both human and animal food as well as vari-
ous nonfood products, such as carrageenan, agar, iodine,
biofuels (biogas, biomethane) and fertilizers. Propagules
Figure 1. Potential carbon sources (the dark text) and sinks (the white text) associated with operational (on-farm)
activities in the bivalve, seaweed and fed finfish mariculture sectors. Carbon sinks are parts of the farming process that
lead to a net uptake of carbon from the environment, whereas sources are processes that lead to a net loss of carbon to the
environment. The outer circles represent external factors that may influence carbon flow through mariculture farms or
modify the magnitude of carbon sinks and sources. Sinks may not represent long term carbon sequestration, depending on
the external influencing factors and the fate of the mariculture product.
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https://academic.oup.com/bioscience February 2022 / Vol. 72 No. 2 •BioScience 125
(early life stages) are produced either in land-based hatch-
eries (largely for cultivation of temperate species) or via
fragmentation of mature seaweed into seed stock on farms
(more common for cultivation of tropical species), and then
fixed to hanging longline systems, staked lines or floating
rafts or racks to mature. Seaweed farming has more than
tripled since 2000, representing 97% of the total 32.4 Mt of
cultivated and wild-harvested seaweed produced globally in
2018 by 48 countries (FAO 2020), although a few countries
in the Asian region dominate production (China, Indonesia,
Republic of Korea, and the Philippines; FAO 2011–2021).
Despite slower growth rates of global seaweed production
in recent years, growth rates remain high in some nations
(e.g., Indonesia), and an increasing number of countries are
engaging with or indicating interest in this sector, including
in temperate areas (FAO 2020). As nonfed organisms that
can be readily grown in a range of conditions and locations,
seaweed mariculture often has fewer environmental impacts
than other types of plant or animal food production (Parodi
etal. 2018).
Over 17.3 Mt of farmed shelled mollusks, mainly marine
bivalves, were produced worldwide in 2018, accounting
for 56.3% of the global production volume of marine and
coastal aquaculture (FAO 2020). Global bivalve production
(marine and freshwater) has more than tripled in the past
30 years because of expanding production in East Asia (FAO
2011–2021), especially in China (85% of global production
in 2017; Wijsman etal. 2019, Willer and Aldridge 2020).
Oysters account for approximately one third of mollusk
production (33% in 2018), with clams, scallops and mussels
accounting for a further 43% of production (FAO 2020).
Most bivalves are farmed in sheltered, shallow near-shore or
intertidal environments on raised infrastructure (e.g., long
lines or racks; Forrest etal. 2009) or grown directly on the
seabed in baskets or on loose shell seeded by natural recruit-
ment (Dumbauld etal. 2009, Forrest etal. 2009). Because
bivalves are low in the marine food chain, filter feeding
on planktonic organisms, they do not require feed inputs
except during breeding and rearing of larvae in hatcheries.
Consequently, similar to seaweed cultivation, bivalve farm-
ing tends to have fewer environmental impacts than many
other forms of food production (Parodi etal. 2018) and may
provide positive ecological functions relevant to the health
and resilience of marine environments (e.g., water clarifica-
tion, nutrient cycling, biodeposition; Petersen et al. 2016,
Rose et al. 2021). Understanding the extent to which dif-
ferent mariculture sectors may positively interact with sur-
rounding ecosystems requires ongoing research. But there is
growing evidence that some sectors and species will provide
specific benefits. For instance, recent research highlighted
the strong positive role of mussel farming on macrofaunal
abundance (Theuerkauf etal. 2021).
Marine fed finfish are commonly farmed using coastal
floating net pens and, to a lesser degree, land-based farming
systems that use recirculating water (FAO 2020), with larval
or juvenile stages supplied from hatcheries or caught in the
wild (Halwart etal. 2007). Fed finfish production via mari-
culture is not yet a major contributor to total global aquacul-
ture production (6.4% in 2018; FAO 2020), but the sector has
comparatively large negative impacts on the marine environ-
ment (Volpe etal. 2013) and significant potential for future
global expansion (Gentry etal. 2017a, Costello etal. 2020).
China, Norway, and Indonesia are the top three producers
of marine and coastal fed finfish mariculture, accounting for
over 50% of global tonnage in 2018 (FAO 2020). Production
is dominated by salmonids (FAO 2020)—in particular,
Atlantic salmon (over a third of fed finfish mariculture in
2016; FAO 2018).
In the present article, we explore opportunities for
these three mariculture sectors to support climate change
mitigation through climate-friendly design and opera-
tional practices that can lead to either avoided emissions
(reducing the quantity of GHGs emitted) or enhanced
carbon sequestration (facilitating the uptake and storage
of carbon, preventing its release to the atmosphere in the
form of carbon dioxide or other GHGs such as methane).
Although we have considered upstream (preproduction)
and downstream (postproduction) activities, because they
are important sources of emissions in the mariculture
supply chain (table 1), we focus especially on identifying
actionable, planning and operational changes that might
provide opportunities for GHG abatement and sequestra-
tion during on-farm production (figure 1). As such, our
assessment contributes to our understanding of designing
Box 1. Ocean carbon cycle.
The oceans are a major driver of carbon cycling and the world’s largest active carbon sink. Carbon in the oceans is either organic or
inorganic and can be present in both dissolved and particulate forms. The main sources of carbon into the oceans are absorption of
carbon dioxide from the atmosphere (resulting in an extremely large carbon sink of dissolved inorganic carbon) and organic carbon
inputs, primarily from coastal runoff and riverine outflow. Dissolved inorganic carbon supports most marine food webs by enabling
marine plants, from phytoplankton to giant kelp, to photosynthesise. Through this process, marine vegetation converts dissolved
inorganic carbon to organic carbon, which is stored in their tissues and then either passed through marine food chains or buried in
marine sediments where it may remain for hundreds to thousands of years (see box 2). Dissolved inorganic carbon can also precipitate
to particulate inorganic carbon through calcification and the development of carbonate minerals, skeletons and shells—for example,
in the formation of bivalve shells.
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126 BioScience •February 2022 / Vol. 72 No. 2 https://academic.oup.com/bioscience
and embedding sustainability in industrial practices, such
as regenerative agriculture (Francis 2016, FOLU 2019), ter-
restrial agroecosystems (Power 2010) and ecoengineering
(Strain etal. 2018).
Mariculture’s greenhouse gas emissions footprint
GHG emissions from mariculture occur via many pathways,
including upstream (e.g., finfish feed production), on-farm,
and downstream (e.g., transportation) emissions (Gephart
etal. 2016, Blanchard etal. 2017). Previous studies indicate
that up or downstream activities contribute a considerable
proportion of GHG emissions in mariculture, often more
than on-farm operations, particularly when feed production
is included as an upstream process (Pelletier and Tyedmers
2007, Volpe et al. 2010, Ziegler et al. 2012, Henriksson
etal. 2013). Because downstream processes, such as trans-
port throughout supply chains, can have a large impact on
overall GHG emissions (Parker 2018), it can be difficult
to generalize an emissions footprint to a sector or species
level. Air transport has been shown to cause GHG emissions
three to five times that of road freight, and 31 times greater
than sea freight (Buchspies etal. 2011, Max etal. 2020). A
specific example from Tamil Nadu (India) found that trans-
port by ship, rail, or road increased the climate impact of
maricultured seaweed by 14%, 51%, and 139% respectively,
compared with the product’s emissions footprint before leav-
ing the farm (Ghosh et al. 2015). Therefore, downstream
accounting heavily depends on where and how the product
reaches the market.
To evaluate trends in the major GHG emitting processes
for the three sectors, we collated data from all the available
literature, focusing on life cycle assessment (LCA) studies
that contained quantitative and comparable GHG emissions
data (fed finfish, 28; bivalves, 14; seaweed, 8). Details of the
systematic review, including all reviewed literature, search
terms, screening and eligibility criteria, and the values used
in our assessment of GHG footprint are provided in supple-
mental tables S1 and S2 and the supplemental text. On the
basis of these studies, we identified typical GHG emissions
sources (table 1) and provided updated estimates of the
total GHG emissions from each sector (excluding emissions
from postharvest transport for the reasons discussed above;
figure 2).
