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Fisheries of the rivers of Southeast Asia

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The lower, potamonic parts of the Ganges–Brahmaputra, the Ayeyarwady (Irrawaddy), the Salween, the Chao Phraya and the Mekong and Lancang Rivers are among the longest and most productive rivers for inland fisheries in the world. Except for the Chao Phraya, they arise on the Tibetan Plateau. All have steep and turbulent upper courses within deep mountain valleys and flat lower courses associated with large deltaic wetlands. Much of the riparian wetlands have been converted to rice culture. They all have rich and diverse fish faunas, comprising >100 families, that are adapted to a wide range of river channel and floodplain habitats. Many species are migratory whitefishes, but more sedentary blackfishes are more important in the fisheries of some rivers. Fisheries may be commercial, artisanal or subsistence and employ a wide range of static and moving gear, some of which requires considerable investment. Most species caught are consumed. Larger fishes are sold for the table; smaller individuals are often processed into a variety of forms including dried products, fish pastes and sauces. Small, low‐value fishes are also utilized for animal feed (mainly for aquaculture) sometimes after processing. There are a wide range of potential threats to the inland fishes and fisheries of Asia including dam development for hydropower and irrigation, overexploitation, pollution, land use change, mining, the introduction of invasive species, and water diversion for agriculture and other purposes. Fisheries are managed either as open access fisheries or lot fisheries which are assigned to particular groups on the basis of auctions. At present, management at local, national, basin and international levels is not meeting the needs of fish and fishery conservation and urgently needs to be reformed to better protect the fisheries in the face of mounting pressures from other users of the aquatic resource.
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Freshwater Fisheries Ecology, First Edition. Edited by John F. Craig.
© 2016 John Wiley & Sons, Ltd. Published 2016 by John Wiley & Sons, Ltd.
363
Introduction
Southeast Asian lowland rivers are among the longest and most
productive rivers for wild‐capture inland fisheries in the world.
They have many elements in common: they mostly arise on the
Tibetan Plateau and have steep and turbulent upper courses
within deep mountain valleys and flat lower courses associated
with large deltaic wetlands. Their lower basins are now densely
inhabited and were the site of early civilizations culminating
in the Mughal (Ganges), Khmer (Mekong), Siamese (Chao
Phraya) and Pagan (Irrawaddy). Historically, these rivers have
been associated with intensive wet rice cultivation, which has
involved modification of the landscape including adjacent
floodplains and associated wetlands, with more recent ongoing
and planned changes due to hydroelectric dams. Some, like the
Ganges, are also grossly polluted and abused by the discharge of
large quantities of domestic and industrial effluents; this repre-
sents a peculiar paradox given the river’s central place in Hindu
mythology. Three of the six systems included in this chapter
have been studied intensively through national institutions and
various government and non‐government projects, so knowl-
edge of the resources and their exploitation, while far from
complete, is better advanced than for many other inland fisher-
ies. This chapter deals with the lower, potamonic parts of the
Ganges–Brahmaputra, especially the extensive deltaic and
flood systems of Bangladesh, the Ayeyarwady (Irrawaddy), the
Salween, the Chao Phraya and the Mekong and Lancang
(Fig.3.24.1).
Fisheries of the rivers of Southeast Asia
Robin L. Welcomme1, Ian G. Baird2, David Dudgeon3, Ashley Halls4, Dirk Lamberts5
and Md Golam Mustafa6
1 Department of Ecology and Evolution, Imperial College Conservation Science, Berkshire, UK
2 Department of Geography, University of Wisconsin‐Madison, Madison, USA
3 School of Biological Sciences, The University of Hong Kong, Hong Kong, China
4 Aquae Sulis (Research) Ltd (ASL), Bradford‐on‐Avon, Wiltshire, UK
5 Laboratory of Aquatic Ecology, Evolution and Conservation, University of Leuven, Leuven, Belgium
6 WorldFish, Dhaka, Bangladesh
CHAPTER3.24
Abstract: The lower, potamonic parts of the Ganges–Brahmaputra, the Ayeyarwady (Irrawaddy), the Salween, the Chao
Phraya and the Mekong and Lancang Rivers are among the longest and most productive rivers for inland fisheries in the
world. Except for the Chao Phraya, they arise on the Tibetan Plateau. All have steep and turbulent upper courses within
deep mountain valleys and flat lower courses associated with large deltaic wetlands. Much of the riparian wetlands have been
converted to rice culture. They all have rich and diverse fish faunas, comprising >100 families, that are adapted to a wide range
of river channel and floodplain habitats. Many species are migratory whitefishes, but more sedentary blackfishes are more
important in the fisheries of some rivers. Fisheries may be commercial, artisanal or subsistence and employ a wide range of
static and moving gear, some of which requires considerable investment. Most species caught are consumed. Larger fishes
are sold for the table; smaller individuals are often processed into a variety of forms including dried products, fish pastes and
sauces. Small, low‐value fishes are also utilized for animal feed (mainly for aquaculture) sometimes after processing. There
are a wide range of potential threats to the inland fishes and fisheries of Asia including dam development for hydropower
and irrigation, overexploitation, pollution, land use change, mining, the introduction of invasive species, and water diversion
for agriculture and other purposes. Fisheries are managed either as open access fisheries or lot fisheries which are assigned
to particular groups on the basis of auctions. At present, management at local, national, basin and international levels is not
meeting the needs of fish and fishery conservation and urgently needs to be reformed to better protect the fisheries in the face
of mounting pressures from other users of the aquatic resource.
Keywords: Lowland river fisheries: Ganges R, Brahmaputra R, Irrawaddy R, Salween R, Mekong R, Chao Phraya R
364 Freshwater resources
Description of habitats
The 2525 km long Ganges is fed by several major tributaries that
drain the Himalayan massif whose upper courses are steep
mountain rivers that debouche onto fertile alluvial plains. The
Brahmaputra (2900 km) originates on the Tibetan Plateau and
combines with the Ganges and the Meghna Rivers to form an
extensive deltaic and flood system lying mainly in Bangladesh
and Bengal (India). The three rivers together drain an area of
1 086 005 km2 and dominate the fluvial geography of the north-
ern Indian subcontinent. The combined deltaic floodplain (the
largest in the world) covers nearly all of Bangladeshs 147 570 km2
area and is formed by a network of the major rivers: the Padma,
the Meghna, the Jamuna and the Brahmaputra. Seventy per cent
of the delta is <1 m above sea level and floods regularly; 10%
remains flooded when water retreats into the numerous lakes,
rivers and canals at river drawdown (Payne et al., 2004).
