DAMS, CHANGES IN SEDIMENT LOAD
AND IMPACT ON FISH RESOURCES
IN THE MEKONG
approach and way forward
BARAN Eric, GUERIN Eric
Project
“A Climate Resilient Mekong: Maintaining the Flows that Nourish Life”
led by the Natural Heritage Institute
June 2012
Table of content
INTRODUCTION ............................................................................................................................... 3
RELEVANT ECOLOGICAL ZONES....................................................................................................... 6
1.1
Initial focus zones ............................................................................................................ 6
1.2
Revised focus zones ........................................................................................................ 7
2 FISH STOCK, FISH PRODUCTION AND FISH CATCHES ............................................................... 9
3 THE MULTIPLE DRIVERS OF THE FISH PRODUCTION .............................................................. 11
4 SIGNIFICANT SUB-BASINS AND DAMS .................................................................................... 12
4.1
Most significant sub-basins ........................................................................................... 13
4.2
Most influential dams ................................................................................................... 14
5 CONCLUSION: PUTTING THE PARTS TOGETHER ..................................................................... 15
6 BIBLIOGRAPHY ........................................................................................................................ 18
Citation
Baran E., Guerin E. 2012. Dams, changes in sediment load and impact on fish resources in the
Mekong: approach and way forward. Report for the Project “A climate resilient mekong:
maintaining the flows that nourish life” led by the Natural Heritage Institute. WorldFish Center,
Phnom Penh, Cambodia. 19 pp.
Acknowledgements
We sincerely thank Mrs Saray Samadee for her assistance during the writing of this report.
2
1
INTRODUCTION
The current report is the last of a series of five papers aimed at assessing the impact on fish
resources of changes in sediment load resulting from dam construction in the Mekong. The
previous studies focussed on the interactions between sediments and fish in tropical rivers and
in the Mekong (Report 1), on the impact of flushing on fishes and the best flushing practices to
minimize these impacts (Report 2), on fish productivity in 32 Mekong hotspots (Report 3), and
on the influence of sediment load on Mekong floodplains and coastal fishery resources
(Report 4). The objective of the current report was initially to assess the impact of predicted
changes in sediment loads due to dam operation on fish resources in 5 main zones of the
Mekong Basin: i) alluvial floodplains; ii) wetlands complexes; iii) deep pools in the river channel;
iv) the delta, and v) and the near-shore fishery.
The five steps of this assessment and their connections are summarized in Figure 1.
Figure 1: Steps in the assessment of the impact on Mekong fish resources of changes in sediment load
due to dam operation.
The first steps of the assessment led to the following conclusions:
Report 1: Interactions between sediments and fish in tropical rivers and in the Mekong:
The combined reduction in flood pulse and sediment load is expected to have a synergetic and
negative effect on the overall productivity downstream of the river. This process of nutrient
rarefaction modifies food web structures and reduces fisheries resources. Sediments load
reduction is also expected to influence fishes’ biological functions, in particular respiration,
nutrition, reproduction and migration, as well as their habitat. However important knowledge
gaps remain, in particular in the Mekong Basin. For instance, the relative role of nutrients and
of turbidity vis-à-vis primary production must be clarified; there is little information in the
literature about the relationship between tropical fish reproduction and the impact of a
reduced sediment load; the role of sediments as a migration trigger is unclear; so is the
3
importance of nutrient transfer through fish migration. Importantly, the coupling between
sediments and nutrients and the role of the fraction of nutrients that are independent from
sediments are two important points that remain to be addressed.
Report 2: Impact of flushing on fishes and best flushing practices to minimize impacts:
Dams in the Mekong Basin should integrate flushing and/or sluicing of sediment to prevent the
reservoir from filling with sediment. However, sediment flushing constitutes a risk for the
downstream ecosystem and results in particular in three main impacts on fish:
- physical impacts (fish gill clogging, changes in riverine habitats and changes in
downstream river temperature);
- chemical impacts (decreased dissolved oxygen levels, chemical contamination), and
- biological impacts (loss of migration or spawning triggers, reduced food abundance,
reduced ability to feed, impact on egg development and increased vulnerability to
diseases).
It is possible to mitigate these impacts to some extent by applying certain flushing procedures
aimed at controlling sediment release and downstream sediment concentration.