Seaweed mariculture GHG footprint. Seaweed mariculture has
lower reported GHG emissions than fed finfish and crus-
tacean mariculture (Hall etal. 2011), although there are
few LCAs for this sector (Froehlich etal. 2019, Halpern
etal. 2019a), and they are biased toward temperate regions
(table S1). This bias contrasts with the dominance of
tropical seaweeds in global production. However, GHG
emissions from on farm processes are likely to be even less
in tropical areas because production typically involves lim-
ited infrastructure and mechanization (i.e., lower energy
inputs) and is closer to shore (i.e., lower on farm transport
and maintenance emissions). On the basis of the available
data, total combined GHG emissions from upstream, on-
farm and downstream processes (excluding postharvest
transport) range from 11.4 to 28.2 kilograms (kg) of CO2e
per metric ton of seaweed produced, with a median of 22
kg of CO2e (mean = 22.3 kg of CO2e; figure 2). Emissions
from farming seaweed are lower and less variable (despite
Table 1. Major greenhouse gas emissions sources at different stages of the production cycle for the three key mariculture
sectors.
Production stage Source of emissions Finfish Bivalves Seaweed
Upstream Production and supply of eggs, larvae, or propagules ~ ~
Terrestrial land-use change and degradation (e.g., for crops or
livestock used in feed)
Feed production and processing (e.g., direct emissions from
crop and livestock farms and wild caught fisheries)
Transport of feed to wholesale and mariculture operations
On farm Fuel use
Energy use
Infrastructure or maintenance
Coastal and subtidal land-use change and degradation ~ ~
Nutrient or effluent impact and water treatment ~ ~
Liquid oxygen and other chemicals used in production ~ ~
Downstream Processing
Packaging and ice
Refrigeration
Transport
Note: and indicate relevance to each sector, with ~ showing where an emissions source is not typical when best practices are implemented,
but may be relevant under some circumstances.
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being based on fewer studies) than emissions from the
bivalve sector and are considerably lower than emissions
from fed finfish. However, including reported postharvest
transport increases the maximum emissions estimate by an
order of magnitude to 231 kg of CO2e per ton of seaweed
(table S1).
With postfarm transport emissions excluded, the most
emissions-intensive aspects of seaweed mariculture are usu-
ally on-farm activities, particularly electricity and fuel use,
although there is variability across the studies in the activi-
ties included as a part of on-farm production (table 1; Hall
etal. 2011, Taelman etal. 2015). In Nordic seaweed farms,
upstream production of propagules combined with on-farm
grow-out has been shown to account for 95% of total energy
usage, with grow-out being the most energy demanding
phase (Alvarado-Morales et al. 2013). In some temperate
European mariculture systems, the infrastructure required
to grow and dry seaweed on the farm (before transportation
and processing) may account for almost 100% of the GHG
emissions from the entire production process (van Oirschot
etal. 2017; note that this study excluded hatchery seeding
and processing). Consequently, although seaweed maricul-
ture may represent a comparably low emissions production
opportunity, attention should be given to sources of energy
for cultivation, especially given efforts to move or expand
seaweed cultivation to potentially energy intensive, offshore
environments.
Bivalve mariculture GHG footprint. As with
seaweed, bivalve mariculture does not
require feed inputs, which minimizes the
associated land-based emissions from
agricultural products. A recent estimate
of emissions from bivalve production
(11.1 tons of CO2e per ton of protein;
Willer and Aldridge 2020) indicated
emissions from this sector were just 7.6%
of the average emissions from terrestrial
(beef, pork, and chicken) protein pro-
duction (Ray etal. 2019). Consequently,
bivalve mariculture is increasingly dis-
cussed as a sustainable, climate-friendly
source of nutrient-rich protein produc-
tion for human consumption (Parodi
et al. 2018, Willer and Aldridge 2020).
Excluding postharvest transport, emis-
sions estimates range widely, from –5
(i.e., bivalves were a net sink of carbon)
to 1874 kg of CO2e per ton wet weight,
with a median estimate of 392 kg of
CO2e, well below the median of fed fin-
fish but higher than seaweed (figure 2).
The inclusion of postharvest transport
increases the maximum emissions esti-
mate to 2744 kg of CO2e per ton wet
weight. The range of GHG emissions
reported for this sector is, in part, due to
the diverse production systems used in bivalve mariculture
(table S1).
Few bivalve studies separate GHG emissions contributions
by production stage (table S1). Those that have, suggest that
on-farm energy and fuel usage contribute most to this sec-
tor’s GHG footprint (Iribarren etal. 2010, Fry 2012), similar
to seaweed mariculture. For example, on-farm operations
contribute 60% and 79% of GHG emissions respectively to
Scottish suspended mussel and intertidal oyster production,
with farming materials (ropes, mesh bags, trestle tables)
generating the remainder. The on-farm process of cleans-
ing oysters before sale (depuration) can account for nearly a
quarter of combined upstream and on-farm GHG emissions
(310 kg of CO2e per ton wet weight; Fry 2012).
Importantly, bivalve shell formation is a net source of
CO2 because, under most growth conditions, bivalves
release more CO2 through respiration and the calcifica-
tion process than the amount stored in their calcium car-
bonate shell (Jiang etal. 2014, Han etal. 2017). However,
most LCA studies have not incorporated CO2 production
from shell building into GHG emissions accounting.
Including CO2 generated from shell formation increased
mean emissions estimates by 219% (Ray etal. 2018; the
data from this study are included in our assessment),
emphasizing the critical importance of accurate GHG
accounting, which incorporates biological and environ-
mental processes.
Figure 2. Comparison of total greenhouse gas emissions across all stages of
the mariculture supply chain for bivalve, fed finfish and seaweed (excluding
post harvest transport emissions). Emissions are reported in kilograms of
carbon dioxide equivalents (CO2e) per metric ton live weight harvested. The
bold horizontal line in each box shows the median value, the box extents are
the upper and lower quartiles, whiskers extend to 1.5 times the interquartile
range and outliers have been omitted from the plot (for ease of viewing). Data
were collated through our literature search for each sector and are available in
supplemental tables S1 and S3.
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Fed finfish mariculture GHG footprint. Our median estimate of
total GHG emissions for fed finfish across the supply chain
(excluding postfarm transport) is 3271 kg of CO2e per ton
wet weight, far greater than that of seaweed or bivalve cul-
ture (figure 2). However, there is large variability in total
emissions estimates (1382–44,400 kg of CO2e per ton wet
weight; figure 3, table S1), which vary among species, the
source and composition of feed, geographic locations, local
or national energy sources (e.g., low-carbon or fossil fuels),
but mainly by farming system (coastal net pens versus closed
or recirculating systems; figure 3). For example, Atlantic
salmon can be produced with relatively low GHG emis-
sions comparable to or only marginally higher than some
mollusks (Pelletier etal. 2009, Hilborn etal. 2018), but such
low-emissions production is rarely reported in the LCA lit-
erature (table S1).
The larger GHG footprint of fed finfish (figure 2) is
commonly attributed to the emissions intensity of feed
supply (Hilborn etal. 2018, Parodi etal. 2018, MacLeod
et al. 2020), and is similar in net pen and RAS farm-
ing (figure 3). Emissions from feed supply include crop
agriculture and associated land-use change, wild-caught
fish meal or oil, as well as feed processing and transport
to farms (table 1; e.g., Ellingsen and Aanondsen 2006,
Pelletier and Tyedmers 2007, Iribarren etal. 2010, Parker
2018, MacLeod etal. 2019). As fed species mariculture is
growing faster than nonfed species (FAO 2020), there is
an urgent need to tackle the emissions intensity of feed
production and the complexity in supply chains that leads
to a greater GHG footprint (Asche
etal. 2018).