The Ayeyarwady (Irrawaddy) River is 2170 km long and has
a drainage basin of 413 710 km2. It rises on the Tibetan Plateau
in China and flows through Myanmar to the sea. It has an exten-
sive delta with some 600 000 km2 of paddy lands on the flood-
plain. The overall north‐north‐east orientation of the delta
contributed to the impact of Cyclone Nargis which on 2 May
2008 destroyed the fisheries sector of the delta, claiming at least
138 × 103 lives and destroying an estimated 100 × 103 boats.
The 2815 km long Salween also flows from the Tibetan high-
lands to Myanmar (Burma) and Thailand through a 324 000 km2
basin. It is a steep river, encased in uplands for much of its length
although there is a small delta.
The Mekong is the dominant river system of the mainland
Southeast Asian region. It rises in China, where it is known as
the Lancang, and flows 4880 km through Laos, Myanmar
(Burma), Thailand, Cambodia and Vietnam. Its basin covers
795 000 km2. Its upper course is confined to a narrow valley and
is steep and rocky. There is a flatter middle Mekong section with
limited floodplains which is interrupted by an anabranching
system of channels around the Siphandone (4000 islands) area
and an extensive series of rapids and waterfalls at the Khone
Falls along the border between Laos and Cambodia (Daconto,
2001). The deltaic region in central Cambodia has a flooded
area of up to 70 000 km2 and includes the Tonle Sap River and
Great Lake. The area of the Great Lake fluctuates from 2200 km2
at low water during the dry season to c. 13 250 km2 at the peak
of 5 months of flooding (Kummu et al., 2014). Its depth also
increases from 0.7 to 1.0 m in the dry season to 6.3 to 9.8 m during
the wet season, with considerable inter‐annual variation. The
Brahmaputra
Ganges
Irrawaddy
Chao
Pharaya
Mekong
Salween
Figure3.24.1 Location of main Southeast Asia rivers.
Fisheries of the rivers of Southeast Asia 365
Mekong channel has numerous deep pools along its length
thatprovide significant dry‐season habitat for fishes, particu-
larly larger species (Poulsen et al., 2002a; Baird 2006a). The
floodplain was formerly heavily forested, but a considerable
proportion has been cleared for rice cultivation.
The Chao Phraya does not arise in the Tibetan Plateau but has its
source in the northern highlands of Thailand. It is smaller than
the other rivers discussed here, at only 372 km long, with a basin
area of 160 000 km2 that occupies the central core of the country.
Many major towns are situated along its length, and it has been
subject to considerable river engineering by successive regimes.
The extensive floodplain was originally forested but has mostly
been cleared for rice cultivation, and large areas are now urbanized.
Main species (diversity) and their uses
The rivers of this region all have rich and diverse fish faunas,
comprising over 100 families, more than are present in either
Africa or tropical America. According to Rahman (1989), 260
species of finfishes belonging to 55 families occur in the fresh
waters of Bangladesh hilsa Tenualosa ilisha used to be the most
important single fishery species, and the Ganges system alone
has 161 recorded species (Payne et al., 2004). The Indo‐Burma
region includes at least 1178 species, including 151 in the
Salween, 328 in the Mae Klong–Chao Phraya, 328 in the
Irrawaddy and over 500 in the Mekong (Kottelat et al., 2012).
The species assemblages are dominated by cyprinids (especially)
and various catfish taxa. Sizes range from the giant stingray
Himantura chaophraya, which can exceed 400 kg, and Mekong
giant catfish Pangasianodon gigas, which reaches over 300 kg, to
small barbs, gouramies and loaches that are valued as aquarium
species (Kottelat et al., 2012).
River fishes are sometimes divided into blackfishes and
whitefishes according to their behaviour and ecology (Hill,
1995). Whitefishes are pelagic, planktivores or piscivores that
migrate between floodplains and channel habitats and espe-
cially along the main channels of the rivers. Such species require
high dissolved oxygen concentrations and avoid low oxygen
condition by migrating; they are egg scatterers that show little or
no parental care. In contrast, blackfishes tend to only migrate
locally within a restricted area and are generally benthonic,
inhabiting floodplain pools and residual channels as well as
inundated forest and rice fields, habitats that are often low in
dissolved oxygen. Many blackfishes practise parental care (e.g.
mouth brooding and bubble‐nest building) and can tolerate low
oxygen levels that prevail when waters far from the main river
channel become stagnant. The care offered by the parent fish
reduces mortality of eggs and fry under such conditions, and it
is notable that Asian blackfishes include more than half of the
fish families known to brood in the mouth. Generally, white-
fishes are more commercially valuable than blackfishes.
Most Southeast Asian river fishes are migratory although,
asthe whitefish versus blackfish comparison shows, there is a
range of breeding behaviours Some species may remain in their
adult habitats, whether floodplain pools or main river channels,
and complete their life cycle without significant migrations, and
others, so‐called greyfishes (e.g. Labeo spp.), have facultative
behaviour with both migratory and static or territorial compo-
nents enabling them to respond readily to changes in flow
conditions. Nonetheless, the key point is that most river fishes
in Southeast Asia make migrations that culminate in reproduc-
tion that coincide with the onset of flow increases in the wet
season; return migrations tend to coincide with falling river
levels. For example, many species (typically blackfishes) migrate
laterally onto the floodplains and riparian flooded forests for
feeding or spawning and adults and young return to the channel
or lake on the falling flood, often at night. Others migrate to an
upstream site in the main river or tributaries and return down-
stream, usually during daylight. The eggs and larvae of such
species drift downstream until they are large enough to move
laterally onto the floodplains. One anadromous species, a large
catfish Pangasius krempfi, migrates over 1000 km from the
South China Sea and the Mekong Delta in Vietnam up the
Mekong to northern Laos (Hogan et al., 2007). In the Ganges,
the mahseers Tor spp. undertake long upstream migrations to
spawn in the rapid headwaters of the tributary rivers. Such
migrations have been extensively studied in the Mekong
(Poulsen et al., 2002b; Baird et al., 2003; Baird & Flaherty, 2004;
Halls & Kshatriya, 2009; Halls et al., 2013a), in particular the
movements of fishes in the Khone Falls area on the Laos–
Cambodia border (Roberts & Baird, 1995; Baird et al., 2001,
2003, 2004; Baird & Flaherty, 2004; Baran et al., 2005; Warren
etal., 1998, 2005; Baird, 2011; Halls et al., 2013a), and in and out
of the Great Lake through the Tonle Sap River where they are
exploited by the ‘dai’ fisheries (Figure3.24.2), and in the Khone
Falls area (Lieng et al., 1995; Poulsen et al., 2002b; Lamberts
2006, 2008a, b; Lamberts & Koponen, 2008; Welcomme et al.,
2012; Halls et al., 2013b).