Report 3: Features of high fish productivity in the Mekong basin:
Our analysis showed that the 32 environmental hotspots identified by the MRC do not
constitute a good reference in view of assessing the impact of sediment load modification on
fish biodiversity and productivity. In particular i) the large range of hotspot sizes (from 400 to
540,000 ha) results in a juxtaposition of systems of different nature, and ii) several hotspots
were selected from an ornithological perspective, with no data available on fish or fish
productivity. An assessment of the fish productivity and impact of sediment reduction in
different zones in the Mekong would require a typology of the different ecozones, an estimate
of the ratio of migratory and non-migratory fish in each zone and a fish stock estimate for each
zone. The approach and model developed by Ziv et al. (2012) offers perspectives for such
assessment.
Report 4: Influence of sediment load on Mekong floodplain and near shore fishery resources:
The Mekong floodplains cover up to 3.9 million hectares and produce more than one million
tonnes of fish each year. Each year the Tonle Sap Lake retains 4.3 to 5.7 million tonnes of
suspended solids and receives 21,500 tonnes of bioavailable phosphorus. Sediment trapping by
dams is expected to reduce that phosphorus input by 10,000 to 18,000 tonnes each year.
Overall a 36% decline in total fish biomass is expected if the Mekong sediment input is reduced
by 80%. However i) the absence of any fish stock assessment is a major impediment to a proper
quantification of the relationship between fish and sediment, and ii) correlations between fish
and sediment do not reflect a number of other factors that also influence the fish production.
In a complex system of multiple drivers, we recommend further development of the BayFish Tonle Sap model of fish production; this multifactor model would allow assessing the relative
weight of nutrients in relation to other drivers such as flooding patterns, floodplain habitats,
connectivity or fishing pressure.
As for the coastal zone, The Mekong River contributes about about 100 million tonnes of
sediments and 16,000 – 17,000 tonnes of attached nutrients to coastal Vietnam, which
4
supports productive fisheries harvesting 0.5 to 0.7 million tonnes of fish per year. In general
river discharge favors coastal fish production and fisheries; however the dynamics of outflows
are as important as their volume. Depending on dam construction scenarios, Mekong outflows
are expected to vary between -8% and +26% in the dry season, and to be almost unchanged in
the rainy season. The impact of such flow variation is not clearly quantified. The role of
nutrients in sustaining primary production and fish production in the coastal zone is well
established; yet that role is under the influence of numerous other modulating factors. In
particular the response of coastal ecosystems to river nutrient reduction varies depending on
local characteristics. River flows also provide carbon inputs to the coastal zone, but the relative
role of this riverine carbon versus that of the coastal vegetation is unclear; in fact it is a
relationship between coastal fish production and Tonle Sap outflow –rather than Mekong
outflow- that is pointed out. Last, the relationship between sediment and fish catches also
depends on fisheries; it is blurred by a variable fishing effort and its analysis in the Mekong is
hampered by a lack of precise catch and effort data in Vietnam. Overall, data currently available
do not allow assessing the possible impact of nutrient load reduction on coastal fish resources.
Assessing the impact of changes in sediment load in the Mekong Basin requires a good
knowledge of the fish resources of each focus zone and their dependency to sediment loads, in
combination with a quantification of predicted changes for each zone (retention, but also
expected flushing events). However, the four first reports of the present series have
highlighted important knowledge gaps currently hampering the assessment of impact of
sediment changes on fish production in the Mekong. The present report will subsequently
focus on the methodological framework and approach required for a detailed and accurate
assessment, once enough detailed data become available. The model described aims to
integrate sediments in a more general perspective linking dam development to fish resources
via the drivers of fish production.
Figure 2: Dams influence fish resources through the multiple drivers of fish production
In the first section of the present report we review the proposed focus zones, and propose
more suitable alternative focus zones for future studies. We then detail the difference between
fish stock subject to changes in sediment load and fish catches resulting from a mixture of
fishing effort and fish stock. In the third part, the main drivers of fish production are
summarized; fish production is influenced sediment load and by these other drivers as well. In
the fourth part, the sub-basins and dams most likely to influence fish production are identified.
Ultimately the above component are brought together into a framework detailing how the
detailed analysis of the influence of sediments on fish production should be approached.
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2
RELEVANT ECOLOGICAL ZONES
Future assessments of the relationship between sediments and Mekong fish resources will have
to focus on relevant ecological zones; we detail below the zones initially propose and review
their relevance.