On-farm GHG emissions are rela-
tively high for all finfish production
systems (median = 1040 kg of CO2e per
ton wet weight), especially if compared
with the total GHG footprint of seaweed
or bivalve farming (figure 2). On-farm
emissions are greatest in closed or
recirculating systems (figure 3), which
are located onshore and require high
energy usage for pumping, filtering, and
maintaining water temperature and oxy-
gen levels (Aubin etal. 2009, Ayer and
Tyedmers 2009, Ahmed and Turchini
2021). However, the most common fed
finfish mariculture system globally is
a nonintegrated (i.e., monoculture),
anchored net pen system, operating over
soft sediment seafloors in coastal waters
less than 30 meters (m) deep (Halwart
et al. 2007, Ayer and Tyedmers 2009,
FAO 2020), within 3 nm of the coast
and not offshore or in the open ocean
(Froehlich et al. 2017). In coastal net
pen farms, upstream and downstream
activities typically contribute more
GHG emissions than on-farm operations (figure 3), with
fuel, energy use, infrastructure construction and mainte-
nance responsible for most on-farm emissions (figure 3,
table S1; Ellingsen and Aanondsen 2006, Hall etal. 2011,
Ziegler et al. 2012). However, few LCA studies consider
environmental emissions for net–pen systems during the
on-farm stage (although these may be considerable; Hu
etal. 2012).
Environmental emissions from fed finfish mariculture
are released via two main pathways: GHGs released from
the decomposition of waste food and nutrient-enriched sea-
water (e.g., methane, nitrous oxide) and carbon emissions
from degraded seafloor habitats that are similar to terrestrial
agriculture’s climate impact through emissions from land-
use change (Hall etal. 2011, Flynn etal. 2012, IPCC 2019b).
Locating net pens in shallow, low energy coastal areas is
preferred for increased accessibility and protection from
damaging waves (Trujillo et al. 2012, Kapetsky etal. 2013).
However, this means that farms are tightly linked with the
surrounding marine environment through water and waste
exchange, which increases the likelihood of degrading sea-
floor habitats (Halwart etal. 2007, Volpe etal. 2013, Abdou
et al. 2018) and the subsequent release of environmental
emissions.
Feed for finfish mariculture is high in nitrogen, and up
to 95% (range of 1%–95%) may be lost as waste to the sur-
rounding marine environment (Findlay and Watling 1994,
Price etal. 2015). The expansion and intensification of fed
finfish mariculture (FAO 2020) have considerably increased
Figure 3. Summary of greenhouse gas emissions from fed finfish mariculture
reported in kilograms of carbon dioxide equivalents (CO2e) per ton live weight
harvested (excluding post harvest transport emissions). Data were collated
from existing studies and, where possible, have been grouped into upstream,
on-farm and downstream activities and separated by production system type
(all systems pooled, closed or recirculating systems and open net pen systems).
The bold horizontal line in each box shows the median value, the box extents
are the upper and lower quartiles, whiskers extend to 1.5 times the interquartile
range and outliers are shown by black points.
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the cumulative nutrient load and subsequent eutrophica-
tion in coastal marine environments (Volpe et al. 2013,
FAO 2020). However, unfortunately, we lack consistent and
comparable data with which to better understand the influ-
ence of farming intensity or stocking densities. Increased
nitrogen and particulates in the water (which contribute to
turbidity) can lead to the loss or degradation of seagrass hab-
itats below and adjacent to the farms (Thomsen etal. 2020).
This can result in GHG emissions through release of stored
blue carbon in the plants and sediments below them (see
box 2) and can reduce the capacity for future blue carbon
sequestration (Jiang etal. 2018, Salinas etal. 2020). Nutrient
enrichment of sediments can also increase microbial activity,
accelerating the sediment carbon cycle and further release of
stored blue carbon (Liu etal. 2017), as well as other potent
GHGs, such as methane (Chen etal. 2016) and nitrous oxide
(Hu etal. 2012).
Although the total global area of mariculture’s built
infrastructure footprint was recently estimated to be at
least 23,000 square kilometers (Bugnot etal. 2021), unfor-
tunately, there are no georeferenced global data available
on the spatial distribution of active fed finfish net pens, nor
their overlap with seagrass habitats. However, this overlap
may be considerable, because seagrass generally thrives in
the same shallow, protected areas that suit net pen maricul-
ture (Short etal. 2007). We estimate that the release of envi-
ronmental emissions resulting from seagrass degradation
and associated blue carbon stock losses could conserva-
tively equate to 4.1%–16.3% of the total global aquaculture
emissions in 2017, depending on the carbon stock loss
scenario and assumed proportion of overlap between farms
and seagrasses (see the worked example in supplemental
file S2 and table S4). There will also be associated losses
of future blue carbon sequestration potential, which may
be up to 0.31% of aquaculture’s GHG footprint per year
(file S2 and table S4). None of the published LCA studies
accounted for these marine environmental emissions in
the on-farm part of the fed finfish life cycle. Neither were
there data on the land-based footprint of closed or recircu-
lating systems or their impact on terrestrial environment
emissions through land-use change, although most studies
included similar types of environmental emissions associ-
ated with terrestrial ecosystem degradation and land-use
change due to farming of finfish feed ingredients. This
omission is likely due to significant knowledge gaps around
the drivers and potential magnitude of environmental
emissions from mariculture-related degradation (Hall etal.
2011, Abdou etal. 2017).
Opportunities for climate-friendly mariculture
There is potential for climate-friendly design and operation
to improve the GHG footprint of seaweed, bivalve, and fed
finfish mariculture. Opportunities to reduce emissions come
through approaches that lead to direct emissions reduction,
actions that reduce indirect (e.g., environmental) emis-
sions (figure 1), farming approaches that enhance carbon
sequestration, and product uses that offset GHG emissions
from other sources (i.e., agriculture and livestock) using
mariculture products. These opportunities are explored in
more detail below.
Box 2. Blue carbon.
Vegetated coastal marine habitats, specifically seagrasses, mangroves and tidal marshes, contain up to 50% of all the organic carbon
buried in ocean sediments (Duarte etal. 2005). These are commonly referred to as blue carbon ecosystems and they can accumulate
and store greater carbon volumes at faster rates than terrestrial forests, because of their ability to trap and bury organic matter (Mcleod
etal. 2011). Their waterlogged soils tend to be low in oxygen (anoxic), decreasing the chance that stored organic carbon becomes oxi-
dized and released as CO2 unless disturbed (Pendleton etal. 2012, Marbà etal. 2015, Sasmito etal. 2019). The effect of salinity on soil
chemistry leads to minimal methane releases, despite the soils being anoxic (Kroeger etal. 2017, Rosentreter etal. 2018). Moreover,
blue carbon ecosystems have the capacity to accrete soil rapidly to keep up with changing sea levels and maintain their preferred posi-
tions in the coastal zone (ensuring adequate light and tidal exposure; Rogers etal. 2019). All these traits make them some of the most
significant biological carbon sinks in the world (Duarte etal. 2013). Unfortunately, blue carbon habitats are often degraded by human
activities (Halpern etal. 2019b). Global mangrove decline due to deforestation and coastal land-use change was estimated at 35% of
their total area up to 1999, with ongoing losses estimated between 0.2% and 8% per year (Friess etal. 2019). In particular, large areas
of mangroves have been cleared to make way for coastal mariculture, (Kauffman etal. 2017), which leads to GHG emissions (Bulmer
etal. 2017) and can halve sediment organic carbon stocks compared with intact mangroves (Kauffman etal. 2018, Arifanti etal. 2019).
Mangrove and tidal marsh conversion for mariculture in China alone is estimated to emit 15–82 million tons of CO2e per year (Wu
etal. 2020). Over 29% of the world’s seagrass area has also been lost since 1879, with ongoing annual losses estimated at up to 7%
(Waycott etal. 2009). Global seagrass loss may cause annual blue carbon emissions of 0.15 billion–1.02 billion tons of CO2e (Pendleton
etal. 2012). Coastal mariculture operations can impact seagrass ecosystems through direct disturbance or shading, leading to losses of
up to half the sediment carbon stock (Lovelock etal. 2017, Trevathan-Tackett etal. 2018). In addition, localized nutrient enrichment
commonly associated with fed finfish mariculture causes metabolic stress and reduced seagrass growth (de Kock etal. 2020), poor
recruitment (Díaz-Almela etal. 2008), and, ultimately, seagrass loss around pens (e.g., Delgado etal. 1999, Ruiz etal. 2001, Herbeck
etal. 2014, Cullain etal. 2018, Thomsen etal. 2020), leading to decreases in both stored blue carbon and ongoing sequestration poten-
tial (Apostolaki etal. 2011, Jiang etal. 2018, Liu etal. 2020).