Figure3.24.2 Dai fixed trawl nets in Tonle Sap, Cambodia. Photograph by
Robin L. Welcomme.
366 Freshwater resources
Fish monitoring programmes and fish consumption studies
indicate that whitefishes form 30–40% of the inland fish bio-
mass in the Lower Mekong Basin. Blackfishes and greyfishes
form c. 50% and 10–20% (Halls & Kshatriya, 2009). Blackfishes
contribute 88% by mass of catches in the rice field fisheries of
the Lower Mekong (Hortle et al., 2008), and in Bangladesh
floodplains, resident blackfishes provide the majority of the
national freshwater fish production (Craig et al., 2004).
Status of freshwater and estuarine fisheries
Capture methods and types of fisheries
Southeast Asian river fisheries may be commercial, artisanal
or subsistence. Commercial fisheries are usually operated
from large boats or fixed engines, such as the ‘dai’ fishery of
the Tonle Sap, which consists of up to 15 rows of static trawls
installed across the river to catch fishes migrating out of the
Great Lake, or the extensive and large‐scale fish fence sys-
tems in the Great Lake that catch fishes migrating from the
drying flooded forests. Such fixed gear and traps are expen-
sive to install and operate and are regulated by strict licens-
ing under the lot fishery system (Figure3.24.3) but can be
very remunerative. For example, Halls etal. (2013a, b) esti-
mated that 1.6 × 106 fishes migrate from the Great Lake each
day of the fishing season (October to March). Most (c. 80%)
are caught by the rows of nets that form the ‘dai’ fishery in the
Tonle Sap River.
Artisanal fishers are far more widespread and range from
dedicated fishers, whose whole time is spent fishing, to others
to whom fishing is an essential element in a more general
rural lifestyle. Subsistence fishers are also widespread and
children and women, in particular, fish rice paddies and
small watercourses for vital contributions to their diets. The
Bangladesh fisheries sector employs almost 15.6 × 106 whole
time and subsistence fishers, yet the majority of fishers are
poor (DoF, 2012).
Fishing is widespread in rice fields, and natural levels of
production may be enhanced by specially dug sump ponds to
retain fishes during the dry phase of culture or may be
stocked. Even without such enhancements, natural produc-
tion from these habitats is high (Hortle et al., 2008) and con-
tributes substantially to the catch from the various river
systems.
In recent years, there has been an increasing trend towards
stocking various habitats including rice fields, floodplain lakes
and small impoundments associated with the river (De Silva &
Funge‐Smith, 2005). This has created a special class of fishers
who are usually grouped into cooperatives and become effective
owners of the stocked material and waterbodies.
The diversity of fish species, waterbody type and seasonal
conditions means that fishing methods are equally diverse. For
example, up to 150 different gears are described by Deap et al.
(2003) for the Mekong. Gears range from large active gears such
as lift nets (Figure3.24.4) and large passive fish fences and traps
that require teams of operators to small individually operated
nets, traps (Figure3.24.5) and hook lines. In the lowwater sea-
son, fish aggregating devices (FADs) or brush parks are com-
mon, albeit often illegal, particularly in the Mekong (‘samras’)
and Bangladesh (‘kathas’) (Figure 3.24.6). Fishing pressure in
Bangladesh particularly has been increased through the deploy-
ment of ‘kathas’ and bamboo fence‐trap systems in canals
between the river and the floodplain by non‐traditional fishers,
which catch broodfishes at the beginning of the monsoon in the
Meghna River and the lower Indo‐Gangetic Plains (Mustafa,
2009). Although usually illegal, fishers also drain (dewater)
floodplain ponds and pools, canals and sections of river chan-
nels during the dry season to remove refuge‐seeking fishes
(Craig et al., 2004).
Figure3.24.3 Entrance to fishing lot, Tonle Sap, Cambodia. Photograph by
Robin L. Welcomme.
Figure3.24.4 Lift net on Bangladesh floodplain. Photograph by Robin
L. Welcomme.
Fisheries of the rivers of Southeast Asia 367
Catch disposal
Most species caught are consumed. The larger individuals are
sold for table fish, especially to the rich urban environments,
such as Bangkok, Phnom Penh, Yangon and Dacca. Smaller
individuals are often processed into a variety of fish pastes
and sauces that are fundamental elements in local diets.
Significant quantities of small, low‐value ‘trash fish’ are also
used to manufacture farm‐made and commercial pellet feed.
In Cambodia, for example, an estimated 60 × 103 t of trash
fish was fed to cultured snakehead mostly Channa micropeltes
and Channa striata and catfish (Pangasius conchophilus, Pangasius
larnaudiei, Pangasianodon hypophthalmus, Pangasius bocourti,
Hemibagrus wyckioides and Clarias batrachus × C.gariepinus)
in 2004 (So et al., 2005). In the Vietnamese delta, an estimated
1.4 × 106 t of marine and freshwater trash fish is required as
feed to culture 1 × 106 t of striped catfish (P.hypophthalmus).
With future supplies of trash fish vulnerable to development
and climate change, and with rising demand and price for
fishmeal on the international markets, alternative sources of
protein and oil will need to be identified. Juvenile fishes, espe-
cially Pangasius spp. and C. micropeltes, may be stocked into
cages for growing on, to be marketed when prices rise. Many
of the floodplain fisheries show strong seasonal fluctuations
in supply and availability of fishes, with matching variations
in prices and differences in use.
Much of the fish trade in the Mekong is across borders. For
example, Bush (2004) documented extensive fish trade between
Cambodia and Thailand via Laos. Added value is often exported
to countries with better processing capabilities and established
access to demanding overseas markets.
Recent trends in the fishery
Trends in abundance
The wild fish catches from the rivers of Southeast Asia are
among the highest in the world and together contribute c. 27%
of the global total (Table3.24.1). There are some difficulties in
establishing exactly how much fish is being caught since most
subsistence catch remains, by its very nature, unreported. For
example, the nominal catch listed in Table3.24.1 is based on
figures provided by member governments to FAO. In the case
of the Mekong countries, the total catch from all inland systems
in the countries is 839 900 t in 2010, whereas independent
assessments by the Mekong River Commission (MRC) (Hortle,
2007; Lymer et al., 2008a) are in excess of 2 × 106 t year−1. In par-
ticular, Vietnam, with a reported catch of only 194 200 t in
2010, was estimated to have catches nearer to 852 000 t. Lymer
et al. (2008b) estimate that actual catches in Thailand are up to
five times greater than those reported in the official statistics.