2.1
Initial focus zones
In the Terms of Reference of the current study, the analysis of the impact on fish resources of
predicted changes in sediment loads was to focus on 5 main zones of the Mekong Basin:
i) alluvial floodplains; ii) wetlands complexes; iii) deep pools in the river channel; iv) the delta,
and v) the near-shore fishery. Actually, these five zones are not optimal for the following
reasons:
- lack of consistency between zones: while the 430 known Mekong deep pools consist of
small to medium-size habitats (a few hectares each, Poulsen et al. 2002a, Chan Sokheng
et al. 2008, Conlan et al. 2008), the other zones constitute large bio-geographical areas
or ecosystems (e.g. 39,000 km2 for the Mekong Delta);
- overlap between zones: the “wetlands complexes” encompass, by definition1, the
Mekong delta as well as the alluvial floodplains, the latter being themselves part of the
delta;
- information gap in two of the proposed zones: information on fish resources is quasiabsent in wetlands (understood as marshes and ponds in plains) as detailed in Report 4,
and fish biology in deep pools has only been addressed, to our knowledge, through two
studies (Baran et al. 2005 and Viravong et al. 2006); these two studies respectively used
gill net catch data and hydroacoustic sampling to give preliminary insights about species
composition and fish biomass in some of these deep pools, but they do not allow
drawing generic conclusions about deep pools as a fish habitat;
- non-integration of the migratory dimension in zone selection: as mentioned in Report 3,
the Mekong Basin is a global system in which migrations are an essential component of
fish ecology and productivity (review of 28 Mekong fish migration studies in Baran
2006). An important fraction of the fishes caught in one area consists of migratory fishes
partly grown in another area. In the Mekong, migratory fishes represent at least 39% of
the overall fish biomass (Baran 2010), which leads to the definition of fish production
zones distinguishing the generation of juveniles from the harvest of adults (Figure 3).
According to the “Ramsar international wetland conservation treaty”, wetlands are defined as “areas of marsh,
fen, peat land or water, whether natural or artificial, permanent or temporary, with water that is static or flowing,
fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six metres."
1
6
Figure 3: Recruitment areas vs. harvesting areas for migratory fishes. Source: Baran 2010
2.2
Revised focus zones
In a fish biology perspective we propose a different zoning, integrating main habitats, migration
issues and data availability. Given the current level of knowledge and data available, a detailed
study of relationships between sediments and fish production in the Mekong should focus in
priority on three main ecological zones:
- the coastal zone, corresponding to the near shore area influenced by the Mekong
discharge and identified as the Mekong plume;
- the downstream zone of the Lower Mekong Basin, including all the Basin floodplains
(Mekong delta, Cambodia lowlands and Tonle Sap) and limited upstream by the Khone
Falls at the border between Cambodia and Laos;
- the upstream zone of the Lower Mekong Basin, corresponding to the Mekong section
between Khone Falls and the Chinese border (or alternatively the Jinhong dam).
7
This zoning reflects to a large extent the fish migration zoning proposed by Poulsen et al.
(2002b) and the MRC ecological zones (MRC 2005), but differs from all previous zoning
attempts by:
- the inclusion of a coastal zone as a part of the Mekong area of influence (Report 4);
- one single upstream zone from Khone Falls to Jinglinqiao in Yunnan. This integration
reflects i) the fact that the upper limit of the Lower Mekong (i.e. the Chinese border) is a
political and administrative boundary but not a hydrological or ecological one; ii) the
findings of Bin Kang et al. 2009 and Kang Bin et al 2009 regarding the similarity of Lower
and Upper Mekong fish faunas up to Jinglinqiao; and iii) the fact that the Vientiane –
Khone Falls segment produces around one million tonnes of fish a year while the
Chinese border – Vientiane segment generates 60,000 tonnes only and thus does not
justify a special case from a fish production perspective.
Figure 4: The three zones proposed for the study of the relationships between sediments and fish
production
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3
FISH STOCK, FISH PRODUCTION AND FISH CATCHES
Future assessments of the relationship between sediments and Mekong fish resources will have
to focus, in addition to sediment loads, on relevant descriptors of the fish resource and drivers
of the fish production, which we detail below.
In absence of any Mekong fish stock assessment, the relationship between sediments and fish
production is to be assessed using fish yield as a proxy of fish production. However the
variability of the fishing intensity introduces a substantial bias in the relationship, as detailed
below.