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Reducing direct and indirect GHG emissions. We have identi-
fied some common approaches to reducing emissions from
on-farm energy and fuel use across all three sectors. These
include shifting to low-emissions energy sources and biofu-
els, as well as sustainable building materials where available
(Myrvang 2006, Aubin etal. 2009, Mungkung etal. 2014).
Unless low-emissions alternatives to fossil fuels can be read-
ily adopted, the potential socioecological benefits of large,
offshore mariculture development (Gentry et al. 2017b)
could be diminished by a need to increase fuel use to enable
distant production at scale. In fed finfish mariculture, chang-
ing from diesel oil to natural gas (a lower emissions fuel) has
been shown to reduce nitrous oxide (a potent GHG) emis-
sions from farmed salmon by 85% and CO2 emissions by
20% (Ellingsen and Aanondsen 2006). In addition, multiple
studies identify that the reuse of materials (rafts, ropes, etc.),
and the specific energy sources used have significant poten-
tial to reduce emissions (e.g., Langlois etal. 2012, Jung etal.
2016). For example, emissions from on-farm energy use are
four times lower using nuclear than coal generated electric-
ity (Aubin et al. 2009). Consideration of on-farm energy
requirements and supply sources are particularly relevant
when establishing large-scale farming sites, especially those
located offshore (e.g., Taelman etal. 2015). The colocation
of offshore mariculture farms with energy generation (e.g.,
windfarms; Buck and Langan 2017), increased use of low-
emissions energy supplies, and the on-site use of seaweed-
derived biofuel products for energy production (Aitken etal.
2014) will be critical to achieving sustainable expansion.
However, changes in a country’s energy portfolio and the
market forces driving the availability and affordability of
biofuels are likely to occur at a national or regional level,
with single farm operators having little control over these
overarching drivers of on-farm GHG emissions (Hall etal.
2011). Therefore, we focus the rest of this overview article on
providing recommendations for readily actionable changes
to the operation and design of on-farm activities.
Opportunities for avoided emissions in finfish mariculture. Site
selection for coastal fed finfish mariculture should exclude
areas of seagrass and other sensitive blue carbon habitats
where possible (figure 4), although complete avoidance may
not be practical in some regions, due the widespread distri-
bution and seasonal variations in the presence and density of
seagrasses. Moving net pens into deeper coastal water (e.g.,
less than 30 m) helps reduce overlap with seagrass habitats,
which are more common in shallower water with greater
light penetration (figure 4). However, reducing blue carbon
emissions through a shift to deeper water will need to be
traded off against the potential for increased operational
GHG emissions from fuel use and maintenance, as well as
the need for more robust farm infrastructure in offshore
conditions (Holmer 2010). Similarly, moving production to
onshore closed or recirculating systems can reduce impacts
on local marine environments. However, onshore systems
have considerably higher emissions from operational energy
use (figure 3, table S1), and the disposal of waste water into
coastal seas (even though it is often treated) and sludge
effluents on land can still result in environmental GHG
emissions. Better accounting for environmental emissions
from mariculture is crucial to effectively compare GHG
footprints across scales and geographies and to monitor the
climate costs and benefits of moving production offshore or
into closed or recirculating onshore systems against coastal
net pen operations.
Where seagrass avoidance is not possible when siting
coastal net pens, reduction in environmental GHG emis-
sions may be achieved through coastal farming practices
Figure 4. Potential pathways for greenhouse gas emissions reductions from fed finfish mariculture.
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such as fallowing or regularly shifting the location of infra-
structure within the broader farm area (figure 4; Lauer etal.
2009, Kletou et al. 2018). Although, even with fallowing,
chronic nutrient enrichment of sediment and water and the
associated potential for environmental emissions of stored
blue carbon may persist (Carroll etal. 2003, Díaz-Almela
etal. 2008, Thomsen etal. 2020). In any case, actions that
reduce the exposure of seagrasses to negative impacts are
preferable to ecosystem recovery attempts after damage has
occurred. If seagrass meadows are lost, recovery can take
decades and may not occur at all if mariculture activities
remain or if the environment has changed significantly
since seagrass loss (Tanner etal. 2014). Regulated baseline
surveys (before farm installation) and ongoing monitoring
of seagrass condition (e.g., benthic videos) along with water
quality monitoring can help minimize harm to seagrasses
(EPA 2020 2021), reducing the potential for environmental
GHG releases from seagrass blue carbon stores.
Leveraging marine environmental modifiers such as local
hydrodynamics when choosing fed finfish mariculture sites
can help to reduce eutrophication-related environmental
GHG releases (figures 1 and 4; Price etal. 2015). Siting of
pens in areas with stronger currents and low water residence
times (high flushing rates) reduces the concentration of
nutrients and accumulation of particulates on the seafloor
(such as uneaten food and feces; Middleton and Doubell
2014, Duman etal. 2020). However, stronger tides, currents,
and wave action can also pose a risk to farm infrastructure
and moorings (Cromey etal. 2002, Bravo and Grant 2018)
and can shift the spatial impact of eutrophication to down-
stream areas (Henderson etal. 2001). Carefully designed dis-
tribution of fed finfish stock within a farm site, on the basis
of knowledge of local marine environmental conditions
(e.g., flow speed and water depth), can reduce both extreme
and cumulative impacts on the seabed, potentially being
more important than stocking density alone (Burić et al.
2020). However, it is difficult to produce generic guidelines
on depths, currents, and tidal conditions that reduce impacts
while optimizing yield, because this is strongly affected by
the species farmed, feeding practices, local marine envi-
ronmental conditions, stocking densities, and the spatial
arrangement of pens (Ali etal. 2011, Cardia and Lovatelli
2015, Doubell etal. 2015, Abdou etal. 2018). Evidence from
pilot studies shows potential for nutrient removal through
farming kelp (seaweed) next to salmon pens, which removes
excess nitrogen inputs while also boosting seaweed growth
rates and harvestable biomass (Wang etal. 2014). This nutri-
ent removal strategy may indirectly reduce environmental
GHG emissions caused by eutrophication from the salmon
farm. However, there are issues of scaling, with each addi-
tional hectare of seaweed cultivation absorbing less than
0.5% of the excess nutrients introduced from the salmon
farm (Fossberg etal. 2018).
Environmental GHG emissions from fed finfish mari-
culture may be decreased by shifting to species that require
less feed or by altering the composition of feed, reducing
eutrophication (Ellingsen and Aanondsen 2006, Han etal.
2018, Little etal. 2018, MacLeod etal. 2019). Improvements
in feed conversion ratio (FCR, the amount of feed required
for each kilogram of fish produced) for fed finfish species
can be achieved through genetics (either selective breeding
or genetic modification; Besson etal. 2016) and innovation
in feed composition (Parker 2018, Hua etal. 2019). These
actions can lead to improved protein transfer efficiency
and reductions in food waste to the environment (Ballester-
Moltó etal. 2017), as well as reducing the total volume of
food required and the GHG emissions directly associated
with feed production (Hall etal. 2011, MacLeod etal. 2020,
Maiolo etal. 2020).
Implementing practices to avoid overfeeding reduces par-
ticulates and nutrient waste lost to the marine environment
(Alongi etal. 2009, Besson etal. 2016, Ballester-Moltó etal.
2017), decreasing the direct and indirect (i.e., environmen-
tal) GHG emissions while boosting economic efficiencies.
