## FISH BIOECOLOGY IN RELATION TO SEDIMENTS IN THE MEKONG AND IN TROPICAL RIVERS

BARAN Eric, GUERIN Eric

Project “A Climate Resilient Mekong: Maintaining the Flows that Nourish Life” led by the N atural Heritage Institute

March 20122 Table of content

1 INTRODUCTION

................................ ................................ ................................ ........................ 5 2 SEDIMENTS: CORE NOTIONS

................................ ................................ ................................ .... 7

2.1 Sediment particle size

................................ ................................ ................................ ....... 8 2.2 Sediments and organic matter

................................ ................................ ....................... 11 2.3 Sediments and inorganic matter

................................ ................................ .................... 12 2.4 Sediment loads

................................ ................................ ................................ ............... 13 3 MEKONG FISH: CORE NOTIONS

................................ ................................ .............................. 15 4 SE DIMENTS AND FISH BIOECOLOGY

................................ ................................ ....................... 17

4.1 Sediments and fish respiration

................................ ................................ ....................... 17 4.2 Sediments and fish nutrition

................................ ................................ .......................... 19 4.3 Sediments and reproduction

................................ ................................ .......................... 22 4.4 Sediments and migration

................................ ................................ ............................... 29 4.5 Sediments and fish habitat

................................ ................................ ............................. 30 5 SEDIMENTS AND FISH PRODUCTION

................................ ................................ ...................... 36 6 CONCLUSIONS

................................ ................................ ................................ ......................... 39 7 BIBLIOGRAPHY

................................ ................................ ................................ ........................ 41

Citation

Baran E. and Guerin E. 2012 Fish bioecology in relation to sediments in the Mekong and in tropical rivers. Report for the Project “A Climate Resilient Mekong: Maintaining the Flows that Nourish Life” led by the Nat ural Heritage Institute. WorldFish Center, Phnom Penh, Cambodia.

Acknowledgements

We sincerely thank Saray Samadee for her overall assistance , in particular with the literature (gathering, referencing, checking)

, Yivchhoy C hhuong for illustrating this report ( cover and f igures 4 and 10) and Chris Myschowoda for editing it

.3 

### EXECUTIVE SUMMARY

The Mekong Basin is experiencing rapid development, including plans for the construction of 88 hydropower projects by 2030, which will have a ve ry substantial impact on the river’s sediment load (sediment trapping by dams). The current report reviews the interactions between sediments and fish in tropical rivers and in the Mekong, and focuses more specifically on a
reduction of sediment loads foll owing dam construction.

In the Mekong, the total annual sediment discharge is about 160 million tonnes per year on average, with a wide range of intra

- annual as well as inter

- annual variability. Sediments in rivers are found in two main forms (suspended and bedded sediments) and they consist of a variety of particle sizes (roughly categorized as gravel, sand, clay and silt)

. Most of the sediment load in the Mekong is suspended.

One property of sediments is that they adsorb and thus facilitate the transport of nutrients (in particular phosphorus and nitrogen) and organic particles. As dissolved nutrients in the water are generally low, the main nutrient load is associated with sediments. T he dominant sediment categories in t he Lower Mekong Basin are fine sands, silts and clays ; the proportion of organic particles represents 6

- 8% of the total sediment load, which is considered high by global
standards.

The Mekong is a place of exceptional fish biodiversity

. Fish biodiversity increases downstream and the 3S system is a hotspot. The Mekong produces about 18% of the world’s freshwater fish yield. Mekong fish resources are essential to food security in the basin

. Fish migrations are an essential feature of the Mekong

. Sediments in fluence fishes’ biological functions, particularly respiration, nutrition, reproduction and migration, but also their habitat. Nutrient load is also a dominant factor explaining the overall productivity of tropical river ecosystems (including floodplains, estuaries and coastal areas).

Respiration. The organic component of sediments has a direct negative impact on dissolved oxygen through its Oxygen Demand. Suspended sediments and adsorbed nutrients also influence the amount of dissolved oxygen in the water through their interference with plant photosynthesis

.

Nutrition. Sediments are the basic input for the food chains and trophic webs that fish are part of at a higher level, and thus are an essential component of fish nutrition. Suspended sediments and the turbidity they cause reduce photosynthesis and thus decrease prim ary production. Suspended sediments also play a role in predator

- prey relationships. In the Mekong, a reduction of sediment loads is likely to result in decreased primary production, which might in turn change the dominant fish species and drive down fish productivity. But the relative influence of nutrients loads and turbidity on primary production must be clarified for this potential impact to be assessed.4 Reproduction. The main relationship between fish reproduction and sediments is through spawning gr ounds. In estuaries high turbidity is favorable to the larvae of many coastal fish species. There is little information in the literature about the relationship between tropical fish reproduction and the impact of a reduced sediment load.

Migrations. Changes in turbidity or water color are a migration trigger for at least nine of the migratory species in the Mekong. Fish migrations can play a significant role in the nutrient distribution in the river basin. In the Mekong fish constitute a nutrient tran sfer from nutrient

rich downstream floodplains towards more oligotrophic upstream tributaries.