Total catches from Myanmar were also reported to be low com-
pared to the potential as calculated by area, although reporting
over the last 5 years seems to have rectified this. In general,
catches throughout the region have risen rapidly, particularly
since the early 1990s (Fig.3.24.7), and the dramatic increases
in reported catches in Myanmar are probably due to the better
estimation of the contribution of floodplain fisheries and espe-
cially subsistence fisheries. Similarly, the considerable increases
reported by Cambodia from 1999 probably arose from the
incorporation of floodplain fisheries following reports by the
MRC, although Cambodia itself has declared its catches prior
to this date to be under‐reported. This example highlights the
Figure3.24.5 Wing trap on Mekong, Khone Falls, Lao Democratic
Republic. Photograph by Robin L. Welcomme.
Figure3.24.6 Fishing a Katha brush park, Bangladesh floodplain.
Photograph by Robin L. Welcomme.
Table3.24.1 Nominal inland catches in 2010 for Southeast Asian countries
Country Nominal catch in 2010 (t)
Bangladesh 1 119 094
Myanmar 1 002 430
Cambodia 405 000
Thailand 209 800
Vietnam 194 200
Lao People’s Dem. Rep. 30 900
Total 2 961 424
Source: Data from FAO FishstatJ.
368 Freshwater resources
fact that there are great deficits in catch data collection in all
the inland fisheries of the region (Coates, 2002), which makes
trends in abundance and productivity difficult to identify or
interpret (Lamberts, 2006).
Trends in species diversity
Inevitably, there have been adjustments in the relative abun-
dance of species in the various systems. Unfortunately, the gov-
ernment reports by species in the fishery statistics are often
incomplete or inaccurate. Where more precise records have
been kept, there are indications that some of the larger species
are no longer as abundant as they were formerly. Abundance of
‘megafish’ species such as P. gigas, Pangasius sanitwongsei,
Catlocarpio siamensis and the Mekong giant salmon carp
Aaptosyax grypus, as well as large species such as Probarbus
jullieni and P. krempfi, has fallen to the point where they are
classified as threatened by the International Union for the
Conservation of Nature and included on the Red List (www.
iucnredlist.org/) and special programmes for their management
have been advocated (Hogan et al., 2001; Baird, 2006b). Recently
published time series of abundance indices for several Red List
species, however, show little evidence of any trends in Cambodia
(Halls et al., 2013a). Indeed, there is no compelling evidence to
suggest that species assemblages that seasonally inhabit the
Tonle Sap Great Lake System of Cambodia have changed signifi-
cantly during the past 15 years (Halls & Paxton, 2014).
In Bangladesh, catches of major cyprinids and other large
species have declined to such an extent that the fishery now
depends on fishes that are <1 year old (0+ year fishes) (Halls
et al., 1999). Some migratory species are being impacted by
dams, for example, T. ilisha used to be the most important single
species in the Bangladesh fisheries contributing 25% of the
catch in 1989, but had declined in importance to 11% in 2010.
Most fisheries that have been systematically studied show signs
that they have been fished down (Welcomme, 1999) in that the
mean size and age of fishes caught has declined substantially,
and this has been associated, inevitably, with a decline in larger
species in Southeast Asian rivers (FAO, 1995; Allan et al., 2005).
Caution should be exercised when using mean fish size as an
indicator of exploitation in the short term, however, because
fish growth in floodplain river systems can vary significantly in
response to the extent and duration of flooding each year (Halls
et al., 2008; Halls & Paxton, 2014). The effect on fish size of
several consecutive dry years (e.g. the period 2001–2002 to
2007–2008 in Fig.3.24.8) could be erroneously interpreted as
increasing rates of exploitation. Since these data were published,
record landings and size of fishes were made during the 2011 to
2012 fishing season following one of the largest floods on record
in Cambodia (Phen et al., 2012).
0
0.5
1
1.5
2
2.5
3
3.5
1950
1954
1958
1962
1966
1970
1974
1978
1982
1986
1990
1994
1998
2002
2006
2010
Nominal catch (t × 106)
Year
Figure3.24.7 Trends over time of nominal inland fish catches for
Southeast Asian countries [ , Lao People´s Dem. Rep.; , Vietnam;
, Thailand; , Cambodia; , Myanmar; , Bangladesh;
, Totals-quantity (t)]. Source: Data from FAO FishstatJ.
0
200 000
400 000
600 000
800 000
1 000 000
1 200 000
0.000
0.005
97–98
98–99
00–01
01–02
02–03
03–04
04–05
05–06
06–07
07–08
08–09
99–00
0.010
0.015
0.020
0.025
Flood index (km2 days)
Mass (kg)
Season
Figure3.24.8 Changes in the mean mass
( blue) of fishes caught in the Cambodian ‘dai’
fishery in response to flood extent and duration
measured by the flood index (IF; red) from
1997 to 2009. Source: Data from Halls and
Paxton (2014).
Fisheries of the rivers of Southeast Asia 369
Trends in abundance
Year‐to‐year trends in catch in floodplain fisheries such as those
of the lower courses of Southeast Asian rivers tend to be corre-
lated with flooding. This is because longer and more extensive
floods increase reproductive success, promote better juvenile
fish survival and improve growth (Welcomme & Halls, 2004).
This response is often only detectable for fisheries that are
intensively monitored such as the ‘dai’ fishery of the Tonle Sap
(Halls et al., 2013c; Halls & Paxton, 2014). They also appear as
alonger‐term trend which can be concealed by shorter‐term,
year‐on‐year variations in relative abundance and growth.
Evidence for long‐term declines in mean fish size and abun-
dance of larger species is mixed. On the one hand, overfishing
and the use of explosives have led to a reported decline of 80%
in landings by Lao artisanal fishers between the 1970s and 1990s
(Roberts, 1993). Furthermore, there is considerable evidence for
substantial changes, both on species composition and mean
length of catches, in Bangladesh. On the other, a recent study
(Halls et al., 2013b) found no compelling evidence in fisheries
monitoring data compiled from 54 locations in the Lower
Mekong Basin to suggest that fish abundance, mean fish size or
fish diversity has declined significantly during the past 20 years.