This can be achieved using advanced on-farm feed delivery
systems (e.g., behavior-based automated, precision feeding
systems; figure 3; Føre etal. 2018) Such technologies may
not currently be within reach for operators in some devel-
oping countries, although early adoption in other countries
may help drive down costs and increase accessibility in the
longer term (Gentry et al. 2019). Mariculture may have
greater scope for adopting technological innovation than
other food production sectors, because it is a comparably
young industry (Waite etal. 2014). There is also the poten-
tial to use seafood sustainability certification schemes to
leverage improvements in feed and fish production systems
that lead to GHG abatement (Madin and Macreadie 2015),
particularly associated with the sourcing of sustainable feed
ingredients (figure 4; Mariojouls etal. 2019).
Opportunities for avoided emissions and emissions offsets in bivalve
mariculture. Bivalve culture in shallow waters regularly
occurs in close proximity to or directly intermingled with
seagrass meadows. Therefore, seabed disturbance and sea-
grass loss can occur when bivalve operations are established
(Tallis etal. 2009), although estimates of the resulting envi-
ronmental GHG emissions are not available as these are not
accounted for in LCAs (table S1). Erecting poles or racks
for raised farming can physically disturb seagrass, whereas
ongoing shading beneath raised cultures reduces seagrass
density and percentage cover (though empirical evidence
for this is limited; Forrest etal. 2009). Nevertheless, raised
bivalve culture appears to have fewer impacts on sea-
grasses than on-bottom culture, which displaces seagrass
through disturbance and direct competition for space
(Ferriss et al. 2019). Importantly, the structures used for
bivalve mariculture do not necessarily preclude healthy
seagrass meadows (Crawford etal. 2003, Dumbauld and
McCoy 2015) and have been shown to stabilize sediment,
improve benthopelagic nutrient transfer and boost neigh-
boring seagrass growth under certain conditions (figure 5;
Ferriss etal. 2019).
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The method used to harvest mature bivalves has profound
impacts on local benthic disturbance, seagrass cover and,
therefore, blue carbon burial and storage. The mechanical
dredging of on-bottom bivalve cultures disturbs seagrass
beds and may hinder their recovery (Tallis et al. 2009,
Dumbauld and McCoy 2015). Manual harvesting of raised
mariculture (suspended cultures retrieved by hand; figure 5)
is the practice least likely to disturb seagrass and buried car-
bon. Raised culture also avoids the direct competition with
seagrass for space and reduces sediment resuspension com-
pared with on-bottom culture. This both stabilizes sediment
to allow seagrass recruitment and enhances or prolongs car-
bon storage by reducing oxidation of subsurface sediments
(Forrest etal. 2009).
The volume of valuable, carbon-rich shell waste from
bivalve aquaculture is considerable, estimated at up to
11.9 Mt per year (based on shells accounting for an average
of 68.6% of total bivalve weight; Tokeshi etal. 2000). Shells
can be turned into calcium carbonate (CaCO3) or calcium
oxide (CaO), providing an abundant, cheap, and sustain-
able resource with broad industry application (FAO 2020),
reflected by the market value of shell waste (between US$538
and $1783 per ton; Morris etal. 2019, van der Schatte Olivier
etal. 2020). Unfortunately, given their potential value, most
bivalve shells are discarded in landfills (10 Mt per year in
China alone; Yao etal. 2014), where they eventually release
the stored carbon to the environment, and carry a high
disposal cost for the farmer (Yan and Chen 2015). This is
despite calcium carbonate from limestone being mined
in enormous quantities worldwide, primarily for cement
production, at significant environmental cost (Morris etal.
2019). Repurposing shell waste as construction aggregate or
for mortar mixes could potentially lead to long-term carbon
storage while offsetting emissions from energy intensive,
nonrenewable resources (figure 5). Markets already exist
worldwide for whole and crushed bivalve shells as aggregate
in construction (e.g., wall and road construction; Morris
et al. 2019), and their use in cement production does not
compromise performance (Yoon et al. 2004, Kumar et al.
2016), although shell availability may limit large-scale appli-
cation. Other destructive uses of shell waste (e.g., calcium
supplement in poultry grit, agricultural lime release) typi-
cally cause the release of the stored carbon (van der Schatte
Olivier etal. 2020). Even so, the repurposed shell still offers
a less emissions intensive alternative to mined CaCO3.
Furthermore, under certain conditions, pulverized shell
applied as liming agent may mitigate GHG emissions from
agricultural soils and could therefore be considered an emis-
sions offset (Hamilton etal. 2007). Oyster shell is also highly
prized for oyster reef restoration as it provides an optimal
settlement substrate (McAfee and Connell 2020). Although
returning bivalve shells to the marine environment will
eventually release the stored carbon as shells dissolve, there
are considerable positive benefits of bivalve reef restoration,
including indirect carbon sequestration through enhancing
blue carbon habitats (Fodrie etal. 2017, Chowdhury et al.
2019, McAfee etal. 2020).
Opportunities for avoided emissions and emissions offsets in seaweed
mariculture. Although biomass yields from seaweed mari-
culture can be very high, often greater than those from ter-
restrial crops (Hughes etal. 2012), variability in the marine
environment around farm sites affects productivity (i.e.,
growth rates and total biomass yields). This, in turn, can sig-
nificantly affect production efficiencies and GHG emissions
(Froehlich etal. 2019), as well as the potential for negative
Figure 5. Potential pathways for greenhouse gas offsets, carbon storage and sequestration from seaweed and bivalve
mariculture.
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interactions with the marine environment (Campbell etal.
2019). Productivity can be enhanced through careful site
and species selection or scaling up operations to optimize
efficiency (Pechsiri etal. 2016, Seghetta etal. 2016), offering
the potential for climate-friendly farm designs, farm siting,
and species choices to support the delivery of GHG emis-
sions reductions outcomes (figure 5).
Emerging markets for climate-friendly, nonfood seaweed
bio products (such as biofuels and biochar) provide an
opportunity to realize GHG emissions offsets from seaweed
mariculture (figure 5; Seghetta et al. 2016, Seghetta et al.
2017, Laurens etal. 2020), although the magnitude of any
benefit is dependent on the energy requirements and energy
sources used in product processing. The upper limit for CO2
uptake through seaweed production is estimated at 2.48
Mt of CO2e (680,000 tons of organic carbon) per year. This
equates to each square kilometer of cultivated seaweed hav-
ing the potential to reduce fossil fuel emissions by approxi-
mately 1500 tons of CO2e a year (based on 2014 production
quantities) if the biomass produced was diverted to the
creation of biofuels (Duarte etal. 2017). Another innovative
use of farmed seaweed is future feed, animal feed products
(fish and livestock) that can achieve a net reduction in GHG
emissions compared with current feed sources or provide a
functional food value (improve fish health and, therefore,
efficiency in production; e.g., Hua etal. 2019; and ruminant
methane reduction; e.g., Kinley etal. 2020). However, there
are few seaweed species suitable for these applications and
they each have specific culturing requirements. The only
species currently shown to deliver methane reduction when
used in livestock feed is Asparagopsis taxiformis (Roque
et al. 2021), a tropical species that is challenging to grow
and may prove difficult to culture at the scales needed to
generate significant GHG reductions. The production of
seaweed-based biochar for use as a soil improver can also
indirectly support climate change mitigation and offsets
through agricultural soil improvement, because it contains
recalcitrant carbon that facilitates long-term soil carbon
sequestration (Roberts et al. 2015, Smith 2016). However,
biochar is currently energy intensive to produce. Other
barriers to the realization of consistent and scalable emis-
sions reductions and offsets through seaweed bio products
include the limited number of cultivated seaweed species
and production environments, and technological and engi-
neering constraints that reduce economic viability (Laurens
etal. 2020). Should these barriers be overcome, there is great
scope for future spatial expansion of this sector (Froehlich
etal. 2019), which could support sustainable intensification
and climate-friendly production.
Carbon sequestration. There has been a rapidly increasing
interest in mariculture’s potential to support climate change
mitigation via direct carbon storage and sequestration,
especially through seaweed farming (Froehlich etal. 2019,
Oceans 2050 2021) and bivalve farming (Hickey 2008). In
this context, carbon sequestration is the direct movement of
organic carbon in seaweed and bivalve biomass into long-
term marine carbon stores, such as the deep ocean.