Figure 5: Fish resources, biological components and data availability
Fish yield = Fish stock x [Fishing effort and Fishing efficiency]
Fishing effort
The fishing effort represents a combination of number of fishing boats, surface area of nets,
time spent using each gear, etc. Neither the number of gears not the time spent fishing have
been monitored in the Mekong.
Fishing efficiency
That complex factor is permanently evolving thanks to new technologies aimed at maximizing
the catch. This is illustrated by the spreading of sonar systems to locate fish shoals on coastal
fishing boats in Vietnam, or the use of electrified bottom chains targeting flat fish on trawls in
the Tonle Sap. The same boat newly equipped with such devices will harvest more even though
his recorded fishing effort (e.g. gear category and time spent fishing) has not formally changed.
Fishing efficiency in the Mekong has never been studied nor monitored.
Fish yield
The fish yield in the upstream zone of the Lower Mekong – i.e. between Khone Falls and the
Chinese border- ranges between 0.9 and 1.1 million tonnes per year; in the downstream zone
of the LMB –i.e. between the river mouth and Khone Falls- the production amounts to 1.2 -1.5
million tonnes per year (Barlow et al. 2008). Provincial statistics reviewed in 2010 (Baran 2010)
indicate that the Mekong coastal fishery produces 0.5 – 0.7 million tonnes of fish each year. The
extent of the Mekong plume shows that this is to be complemented with the catch statistics
9
from Ba Ria – Vung Tau, Binh Thuan, Kampot and Sihanoukville provinces, although the latter
are not known at this point (Report 4). Overall, the Mekong fish catches (i.e. freshwater plus
coastal yields) range between [0.9 +1.2 + 0.5] – [1.1 + 1.5 + 0.7] = 2.6 – 3.3 million tonnes per
year.
Importantly, no monitoring program allows assessing the annual variability of these catches.
Furthermore, the fish catch is not an accurate proxy of the fish stock, for two main reasons:
- the catch results from a combination of fish stock and fishing intensity; an increase in
fish yields may reflect a change in the fishery rather than in the stock, to the extent that
the overall catch can keep growing while the stock decreases (case of early overfishing).
In fact, the fishing effort has never been accounted for in the Mekong, resulting in an
undervaluation of the fish stock in areas of low fishing pressure such as Laos;
- as detailed in section 1.2, there is a disconnect for long distance migratory fishes
between upstream breeding zones where juveniles are generated but not caught and
downstream feeding zones where adults are caught but not generated. Thus, an
estimate of the standing stock per zone based on catch data would result in an
undervaluation of the ecological role of upstream breeding zones in the sustainability of
the overall production.
One might argue that if fishery data are insufficient, the standing fish stock can be
approximated by an assessment of recruitment, growth and mortality:
Fish stock = Fish recruitment + Fish growth - Fish mortality
Unfortunately that alternative approach would be subject to information gaps even larger than
in the case of the fishing-based approach, as detailed below.
Fish recruitment
With more than 791 fish species identified but quasi-undocumented in term of their
recruitment dynamics and 165 long-distance migrants representing almost 40% of the total
yield but whose recruitment area is geographically disconnected from their main feeding and
growth zone, fish recruitment in the Mekong Basin can be considered unknown.
Fish growth
The growth of some native species cultured in ponds or cages is documented, but in general the
growth of Mekong fish species in the wild, most probably dependent on nutriment loads
(cf. Report 1) and on flood pulse (Reports 1 and 4), is actually now known.
Fish mortality
This parameter is a combination of natural mortality (unknown) and man-driven mortality due
to fishing and environmental changes (e.g. pollution in floodplains, mortality at dam passage,
etc). The fishing mortality is the only component for which a loose proxy exists (i.e. fish catch
data).
The above factors and corresponding levels of knowledge are summarised in Figure 6.
10
Figure 6: Fish stock assessment based on fishery yield or on biological parameters.
In conclusion, the current absence of information on fisheries descriptors or on biological
parameters of the fish populations does not allow characterizing the fish production as needed,
and therefore does not allow, at this point, assessing the relationship between the Mekong fish
production and changes in sediment loads.
11
4
THE MULTIPLE DRIVERS OF THE FISH PRODUCTION
The change in sediment loads is only one among several dam-related factors which can affect
fish resources, dam development itself being one among several human factors affecting fish.