Aquaculture
Aquaculture has been developing rapidly in mainland
Southeast Asian countries for many years but has expanded
considerably since 1995 (Fig.3.24.9). In 2010, the combined
production from the countries covered here was nearly
4.5 × 106 t (Table3.24.2). Not all of this is directly connected
tothe rivers. All levels of culture are represented. At the most
extensive levels are the stock‐recapture systems that are
common in small dams and rice paddies. These are interme-
diated between intensively managed inland fisheries and
aquaculture (De Silva & Funge‐Smith, 2005; Miao et al.,
2010). More intensive pond culture, mainly for catfishes,
cyprinids and tilapias (Table3.24.3), is common, particularly
in areas distant from the major capture fisheries. Cage culture
(Figure 3.24.10) is also widespread. Intensive culture of
Pangasius hypophthalmus has grown from humble origins
in cage culture in the Mekong into an activity producing
>1 × 106 t of fish, mostly in small family ponds on the Mekong
floodplain in Vietnam. Output from the sector is now in the
order of 1 × 106 t year−1, with a value close to US $ 2 × 109
making a significant contribution to the country’s GDP and
supporting the livelihoods of >200 × 103 people in the delta
(De Silva & Phuong, 2011).
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
1950
1955
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
Aquaculture production (t × 106)
Year
Figure3.24.9 Trends over time of nominal freshwater aquaculture production
for the Southeast Asian countries ( , Cambodia; , Lao People’s Dem.
Rep.; , Thailand; , Myanmar; , Bangladesh; , Vietnam).
Source: Data from FAO FishstatJ.
Table3.24.2 Nominal freshwater aquaculture production in 2010
forSoutheast Asian countries
Country Production in 2010 (t)
Vietnam 1 889 770
Bangladesh 1 226 427
Myanmar 772 396
Thailand 432 378
Lao People’s Dem. Rep. 82 100
Cambodia 57 780
Total 4 460 851
Source: Data from FAO FishstatJ.
Table3.24.3 Nominal freshwater aquaculture production by species in 2010
for Southeast Asian countries
Species Nominal production in
2010 (t)
%
Pangasius spp. 1 177 609 26.40
Roho labeo 808 199 18.12
Cyprinids nei 486 362 10.90
Catla catla 248 526 5.57
Hypophthalmichthys molitrix 213 716 4.79
Oreochromis niloticus 201 520 4.52
Cyprinus carpio 190 458 4.27
Cirrhinus cirrhosus 186 128 4.17
Rita rita 143 702 3.22
Tilapias nei 139 845 3.13
Catfishes, hybrids 116 875 2.62
Freshwater fishes general 92 842 2.08
38 other species 547 911 10.20
Total 4 460 851 100.00
Source: Data from FAO FishstatJ.
nei, not identified elsewhere.
370 Freshwater resources
In Bangladesh, aquaculture provides a more lucrative use
ofland and water than alternative activities, and a hectare of
land under aquaculture generates at least 43% higher income
than a hectare of land under crop cultivation (Hasan &
Talukder, 2004). The most important practices and species
cultured are carp polyculture, Pangasius monoculture,
shrimp and giant freshwater prawn Macrobrachium rosen-
bergii and genetically improved farmed tilapia Oreochromis
niloticus (GIFT).
Threats to fisheries production
There are a wide range of potential threats to the inland fishes
and fisheries of Asia. These include hydropower dam develop-
ment, overfishing and other unsustainable exploitation prac-
tices, pollution, land use change, mining, the introduction of
invasive species, and water diversion for agriculture and other
purposes, and habitat alteration by dams. These will interact
and their synergistic effects can be unpredictable, but in general
terms, the outcomes are likely to be continued decline in fishery
yields and in the abundance of large species. Such declines have
been occurring for some time (Dudgeon, 1992) and have been
reviewed at some length in recent publications (Dudgeon, 2011,
2012; Kottelat et al., 2012), and thus, they will be described here
in outline only. It must be stressed that because of the migratory
habits of many Southeast Asian river fishes, and their use of the
floodplain to complete their life cycles, they are especially sensi-
tive to any human intervention that alters the connectivity of the
main channels of rivers and their tributaries and limits access
tofloodplain habitats. Catches for floodplain fisheries are high
mainly because of the richness of the floodplain habitats arising
from the flood pulse effect (Junk et al., 1989). The annual yield
for Bangladeshi floodplains ranges between 50 and 400 kg ha−1
(Craig et al., 2004). Yields from wild rice paddies are
c.120 kg ha−1 year−1 (Hortle et al., 2008). Thus, any changes that
affect the extent and duration of flooding are detrimental to
fishproduction.
Dams
One of the most serious potential threats to fishes and fisheries
in many of the river basins in the region, especially the Mekong
Basin, is the proposed construction of large numbers of hydro-
power dams, both on the river mainstreams (Cronin, 2009;
Baird, 2011; Kottelat et al., 2012) and on their tributaries (Ziv
etal., 2012). China has already built a number of dams on the
upper course of the Mekong (along the Lancang Jiang), and the
effects on fisheries are not known in detail although this section
of the river does not have an extensive floodplain. Dams on the
Lower Mekong mainstream will have very serious consequences
as they have the potential to block migrations or diminish sur-
vival and reproductive success (both upstream and down-
stream), destroy habitat, alter water quality and dramatically
change hydrological patterns both upstream and downstream.
Changes to flow conditions not only affect fish growth but also
have the potential to reduce reproductive success, particularly
the transport of developing early life history stages of fishes to
suitable habitat (Halls et al., 2013b). Changes to hydrological
conditions also have the potential to disrupt reproductive
behaviour because the appropriate recruitment cues (changes in
flow, temperature or turbidity) are lacking. Predicting the
impact of dams on the highly diverse and dynamic fisheries is
made more complex by the limited availability of baseline infor-
mation on drivers of natural variability in fisheries stock, on the
precise effects of dam construction and on the consequences
ofdam operation on flood pulses (Lamberts & Koponen, 2008).
In some cases, dams act as ecological traps. In this context, it is
regrettable that construction of one mainstream dam on the
Lower Mekong in Laos, the Xayaburi Dam, began in early 2012;
most of the estimated US $ 3.5 × 109 cost of this dam will be met
by Thailand, which will be the primary market for the electricity
generated by it. Information on how the local fishery impacts in
Lao PDR will be mitigated or offset and the implications for
fisher livelihoods are lacking thus far.
Ziv et al. (2012) speculate that the 11 mainstream dams in the
Lower Mekong Basin may reduce biomass of migratory species
by 51%. If built, the 78 planned tributary dams could reduce
biomass by a further 19% by 2030, and if all projected dams are
built, the combined loss could exceed 70% of migratory fish bio-
mass. The cumulative impacts of cascades of mainstream dams
have been examined by Halls and Kshatriya (2009). The study
concluded that fish passes would need to be highly efficient to
ensure the persistence of those populations of fishes selected for
study, particularly those with a large body size. Such required
levels of fish pass efficiency have rarely been achieved elsewhere
[see Agostinho et al. (2002) for South American rivers].