Potential for carbon sequestration through seaweed mariculture. In
natural environments, seaweed plays an essential and major
role in ocean carbon regulation and carbon flux (box 1;
Queirós etal. 2019). Natural seaweed habitats are typically
associated with hard substrates, as opposed to soft sedi-
ment and, therefore, have lower inherent potential for direct
transfer and sequestration of carbon to the sediment than
blue carbon habitats (box 2). However, naturally growing
seaweeds do donate organic carbon in the form of detritus
to nearby receiver blue carbon habitats, where the material is
trapped and buried in the sediment (Chung etal. 2011, Hill
etal. 2015, Trevathan-Tackett etal. 2015). Net sequestration
of particulate organic carbon from natural seaweed beds is
estimated to equate to 4%–5% of the blue carbon seques-
tered into some mangroves (Queirós etal. 2019). Receiver
habitats are also not exclusively shallow, vegetated blue car-
bon ecosystems; in fact, offshore export of seaweed detritus
to deep (less than 1000 m) ocean sediments may account
for approximately 90% of carbon sequestration from natural
seaweed biomass (Krause-Jensen and Duarte 2016).
The transfer of organic carbon to receiver habitats (both
deep sea and shallow blue carbon environments) can also
occur from seaweed mariculture farms, although there are
many unknowns around this process including the magni-
tude, consistency, and potential benefits or negative impacts.
This process occurs through detachment of seaweed from
lines, breaking off of fronds, or erosion and decay of culti-
vated plants (figure 5; Hyndes etal. 2012, Hill etal. 2015).
Carbon from maricultured seaweed may also be moved
indirectly into nearby coastal sediments through grazing
organisms that consume biomass at the farm and move
into neighboring marine ecosystems, although, again, the
magnitude of this transfer and its ultimate impact on carbon
sequestration are currently unknown (Sondak etal. 2017).
Generally, seaweed losses could be considered a small pro-
portion of the total farmed biomass (approximately 4.5%),
because this transfer is minimized through operational
practices (e.g., siting choice, orientation and maintenance)
that aim to improve production efficiency (Campbell etal.
2019). Also, in some jurisdictions, there are guidelines or
regulations around seaweed farming that require siting away
from sensitive habitats, such as seagrass and rocky reefs (e.g.,
Eklof et al. 2006). Although this requirement may serve a
useful purpose in reducing the risk to marine habitats from
farm shading, disturbance, introduced species, farming
infrastructure, or operations and waste discharges, it may
also reduce the opportunity for seaweed farms to act as blue
carbon donors. To establish seaweed farming as an effective
blue carbon donor, we need to better understand the rate
of carbon transfer from farms to nearby receiver habitats,
and the environmental modifiers (e.g., flow rates and expo-
sure; figure 1) that influence this connectivity (figure 5).
Alongside efforts to design seaweed farms that enhance blue
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carbon donation, there would need to be careful monitor-
ing of the potential benefits and the magnitude of carbon
sequestration achievable, as well as the potential for negative
impacts from movement of biomass to blue carbon habitats,
such as shading or smothering.
The Intergovernmental Panel on Climate Change (IPCC)
has recommended macroalgal production as a research area
for climate change mitigation (IPCC 2019a), an ocean-based
climate change mitigation also suggested by the High-Level
Panel for a Sustainable Ocean Economy (Hoegh-Guldberg
et al. 2019, Stuchtey et al. 2020). Although seaweed is a
low-emissions mariculture sector (figure 2) that stores large
amounts of carbon in the farmed plant biomass, once the
product is harvested, carbon is either released or transferred
up the food chain. Therefore, this sector should not, by
default, be considered a source of long-term carbon seques-
tration (Duarte etal. 2017).
The intentional farming of seaweed as a means to capture
and sequester anthropogenic CO2 could function in a similar
way to carbon farming initiatives on land (Froehlich etal.
2019, Hoegh-Guldberg et al. 2019). This approach would
rely on a nonharvest mariculture model, where biomass is
either retained in situ or allowed (or facilitated) to sink to
the deep sea (less than 1000 m), where the carbon can be
sequestered for long periods of time (figure 5; Froehlich
etal. 2019). Noting that passive sinking requires less energy
and labor compared with facilitated sinking, but the lat-
ter provides greater volume capacity and more certainty of
long-term sequestration. Both approaches may result in
unexpected negative consequences, not least the potential for
impact on deep sea ecosystems. Therefore, further research
and thorough risk assessments of any proposed seaweed
sinking activities (at scale) still need to be undertaken. In
this nonharvest mariculture model, income would be gener-
ated through alternative funding streams such as payments
through carbon markets, as opposed to markets based on
harvested seaweed products. If 14% of current seaweed pro-
duction (farmed within only 0.001% of the suitable area glob-
ally) were directed toward such carbon markets, this could
offset all of the emissions from global aquaculture, enabling
carbon neutrality for this industry (Froehlich etal. 2019).
A nonharvest carbon sequestration-based model for sea-
weed mariculture would necessitate large-scale operations
and appropriate siting, likely in offshore areas (Pechsiri etal.
2016, Fernand etal. 2017). Such siting may also allow for
proximity to deeper waters in which seaweed could be sunk
and carbon sequestered (figure 5). However, the potential
benefits of such offshore farms have to be weighed up
against the additional transport and infrastructure related
emissions from more distant farming operations. Ideal
placement would likely be close to both deep waters and the
coast (for maintenance and service access), meaning this
industry may be most feasible in coastal waters in which the
continental shelf is closest to the land and the biogeochemi-
cal and hydrodynamic environment supports productive
seaweed growth and offshore transport of organic material.
If all the organic carbon from farmed seaweed was
sequestered (i.e., not harvested) and the seaweed farming
operations were carbon neutral; global seaweed mariculture
could sequester between 0.05 and 0.29 Gt of CO2e per year
(Hoegh-Guldberg etal. 2019). This is similar to the volume
of carbon (0.16–0.25 Gt of CO2e per year) that could be
sequestered through the restoration and protection of the
world’s mangroves (Hoegh-Guldberg etal. 2019). Although
these seaweed carbon sequestration estimates could be
viewed as a theoretical maximum for sequestration poten-
tial, they are arguably unrealistic, given the relatively limited
current scale of seaweed mariculture. Therefore, attaining
this amount of sequestration is unlikely in the short term.
We are also not aware of any seaweed mariculture opera-
tions currently operating, at scale, under a carbon offsetting
or trading model (although see Oceans 2050 2021, Running
Tide 2021). Therefore, the realization of true carbon seques-
tration from this sector, although it is promising, remains an
important area for action if ocean-based solutions to climate
change through climate-friendly mariculture are to be real-
ized (Hoegh-Guldberg etal. 2019).
Is carbon sequestration achievable through bivalve mariculture? The
carbon embedded in bivalve shells during calcification
equates to significant volumes when farmed at scale (e.g.,
an average of 0.67 (sd = 0.06) Mt of C stored in the shells
of the 9.94 Mt of bivalves produced by Chinese mariculture
in 2007; Tang etal. 2011), and there has been enthusiastic
support for the potential of carbon sequestration from
shells after harvesting (Hickey 2008). However, because
bivalve shell formation and respiration are a net source of
CO2 from sea to atmosphere (see above), the potential for
bivalve monocultures to directly sequester carbon is limited
(Filgueira et al. 2015, Munari etal. 2013). The cofarming
of bivalves and seaweed can offset the carbon released by
bivalves, which, under optimal conditions, can switch pro-
duction from the net carbon source of bivalve monocultures,
to a net carbon sink as bivalve–seaweed cocultures (Han
etal. 2017). Seaweed primary production is usually carbon
limited, but when grown close to bivalve mariculture, CO2
released by the bivalves can enhance seaweed photosynthe-
sis. This in turn releases oxygen and improves conditions
for bivalve cultivation (figure 5; Yang etal. 2005, Jiang etal.