Dam development will reduce the downstream sediment/nutrient load (which will alter the
river geomorphology, habitats and productivity), but will also reduce the flood pulse, alter
water quality and obstruct fish migrations (McCartney et al. 2000, Marmulla 2001). Nutrients,
flood pulse, migrations and water quality can be considered as the four most important factors
influencing fish production in tropical systems (Junk 1999, Welcomme 2001, Arthington et al.
2004). Furthermore dam development is not the only anthropic factor influencing Mekong fish
resources. Thus:
- massive sediment extraction (estimated to reach 27,000,000 m3 = 43,2 millions tonnes a
year, Bravard and Goichot 2012) modifies the river bed and riverine habitats;
- agricultural development in the Mekong results in intensive use of fertilizers (e.g.
378 kilos of NPK per hectare and per year in average in the delta over 2 million hectares
of rice fields, Huan et al. 2005); these nutrients partly contribute to the nutrient load of
the Mekong outflows (225,000 tonnes of nitrogen and 37,000 tonnes of phosphorus
from agricultural fertilizers run off into the Mekong rivers each year; MRC 2008);
- urban effluents negatively influence water quality and aquatic biodiversity (but not
necessarily fish productivity), in particular downstream or large cities such as Siem Reap,
Phnom Penh or in the delta (150-170,000 tonnes of organic effluents discharged
annually from urban areas, MRC 2008).
The interactions between the multiple drivers of fish production are illustrated in Figure 7.
Figure 7: Main factors influencing fish resources in the 3 focus zones. Boxes corresponding to the initial
focus of the current review are Dams and Sediments; the other boxes correspond to factors also
influencing fish production in the three main focus zones.
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5
5.1
SIGNIFICANT SUB-BASINS AND DAMS
Most significant sub-basins
To our knowledge, only one published2 study has tried to assess the relative importance of the
different Mekong sub-basins to fish production. Ziv et al. (2012) reviewed the role of floodplain
fish versus that of migratory fishes in total fish production, compiled 70 species distribution lists
to model the carrying capacity in migratory fishes of each sub-basin, and plotted the results
(Figure 8). This approach highlights the biological importance of:
- the 3S sub-basin, being very rich in biodiversity and accessible to fishes feeding in
lowland floodplains (Baran et al. 2011);
- the Mun/Chi sub-basin, rich in biodiversity and also the largest Mekong sub-basin
(Poulsen et al. 2002b; Baran 2010); this watershed represents the Thailand share of the
LMB but is already heavily blocked by multiple irrigation dams;
- the multiple Lao tributaries used as breeding sites for a number of migratory species
feeding in the lowlands (Poulsen et al. 2002b); they can be collectively identified as the
Lao sub-basins.
Figure 8: Dry-season carrying capacity of migratory fishes. Source: Ziv et al. 2012.
The results of the MRC IBFM (Integrated Basin Flow Management) project have never been publicly released and
the current BDP works on the significance of tributaries are in progress.
2
13
In addition to the above sub-basins or clusters, China plays an important role in terms of
sediment supply and sediment retention (Kummu and Varis 2007, Hongjuan Zhai et al. 2010).
This leads to recommending Srepok, Sesan, Sekong, Mun/Chi, Lao tributaries and the Upper
Mekong as most significant sub-basins from a fish perspective.
5.2
Most influential dams
The impact of a given dam on fish resources depends on the characteristics of the dam but also
on its location and on the ecological significance of the tributary river it is built on. A recent
analysis (Thomas 2012) shows that the 3S Rivers (i.e. Sekong, Sre Pok and Sesan Rivers) are
among the top five rivers for their contribution to the sediment load of the Lower Mekong
Basin (Figure 9). Multiple studies also identified the Mekong mainstream between Khone Falls
and Kratie as an important fish migration corridor between downstream feeding zones and
upstream breeding sites (Sao Leang and Dom Saveun 1955, Roberts and Baird 1995, Poulsen et
al. 2002b, Timmins 2006, Bezuijen et al. 2008). The most influential dams corresponding to
these ecologically significant river stretches are detailed below.
Figure 9: Contribution (in percent) of Mekong sub-basins to the sediment load in the Lower Mekong
Basin.
i.
ii.