Given the dependence on many inhabitants of the Lower
Mekong Basin on fisheries for their livelihood, these studies
provide grounds for grave concern. In Laos and, especially,
Figure3.24.10 Culture cages in Tonle Sap, Mekong. Photograph by Robin
L. Welcomme.
Fisheries of the rivers of Southeast Asia 371
Cambodia, for example, fishes are a major source of animal pro-
tein for humans. If fish stocks are reduced significantly by dams,
then the shortfall in protein will need to be met from alternative
sources, such as pigs or other livestock, and such a shift from
fishing to animal husbandry may be neither environmentally
nor economically sustainable nor even practical for many
communities.
The complexity in predicting the impact of dams on the
highly diverse and dynamic fisheries is exacerbated by the lim-
ited availability baseline information of drivers of natural vari-
ability in fisheries stock, and the precise effects of dam
construction and subsequent operation on flood pulse and con-
sequences thereof (Lamberts & Koponen, 2008) acting, in some
cases, as ecological traps.
The impacts of dams on the Ayeyarwady have yet to be
assessed, in part because the location and operation of a number
that have been proposed have yet to be finalized. Details are
scarce, but a partnership with Chinese engineers involves plans
for at least five dams along the mainstream including the mas-
sive 228 m tall Tasang Dam. The lower Salween remains the only
undammed river in Asia, but preparations for some of a cascade
on 13 dams along the Nujiang, the upper course of the Salween
River in China, began illegally in 2003 and were suspended in
2004 only after the intervention of the Chinese Premier, but
these dams are likely to be constructed within the next few years
as part of Chinas attempt to reduce the intensity of carbon‐
based energy sources (Dudgeon, 2011).
The Chao Phraya Dam started regulating the Chao Phraya
River in 1957 and is mainly a flood control mechanism for regu-
lating flow in the delta. Systematic studies have not been done,
although it is likely to be a barrier to fish migration within the
river and, through reduced flooding of the plain downstream,
has blocked access to the normal breeding, feeding and nursery
grounds of many species; only an estimated 30 of the 190 native
species can reproduce in the river mainstream (Compagno &
Cook, 2005). The major dam in the Ganges–Brahmaputra sys-
tem is the Farakka Barrage on the Ganges River. It has been in
operation since 1975 and has been the source of much dispute
between Bangladesh and India. Its main impact on fisheries has
been the dramatic decline in catches of the anadromous T. ilisha
in the Bangladesh fishery (Payne et al., 2004), and catches of the
species have reduced from 24% in 1985 to 11% in 2010–2011
(DoF, 2012). This decline occurred despite warnings, dating
back decades, of the damaging potential of dams for this species
(Hickling, 1961).
Floodplain drainage and management
In some systems, particularly the Bangladesh rivers and the
Ayeyarwady and increasingly also the Tonle Sap Lake, the
floodplain is modified by bunding to enclose areas for greater
flood control (Craig et al., 2004). Such enclosures limit access
of fishes to their habitual spawning and nursery grounds.
The negative impacts of this can be mitigated to some extent
by appropriate management of sluice gates for the benefit of
both fisheries and agriculture (Sultana & Thompson, 1997;
Halls et al., 2008). In Bangladesh, particularly, floodplain
aquaculture systems within the enclosures are filled with
flood water from adjacent rivers or canals. Water is then
trapped in the system, and the connectivity of waterbodies to
the river is hampered in such a way as to obstruct natural fish
life cycles. Wild species that enter the enclosures may subse-
quently grow too large to escape. While these may be a valu-
able by‐catch, the isolation of large broodfish precludes free
recruitment of fishes into the capture fishery, and thus, there
is a need for careful control of natural reproduction (Blake &
Barr, 2005). Craig et al. (2004) estimate that some 5.74 × 106 ha
of the Bangladesh floodplain is liable to be under flood con-
trols of this type, resulting in the loss of c. 151 × 103 t of fishes.
Pollution
Many rivers in the region suffer from low water quality. Pollution
from industrial sources and eutrophication from untreated
urban discharges are common, and mining effluents may also
be locally important. Smaller tributaries that lack the dilution
potential of larger rivers are especially affected to the extent that
fish stocks can be drastically reduced and fishes may even be
entirely eliminated under low‐flow conditions in the dry season.
Even in some large rivers, such as the Ganges, however, much of
the mainstream is in very poor condition, and fish populations,
especially major carps, have been greatly reduced (Natarajan,
1989). The Chao Phraya is also highly polluted in places, and
increasing industrial activity on other rivers is reducing water
quality especially adjacent to major urban centres. Thus far, pol-
lution does not pose a major threat to the Mekong, although
some local impacts from aquaculture and soil acidification have
occurred in the delta.
Water abstractions
Water abstraction for irrigated agriculture is common in some
rivers. In the Ganges, in particular, several of the major tributar-
ies have very reduced flows. Water abstraction can also be detri-
mental to the duration and intensity of flooding of the riparian
wetlands impacting on fish reproduction and growth (Shankar
et al., 2005).
Introduction of nonnative species
The FAO Database for Introduced Aquatic Species (DIAS)
(http://www.fao.org/fishery/topic/14786/en) lists many spe-
cies as having been introduced into the countries of the region,
and a summary of major invasive freshwater species in
Southeast Asia is given by Dudgeon (2012). Reasons for the
introduction vary, but most of the fishes that are listed are
aquaculture species. Many other small species, often undocu-
mented, are for the aquarium fish trade and are reared in
Vietnam and Thailand for export elsewhere. Some invasive
species originate from other river basins in the region, such as
the Indian major carps or the Chinese cyprinids. Others are
from as far afield as Africa or Latin America. Many of the
372 Freshwater resources
species were initially introduced into Thailand and dispersed
to other countries from there. Despite the high number of spe-
cies introduced, very few appear to have been successful in
establishing wild populations, although there have been wide-
spread escapes from aquaculture installations. Even fewer con-
stitute a nuisance (Welcomme & Vidthayanon, 2003); although
there are some exceptions, and invasive species such as South
American loricariids and mosquito fish Gambusia affinis have
the potential to impact native biota (Dudgeon, 2012). By con-
trast, tilapias, in particular Oreochromis niloticus, O. mossam-
bicus and the GIFT tilapia, which form a mainstay of pond
aquaculture and stocked lake fisheries, have brought liveli-
hood benefits for humans, although the ecological effects of
widely established feral populations have not been investigated
in the region.