2014), with an optimal ratio for carbon capture by seaweeds
of 4:1 (bivalves to seaweed wet weight; Han etal. 2017). In
China, where bivalve–seaweed cocultures are the primary
form of bivalve mariculture, annual carbon removal is
increased relative to bivalve monocultures by an average of
28.4% (sd = 0.1%; Tang et al. 2011). However, these mutual
benefits are rarely integrated into bivalve mariculture out-
side of East Asia (Neori 2007). The labor, infrastructure
and finance required to make such a change to cultivation
practices and farm design could be a significant barrier, but,
depending on the existing culture method and product mar-
kets, it may only require minor operational adjustments. For
example, suspended bivalve farms could incorporate vertical
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seaweed cultures between bivalve stocks (e.g., baskets, line
cultures), whereas the infrastructure for offshore bivalve
cultures can readily incorporate seaweed longlines (Buck
and Langan 2017). Of course, the capacity of such cocultures
to truly sequester carbon (i.e., store it for hundreds to thou-
sands of years) depends on the fate of the harvested bivalve
shells (van der Schatte Olivier et al. 2020) and seaweed
(Duarte etal. 2017), as was discussed in previous sections.
The various ecosystem regulating services associated with
bivalve mariculture (Petersen etal. 2016, Alleway etal. 2019,
van der Schatte Olivier etal. 2020) can influence the dis-
tribution and performance of blue carbon habitats, such as
seagrasses, generating another potential source of indirect
enhancement of marine carbon sequestration. Bivalve filter
feeding on particulate matter encourages seagrass and mac-
roalgal growth by reducing water turbidity, thereby increas-
ing sunlight penetration (Wall et al. 2008, Humphries etal.
2016, Han etal. 2017). This filtering activity can also buffer
coastal waters from eutrophication events by assimilating
excess nutrients (Peterson and Heck 2001, Higgins et al.
2011, Willer and Aldridge 2020), some of which are depos-
ited on the seabed as bivalve feces or pseudofeces, which
acts as fertilizer and enhances seagrass growth (Reusch etal.
1994, Peterson and Heck 2001, Kent et al. 2017, Gagnon
etal. 2020). However, in areas with particularly high nutrient
loads, the additional fertilization of the benthic sediments
by bivalves has been shown to limit the growth of seagrass
(Wagner etal. 2012).
The influence of bivalve mariculture on blue carbon
habitats, whether positive or negative, will be mediated by
environmental setting, hydrodynamics and farm design
(figure 1). Sheltered coastal systems that are slow flushing,
shallow, and fringed by urban development are particularly
vulnerable to nutrient enrichment (Ferreira and Bricker
2016). Such locations commonly support seagrass meadows
and are suitable for bivalve mariculture, presenting opportu-
nities to use bivalves as a cost-efficient means of extracting
coastal nutrients through harvesting and, therefore, mitigat-
ing the impacts of eutrophication on blue carbon ecosystems
(Newell 2004, Petersen etal. 2016, Theuerkauf etal. 2019).
Maximizing the demonstrated potential of bivalve mari-
culture to cycle carbon and other nutrients to the seafloor
(Sui et al. 2019) will require lease designs that encourage
biodeposition and sediment stabilization, such as those that
direct water over lease structures or away from the sediment
surface to minimize resuspension (Comeau et al. 2014).
Therefore, adopting mariculture designs that promote the
regulating services and boost seagrass performance is nec-
essary if bivalve mariculture is to indirectly enhance blue
carbon sequestration (figure 5) as well as reducing potential
for environmental GHG emissions.
Is carbon sequestration achievable through fed finfish maricul-
ture? There is evidence of sediment accumulation and
organic carbon enrichment under fed finfish mariculture
net pens (figure 1; Carroll etal. 2003, Holmer etal. 2005,
Tanner and Fernandes 2010, Yang et al. 2018), which is
potentially significant in terms of sediment carbon seques-
tration (MacLeod etal. 2019). However, the spatial extent
and magnitude of this process and whether it leads to
meaningful increases in sediment organic carbon stocks
are poorly studied and difficult to quantify (Henriksson
etal. 2014a, 2014b). Some studies suggest that organic car-
bon accumulated in surface sediments under pens is highly
labile, with increased carbon turnover rates (Liu etal. 2016,
Eiríksson etal. 2017, Liu etal. 2017) and returns to base-
line levels after fallowing (Lauer etal. 2009). This implies
that organic carbon enrichment under sea pens is not a
feasible mechanism for long-term carbon sequestration. In
addition, the plethora of negative, ecological consequences
associated with mariculture-derived sediment enrichment
would likely negate any potential for emissions abatement
benefits. These negative impacts include seagrass loss (as
was described above) and the associated releases of blue
carbon and other GHGs (López etal. 1998, Milewski 2001,
Alongi etal. 2009, Liu et al. 2017, Moncada etal. 2019),
oxygen depletion and anoxic sediments (Hargrave 2010),
altered pore-water chemistry (Tanner and Fernandes
2010), a reduction in the abundance and diversity of ben-
thic fauna, and a shift in the community composition of
benthic microbes potentially increasing organic matter
decomposition, a further release of GHGs and a break-
down of benthic–pelagic nutrient cycling (Weitzman 2019,
Weitzman etal. 2019).
Summary and recommendations
Although mariculture is typically reported as having a
smaller GHG footprint than other food production indus-
tries (e.g., Hilborn etal. 2018, MacLeod et al. 2020), there
are large differences in the median GHG emissions footprint
of the fed finfish, bivalve and seaweed sectors and consider-
able variability within sectors (figures 2 and 3, table S1).
This variation, and the critical need to rapidly reduce GHG
emissions at a global level substantiate the need to identify
and implement strategies that advance the climate-friendly
capacity of mariculture, regardless of its current GHG emis-
sions profile. The greatest opportunities for high-volume
reductions in GHG emissions are likely to come from
changes in upstream and downstream parts of the sup-
ply chain (figure 3; Hilborn etal. 2018). In particular, the
method of postharvest transport has a large impact on the
final GHG emissions footprint of maricultured products.
High-value products (e.g., tuna and salmon) represent glob-
ally significant export industries reliant on rapid air freight
to international markets. As the importation of seafood
products becomes more viable in emerging economies there
is the potential for GHG emissions associated with trans-
port to increase substantially. Shortening supply chains and
building regional markets could reduce GHG emissions at
the same time, potentially contributing to greater food secu-
rity (Belton etal. 2018) and industry resilience in times of
crisis (Froehlich etal. 2021).
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136 BioScience •February 2022 / Vol. 72 No. 2 https://academic.oup.com/bioscience
Because there is greater potential for mariculture farm
operators to influence on-farm reductions in GHG emis-
sions, we focused on on-farm activities and carbon exchange
with the surrounding marine environment, aligning our
climate-friendly assessment with the principles of designed
industrial solutions (Francis 2016) and the FAO’s Ecosystem
Approach to Aquaculture, which seeks greater industry
sustainability (Soto et al. 2007, Brugère et al. 2019). For
mariculture to move decisively toward climate-friendly
on-farm operations, we will need a concerted effort to
reduce direct emissions (particularly through leveraging
industry–environment interactions that alter local carbon
flows; figure 1), and the development of market opportuni-
ties that support carbon sequestration and carbon offsetting.
Unsurprisingly, there is not a single silver bullet solution that
works in all sectors and situations. Rather, the sectors often
have different interactions with the surrounding marine
environment and operational practices that offer bespoke
opportunities for either avoiding GHG emissions or enhanc-
ing carbon sequestration (figures 1, 4, and 5). Nevertheless,
in synthesizing across the three sectors we identified six
principles that can help enable climate-friendly mariculture
approaches. These are theoretical and, while couched in the
literature, they still require future research and development
regarding cost–benefit analyses, detailed assessments of
trade-offs with other environmental or socioeconomic fac-
tors, and investigations of the feasibility of scaling up.