Lower Sesan 2 dam (LSS2): located just downstream of the confluence between
Sesan and Srepok Rivers, this single dam will alter 16.4% of the sediment flow
(Thomas 2012) and 9.3% of the fish biomass (Ziv et al. 2012) of the Lower Mekong
Basin;
Stung Treng dam: located just upstream of the confluence with the 3S, this
mainstream dam will block access to 646,000 km2 or 79.9% of the Mekong Basin to
migratory fishes present of lowland floodplains in the wet season (Baran 2010), and
14
iii.
will alter the sediment flow of sub-basins contributing at least 39.9% of the total
sediment load in the LMB; however this dam will not alter fish migrations to and
sediment flows from the 3S sub-basin;
Sambor dam: located on the mainstream near Kratie, downstream of the confluence
with the 3S, the Sambor dam will block access of migratory fishes to 81.3% of the
Mekong basin (i.e. a potential impact on 39% of the Mekong fish biomass) and will
alter the sediment flow of sub-basins contributing at least 66.2% of the total LMB
sediment load.
Thus, the analysis of the joint influence of dams on sediment and on fish resources should be
initially focussed on the components of the system whose role or influence is clear and distinct:
Upstream Mekong, Mun/Chi, Laos, Sekong, Sesan, Srepok sub-basins, and Lower Sesan 2, Stung
Treng and Sambor dams (Figure 10).
Figure 10: Sub-basins important to fish production and dams most influential to fishes
15
6
CONCLUSION: PUTTING THE PARTS TOGETHER
The above sections lead to the conclusion that identifying the impact of sediment retention by
dams on the fish stock and its production requires an approach covering:
i. most significant sub-basins in terms of sediments and fish resources;
ii. dams most influential to sediments and fish.
For each of these sub-basins and dams, analyses should focus on:
iii. changes in main drivers of the fish production, and
iv. monitoring of the fish yield in each focus zone
v. monitoring of the fishing effort;
Ultimately, it is by combining fish yield and fishing effort that one can approximate:
iv. fish stock and changes in fish production in each focus zone
This progressive, detailed and systematic approach, illustrated in Figure 11, is probably the only
way to assess the impact of sediment retention at a given dam on the fish stock and production
in a given zone.
Figure 11: Conceptual model for an assessment of the impact on fish of sediment changes due to dam
construction
In the coastal zone, fish resources depend upon sediment loads, flood pulse and water quality.
Relationships between coastal fish resources, sediment loads and floods have been reviewed in
Report 4. Sediment loads determine the nutrients input into the coastal zone as well as the
turbidity of the plume. The influence of river floods on coastal fish production is clearly
established, both volume and dynamics of the flood being important. Thus, flood regulation by
dams will have an effect on coastal fish resources. Water quality is mainly dependent upon the
input of organic matter from cities and fertilizers from agriculture. Migrations are not
16
considered quantitatively important in this zone and will not be significantly affected by
upstream dams.
In the downstream zone of the Lower Mekong Basin, fish resources depend mainly upon
sediment loads, flood pulse, migration and water quality. As described in Report 3, the flood
pulse plays a major role in floodplains productivity, and river discharge regulation by dams is
expected to change the timing and intensity of the flood, thus affecting floodplains
productivity. In this zone sediment also contribute to the exceptional productivity of the
floodplain system. Migration is another important factor for the fish resource of this zone, and
dams will largely block the transfers of long distance migratory fish between this downstream
zone where they feed and the upstream zones where they breed (40% of the Mekong fish
biomass is concerned by that factor). In absence of substantial pollution at this stage in that
zone, water quality refers mainly to dam effluents and agricultural fertilizer outflows.
In the upstream zone of the Lower Mekong Basin, fish resources depend mainly upon sediment
loads and migrations. The influence of the riverine flood pulse is less marked than in floodplains
and in the coastal area. Sediment loads become all the more important here and determine
nutrient input into the food web. Dam construction will also affect fish migrations here. Water
quality is an issue downstream of dams only.
In all zones, an assessment of the fishing effort is required in order to relate fish catches to fish
stock, before the latter can be related to the fish production drivers, including sediments. The
impact in the three focus zones will also depend on which of the 3 most influential dams (i.e.
Sambor, Stung Treng and Lower Sesan 2) is built, and subsequently on which of the six most
significant sub-basins (i.e. Upper Mekong, Mun/Chi, Lao tributaries, Srepok, Sesan and Sekong)
gets disconnected from the downstream floodplains.
Overall, dams will impact fish resources in multiple ways that should not be related to
sediments only, and dam development itself is one among several human factors impacting fish
resources. Assessing the impact of changes in sediment load on the Mekong fish production will
require the quantification and integration of the most of these factors before an assessment of
the relative role of sediments can be achieved.
17
7
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