Climate change
Most of the major rivers of Southeast Asia are fed by precipita-
tion and glacial melt from the Tibetan Plateau and Himalaya,
which determines their discharge patterns. As climate change
has the capacity to alter both the persistence of glaciers and the
rainfall patterns, long‐term effects on the timing and duration
of river flooding can be anticipated (Xu et al., 2009). Potential
scenarios and consequences for the Mekong have been analysed
by Schipper et al. (2010) and Chu Thai Hoanh et al. (2010).
Most climate change scenarios for Southeast Asia are that
extreme flow events will become more common and the wet‐
season floods will become more intense and dry‐season drought
will lengthen (Dudgeon, 2012). Other specific projections for
the Mekong include elevated mean annual temperatures and
greater duration of warm periods, as well as an increase in
annual precipitation and greater river flows (Bezuijen, 2011).
Monsoonal flows in the Salween also are expected to increase
over the latter part of this century (Xu et al., 2009). Allison et al.
(2009) ranked Vietnam and Cambodia as two of the most
vulnerable countries in tropical Asia to suffer the impacts of
climate change on their fisheries along with Bangladesh,
Pakistan and Yemen. The extent to which fishes will adapt to
such changes and the implications for fisheries remain largely
speculative. Higher temperatures and more climatic extremes
will lead to adaptation by humans, which will involve increases
in water use and abstraction for agriculture and dam building
(for water storage, flood protection and hydropower genera-
tion) leading to additional fragmentation of rivers. None of
these adaptation measures are likely to augur well for fishes or
fisheries, and adaptation to climate change may well have greater
impact on fishes than the change itself. Modelling studies
described by Mainuddin et al. (2010) concluded that climate
change impacts on fisheries yield from the Mekong River would
not be detectable because of the significant natural variation in
hydrological conditions in the system that would overshadow
climate change‐related effects.
One of the most severe future impacts of climate change is
the anticipated rise in sea levels. This would place several of
thelow‐lying deltaic areas at risk of permanent flooding, par-
ticularly parts of Bangladesh and the Chao Phraya delta. Delta
shrinkage and reductions in the rate of aggradation (due to
sediment trapping by upstream dams) have already been
reported for the Chao Phraya (Thailand) and Mekong (Syvitski
et al., 2009). The extension of saltwater intrusion in freshwater
or brackish delta waters will further alter habitats.
Management activities
Two main types of fishery management are practised across
Southeast Asia: leasable fisheries and open fisheries. An example
of these management systems as practiced in Myanmar is given
by FAO (2003). Leasable fisheries (also known as fisheries
Figure3.24.2) consist in the assignment of sections of the envi-
ronment, usually floodplain waterbodies or particular main
channel fishing locations, to individual fishers or groups of fish-
ers. The assignment is usually by auction which confers on the
successful bidder exclusive exploitation rights for a specified
time. This system, while efficient at gathering revenue, may
result in heavy overexploitation if the lease period is too short.
Longer lease periods result in better management and conserva-
tion, such as the Bangladesh system for stocked floodplain
enclosures. Open fisheries are freely accessible to all, in princi-
ple, although local institutions may limit access to certain groups,
particularly under current trends towards co‐management. In
many places, leasable and open fisheries coexist, and the man-
agement type depends on the nature of the fishery (commercial
v. subsistence) or the season.
Despite some trends to liberalization of inland fisheries in
Southeast Asian countries, the Fisheries Departments remain in
control of regulation and enforcement. This they do by manag-
ing the leasing process and by setting regulations governing
thetype and specifications of gear used, setting closed seasons
and areas and regulating markets. They also are responsible
for running hatcheries to support stocking programmes into
reservoirs and rice fields. Community‐based management and
other forms of co‐management are gradually gaining momen-
tum in the region, replacing large private leases as in Cambodia.
In the Tonle Sap floodplain, the leases play an important but
largely unintended role in biodiversity conservation (Lamberts,
2008b).
Most of the rivers of the region flow through more than one
country. Some international river organizations exist to harmo-
nize fisheries interests between the various countries. In the
Mekong in particular, the MRC brings ministerial represen-
tative of Cambodia, Thailand, Laos and Vietnam together,
although part of the basin is in Myanmar and China. It provides
a forum for discussion of fisheries matters, sustainable develop-
ment of the river, and the MRC is responsible for commission-
ing much basic research. The MRC does not have a direct role
in national fisheries management, however, and its advice is
sometimes ambiguous and is frequently ignored when activities
Fisheries of the rivers of Southeast Asia 373
such as dam building are at issue. The recent commencement of
construction of the Xayaburi Dam by Lao PDR was in the face
of strenuous objections by Vietnamese and Cambodian repre-
sentatives at the MRC, and a declaration by the MRC secretariat
that a 10 year moratorium on dam building on the river main-
stream would be desirable to allow investigation of the potential
environmental impacts of proposed dams (Dudgeon, 2011).
The fisheries sector in Bangladesh has multiple stakeholders,
of which government agencies play a vital role in terms of
aquatic ecological management, administration and power rela-
tions. The Ministries of Fisheries and Livestock (MoFL), Land
(MoL) and Youth (MoY) play crucial roles in natural resources
management. The MoL owns all lands and waterbodies in
Bangladesh, while the Department of Fisheries (DoF) is the only
implementing agency under the key ministry, MoFL, and
authorization must come to MoFL from MoL for management
of aquatic ecology (Mustafa & Brooks, 2009). The management
of these resources, based upon a combination of short‐term
leased access to waterbodies or supported by a combination of
conventional management interventions, often excludes the
poorest fishers and encourages leaseholders to effectively ‘mine
resources at non‐sustainable levels of exploitation.
Conclusions
The fisheries of the lower parts of the rivers of the Southeast
Asian region are among the most productive and intensely
exploited in the world. The fisheries are extremely complex
socially and politically, and there is a wide range of gears and
approaches to catching, marketing and regulating the fisheries.
As well as the high fishing pressure, the inland water environ-
ments with their many river channels and extensive riparian
wetlands are under considerable pressure for a range of other
human activities. Of these, dam building for hydropower, pollu-
tion, water abstractions, deforestation and drainage and modifi-
cation of floodplains are increasingly serious and, together with
overexploitation, pose threats to the continued productivity and
sustainability of catches. As human populations continue to
grow and economic development proceeds apace over much of
the region, pollution, environmental degradation and intensity
of threat to river fishes can be expected to increase. Without
appropriate national, regional and international mechanisms to
ensure that greater care is taken of the aquatic environments, a
decline in the important fisheries yields and significant changes
in fish community structure are to be expected. To a certain
extent, such declines may be offset by intensifying management
of sectors of the fishery and by a wider diffusion of aquaculture.