On-farm production can be emissions intensive. Climate friendly
operations will need to find opportunities to increase
efficiency (i.e., lower on-farm energy usage, shift to low-
emissions energy sources, the use or reuse low-emissions,
durable materials for farming infrastructure) and reduce
nutrient inputs and wastes that lead to environmental GHG
emissions, while also working toward carbon neutrality
through the use of biofuels and clean energy sources to
power on-farm operations.
Interactions with surrounding marine environments influence GHG
emissions. Our analysis highlights several farm design and
operational changes that provide immediate options to
reduce the impact of finfish and bivalve farming on benthic
habitats and avoid environmental emissions of stored blue
carbon (figures 4 and 5). These include siting fed finfish
operations away from sensitive blue carbon habitats in
deeper or faster flowing waters and minimizing feed waste
to the environment (figure 4), as well as adopting climate-
friendly grow-out strategies for bivalves that minimize
benthic disturbance and sediment resuspension, such as
using suspended bags and trays and manual harvesting
methods (figure 5). There is also potential for both bivalve
and seaweed mariculture to indirectly support or enhance
blue carbon sequestration. For bivalves, this relies on posi-
tive interactions with surrounding marine environments
that support sediment stabilization (and associated carbon
burial), reduce eutrophication via filter feeding and increase
benthopelagic coupling (i.e., transfer of fertilizing nutrients
to the seafloor). Future research on the types of farm designs
that promote the greatest biodeposition, sediment accumu-
lation, and organic carbon burial (within ecological limits) is
needed to understand the full potential for climate-friendly
bivalve mariculture operations to deliver these benefits.
Polyculture can support on-farm GHG emissions reduc-
tions. Opportunities for reducing indirect GHG releases
from bivalve culture (i.e., the release of CO2 associated with
calcification) and environmental degradation resulting from
eutrophication around fed finfish farms may lie in polyc-
ulture approaches. These include cofarming bivalves with
seaweed, which can lead to a net reduction in CO2 emissions
(figure 5; Han etal. 2017) and cofarming fed finfish with sea-
weed or bivalves (Wang etal. 2014, Strand etal. 2019) that
absorb excess nutrients, helping to reduce eutrophication
and related blue carbon habitat degradation. However, the
emissions abatement potential of such cross-sector syner-
gies remains dependent on the fate of the farmed seaweed
and bivalve shell waste, which would need to be repurposed
through one of the potential applications that either enable
ongoing carbon storage or offset emissions from other sec-
tors (figure 5; Morris etal. 2019).
For carbon sequestration and GHG offsets, the fate of the product and
scale of operation are key. There is a low immediate likelihood
of long-term carbon sequestration from the sectors assessed.
Seaweed mariculture holds the greatest potential for long-
term carbon capture if seaweed is left in place (i.e., a carbon
farming approach) and sequestration if transported to ocean
carbon sinks. However, the potential negative impacts of
these practices are unknown, and scaling up will be a chal-
lenge, because large quantities of biomass would need to be
directed toward processes and markets that facilitate deep
sea burial (Froehlich etal. 2019) or blue carbon donation
(Trevathan-Tackett et al. 2015). Even if production was
scaled up to account for 25% of total domestic aquacul-
ture production tonnage in countries currently producing
small volumes of seaweed, this would be unlikely to result
in meaningful amounts of carbon sequestration (see the
worked example in supplemental file S3 and table S5).
Further barriers to the effective implementation and scaling
of both seaweed and bivalve sequestration and offsetting
strategies include a lack of recognition of seaweed carbon in
current carbon accounting and trading markets, the poten-
tially high costs of large-scale seaweed farming (particularly
in offshore environments) compared with terrestrial carbon
farming and the lack of large-volume markets for bivalve
shell waste. To generate both scientific and market confi-
dence in these proposed approaches, pilot projects designed
to evaluate the potential sequestration and offsetting bene-
fits, operational feasibility, and scale are needed (e.g., Oceans
2050 2021). These may include collecting data on on-farm
nutrient fluxes, operational approaches to seaweed deep-sea
transport and shell repurposing, potential for long-term
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sequestration, and full LCAs. Climate-friendly approaches
will apply differently to different environmental and sectoral
settings, and farms may need to pursue a portfolio of options
to gain the greatest climate benefits while maintaining cost-
effective operations.
Thorough carbon accounting is critical. Unfortunately, variable
methods and scope in mariculture GHG emissions LCAs
mean that truly comparable studies accounting for the full
supply chain are rare even for the same species and culture
systems (Henriksson etal. 2013, Avadí etal. 2018, Bohnes
et al. 2018, Bohnes and Laurent 2019). There is a clear
need for more standardized reporting of GHG emissions to
ensure comparability across studies within and between sec-
tors, a challenge of LCAs that is not limited to the maricul-
ture industry (Bohnes etal. 2018, Bohnes and Laurent 2019).
Improvements here would provide a robust knowledge base
with which to support mariculture’s efforts to reduce and
regulate its GHG footprint. In addition, the GHG emissions
outcomes of both positive and negative interactions between
mariculture operations and surrounding marine environ-
ments are missing from most LCA studies. We found that
environmental emissions may contribute significantly to
the industry’s GHG footprint (supplemental material S2,
table S4), therefore the true potential for avoided emissions
from mariculture cannot be fully understood until these
missing links are incorporated. Accurate LCAs for a range of
exemplar species and environment combinations (as a start-
ing point) may need to be done through industry audits that
are incentivized by either (or both) regulation and climate-
friendly certification. An increase in the availability of LCA
tools and software that can be more easily used by a wide
range of practitioners and that include appropriate options
for the parameterization of mariculture operations (includ-
ing environmental emissions) would improve the accuracy
and feasibility of GHG accounting in mariculture.
Availability of infrastructure, investment and value of end products
may affect uptake of climate-friendly practices at farm, country,
and regional scales. Some of the more innovative emissions
reduction and sequestration practices suggested in the pres-
ent article may need to be adopted early in regions with
greater resources for research and development and greater
capacity for investment in nascent technologies (Gentry
etal. 2019).
Conclusions
By linking the provision of food from mariculture to
broader environmental benefits, such as GHG abatement,
our study can support the development of climate-friendly
mariculture practices that generate sustainable ecological,
social, and economic outcomes (Tlusty etal. 2019). We also
hope to assist in aligning the mariculture industry with
carbon accounting, offsetting, and crediting schemes that
are focused on either achieving demonstrable GHG emis-
sions reduction or carbon sequestration. This is currently
hindered by the lack of specific mariculture policy frame-
works and knowledge gaps that prevent effective carbon
accounting, particularly in the context of environmental
(e.g., blue carbon) emissions and sequestration (Froehlich
et al. 2019, Lovelock and Duarte 2019). Considering the
projected global reliance on mariculture for food production
into the future and the industry’s persistently high growth
rate (7.5% per year over the last 50 years; Bene etal. 2015,
FAO 2020), sustainable intensification and the broadscale
adoption of the Ecosystem Approach to Aquaculture will
be critical to mitigating the climate impacts of a scaleup in
mariculture production (Henriksson et al. 2018, MacLeod
etal. 2019, Yuan etal. 2019, FAO 2020). This issue is par-
ticularly pertinent in this, the UN Decade of Ocean Science
for Sustainable Development, which highlights the impor-
tance and opportunity for sustainable ocean provisioning
to deliver equitable food and economic outcomes into the
future (Bennett etal. 2019, Pretlove etal. 2019).
Acknowledgments
This research was supported by a grant from The Nature
Conservancy (USA) to H. Alleway. B. Gillanders and A.
Jones at the University of Adelaide. We are grateful to B.
Gillanders who provided logistical support and funding
to A. Jones over the course of this work. We would like to
thank the three anonymous reviewers of this manuscript
who provided expert technical input and whose construc-
tive and detailed comments helped us to greatly improve the
structure and readability of the article.
We acknowledge and respect the Kaurna people, the
Traditional Owners of the lands on which the University of
Adelaide is built. We pay respects to Kaurna Elders past and
present.
Supplemental material
Supplemental data are available at BIOSCI online.
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Alice Jones (alice.jones01@adelaide.edu.au), Dominic McAfee (dominic.mca-
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