The extent of endangerment of megafishes in the Mekong River,
which is still relatively pristine and unpolluted, could be seen as
an early sign of things to come. Moreover, national imperatives
that prioritize economic development over environmental
protection, exemplified by plans for dam cascades along the
region’s major rivers, do not inspire confidence that the wise
course of careful long‐term management for conservation of
river fishes will prevail over short‐term considerations.
A wider appreciation of the value of river fishes (for instance,
their consumptive value as food) may help to ensure their con-
servation, as would greater consideration of the role that fishes
and fisheries play in sustaining livelihoods and the logistical
difficulties of replacing that role with other sources of animal
protein. A greater awareness of the importance of fishes as com-
ponents of healthy functioning ecosystems that provide irre-
placeable ‘free’ services for humans (for instance, clean, fresh
water) could enhance efforts to conserve and manage river fishes,
and education of citizens and decision‐makers will be key to such
initiatives. The identification of flagship species could be a first
step to enhance public consciousness of river fishes, and such an
approach has been used for the tambaqui Colossoma macropo-
mum, a giant characid that has become a symbol of the need
to protect Amazonian floodplain habitats (Araujo‐Lima &
Goulding, 1997). As it is an important food fish, C. macropomum
embodies the problems that need to be resolved in order to sus-
tainably manage an entire multi‐species fishery in the Amazon.
Identification of suitable candidate species from Southeast Asia
should not be too challenging: for instance, the giant P. gigas or P.
jullieni could be flagship species for the Mekong. This will only
be an initial stage in a more wide‐ ranging process that must
involve innovative partnerships thatcombine the efforts of, for
example, fishery scientists, conservation organizations, citizens’
groups and those charged with managing natural resources in
national governments. It will also require decision‐makers to
commit to action that will sustainably manage an irreplaceable
natural resource. A particular responsibility of fishery scientists
is to ensure that the research they do contributes to management
of fish stocks and the collection of data that allows determination
of trajectories of catch composition and population change so
that the effectiveness of management measures can be assessed.
More must be done to identify and undertake the research neces-
sary to support management strategies that reconcile human use
of Southeast Asian rivers with the preservation of ecosystem
integrity and maintenance of fish stocks.
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Abstract JUNK, W. J., P. B. BAYLEY, AND R. E. SPARKS, 1989. The flood pulse concept in river-floodplain systems, p. 110-127. In D. P. Dodge [ed.] Proceedings of the International Large River Symposium. Can. Spec. Publ. Fish. Aquat. Sci. 106. The principal driving force responsible for the existence, productivity, and interactions of the major biota in river—floodplain systems is the flood pulse. A spectrum of geomorphological and hydrological conditions produces flood pulses, which range from unpredictable to predictable and from short to long duration. Short and generally unpredictable pulses occur in low-order streams or heavily modified systems with floodplains that have been leveed and drained by man. Because low-order stream pulses are brief and unpredictable, organisms have limited adaptations for directly utilizing the aquatic/terrestrial transition zone (ATTZ), although aquatic organisms benefit indirectly from transport of resources into the lotic environment. Conversely, a predictable pulse of long duration engenders organismic • adaptations and strategies that efficiently utilize attributes of the ATTZ. This pulse is coupled with a dynamic edge effect, which extends a "moving littoral" throughout the ATTZ. The moving littoral prevents prolonged stagnation and allows rapid recycling of organic matter and nutrients, thereby resulting in high productivity. Primary production associated with the ATTZ is much higher than that of permanent water bodies in unmodified systems. Fish yields and production are strongly related to the extent of accessible floodplain, whereas the main river is used as a migration route by most of the fishes. In temperate regions, light and/or temperature variations may modify the effects of the pulse, and anthropogenic influences on the flood pulse or floodplain frequently limit production. A local floodplain, however, can develop by sedimentation in a river stretch modified by a low head dam. Borders of slowly flowing rivers turn into floodplain habitats, becoming separated from the main channel by levées. The flood pulse is a "batch" process and is distinct from concepts that emphasize the continuous processes in flowing water environments, such as the river continuum concept. Flooclplains are distinct because they do not depend on upstream processing inefficiencies of organic matter, although their nutrient pool is influenced by periodic lateral exchange of water and sediments with the main channel. The pulse concept is distinct because the position of a floodplain within the river network is not a primary determinant of the processes that occur. The pulse concept requires an approach other than the traditional limnological paradigms used in lotic or lentic systems. Résumé JUNK, W. J., P. B. BAYLEY, AND R. E. SPARKS. 1989. The flood pulse concept in river-floodplain systems, p. 110-127. In D. P. Dodge [cd.] Proceedings of the International Large River Symposium. Can. Spec. Publ. Fish. Aquat. Sci . 106. Les inondations occasionnées par la crue des eaux dans les systèmes cours d'eau-plaines inondables constituent le principal facteur qui détermine la nature et la productivité du biote dominant de même que les interactions existant entre les organismes biotiques et entre ceux-ci et leur environnement. Ces crues passagères, dont la durée et la prévisibilité sont variables, sont produites par un ensemble de facteurs géomorphologiques et hydrologiques. Les crues de courte durée, généralement imprévisibles, surviennent dans les réseaux hydrographiques peu ramifiées ou dans les réseaux qui ont connu des transformations importantes suite à l'endiguement et au drainage des plaines inondables par l'homme. Comme les crues survenant dans les réseaux hydrographiques d'ordre inférieur sont brèves et imprévisibles, les adaptations des organismes vivants sont limitées en ce qui a trait à l'exploitation des ressources de la zone de transition existant entre le milieu aquatique et le milieu terrestre (ATTZ), bien que les organismes aquatiques profitent indirectement des éléments transportés dans le milieu lotique. Inversement, une crue prévisible de longue durée favorise le développement d'adaptations et de stratégies qui permettent aux organismes d'exploiter efficacement 1 'ATTZ. Une telle crue s'accompagne d'un effet de bordure dynamique qui fait en sorte que l'ATTZ devient un « littoral mobile'<. Dans ces circonstances, il n'y a pas de stagnation prolongée et le recyclage de la matière organique et des substances nutritives se fait rapidement, ce qui donne lieu à une productivité élevée. La production primaire dans l'ATTZ est beaucoup plus élevée que celle des masses d'eau permanentes dans les réseaux hydrographiques non modifiés. Le rendement et la production de poissons sont étroitement reliés à l'étendue de la plaine inondable, tandis que le cours normal de la rivière est utilisé comme voie de migration par la plupart des poissons.
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