Habitat. Sediment load plays a major role in river geomorphology and in the riverbed and riverine habitats. Blocking sediment load will degrade the river bed d ownstream and will therefore affect the aquatic organism communities adapted to these specific habitats. In the Mekong, deep pools, an essential habitat for the survival of numerous fish species during the dry season, could be affected by modified water fl ows that result in an infilling of these pools, though this impact remains to be clarified. Water pH is also a characteristic of fish habitats that is related to suspended sediment concentrations.

The river flood pulse concept explains the exceptionally h igh productivity of rivers and their associated floodplains. It has been widely accepted as describing the exceptional productivity of the Mekong. Dam development is expected to reduce `the flood pulse as well as downstream sediment concentrations; this co mbination may have a synergetic and negative effect on the overall productivity of the river downstream, leading to its oligotrophication . This process of nutrient rarefaction, poorly studied, changes food web structures and decreases fisheries

resources. Oligotrophication is most commonly related to phosphorus deficiency, but the nitrogen/phosphorus ratio is also to be considered

. Ongoing hydrodynamic and physical modelling will also clarify the retention rate of dams and the fate of sediments, but the

cou pling between sediments and nutrients and the role of the amount of nutrients that are independent of sediments remain to be addressed.5 

### 1 INTRODUCTION

Sediments are an essential component of rivers and of their biological functioning. In addition to their influence on river geomorphology (maintenance of river forms and habitats such as pools and sand bars), sediments also include nutrients, detritus and organic debris of various
sizes which interact with the river’s different life form s, including fish (USEPA, 2003). The interaction between sediments and aquatic organisms, directly or indirectly through the effects of sediments on physical habitats, unquestionably influences the biodiversity and productivity
of a river

.

The Mekong is t he second river in the world after the Ganges for its load in suspended solids 1
, and is also the second richest river in the world in terms of fish biodiversity (Carling 2009; Baran 2010). Its average sediment discharge is estimated at 160 million tonnes p er year, i.e. 12% more than the Amazon River (Wolanski and Nguyen Huu Nhan 2005, Sarkkula et al. 2010).

The Mekong Basin is experiencing rapid development, including plans for the construction of 88 hydropower dams by 2030, which will have a very substan tial impact on the river’s
sediment load : when river flows enter a dam reservoir they decrease in velocity, which results in a fraction of the transported sediment being deposited in the dam reservoir (Palmieri et al. 2003, Lu and Siew 2005, and Figure 1 ). Thus, the Strategic Assessment of Mekong mainstream dams predicts a 75% reduction of the sediment load at the mouth of the Mekong by 2030 if all planned dams are built (ICEM 2010)

.

Figure 1 : Accumulation of sediment behind dams. Source: Fruchart 2009

An abundant literature covers the effects of increased sediment and nutrient concentrations caused by reservoir sediment flushing or organic pollution on fish; however, very few studies
detail the consequences of reduced sediments or nutrients for fish communities and production. This might be explained by visibility and timing issues: while flushing operations are spectacular and their impact immediately measurable, the effects of reduced sedi ment loads on ecosystems involve long

- term processes such as modification of the river morphology over

1 These figures are some what disputed since Welcomme (1985) indicates 190 and 406 million to nnes of suspended load for the Mek ong and the Amazon respectively, but 560, 812 and 1625 million ton ne s for the Yangtze, Brahmaputra and the Ganges respectively. The Yangtze, Brahmaputra and Amazon are not part of Carling’s comparison.6 decades (as a consequence of sediment loss) and oligotrophication (resulting from nutrient loss) whose impacts on fish communities are gradual, over seve ral years. The consequences of these processes might also be buffered and thus hidden for a while by the resilience capacity of the river (nutrients released by the floodplain and sediments released by erosion to overcome the losses in upstream inputs).

T EXT BOX 1

MRC State of the Basin report, 2003 (excerpts)

At present the MRC monitors chemical water quality once or twice per month at 98 sites. Sampling has been conducted since 1985 in three LMB countries, and since 1992 in Cambodia. The data collecte d under the MRC sampling programme are taken just below the water surface, and give no indication of the bedload transport of sediment. Nevertheless, the measure should act as a reasonable indicator of
the amount of sediment in the water.

The concentratio ns of TSS showed a decrease at 23 sites and an increase at six. Notable regions where TSS concentrations decreased were the entire set of Mekong mainstream sites between Chiang Saen and Pakse in Lao PDR, and eight of the 13 sites in northeastern Thailand. The trends in phosphorus and nitrogen were similar [except in the Delta]..

The most prominent change at mainstream sites was the decrease in TSS which extended from Chiang Saen downstream. At the Chiang Saen site, the decline in TSS concentrations is like ly to be due to changes that have occurred in China. The most likely dam that could have influenced the river is the Manwan Dam. Regression analyses on data before and after the dam started filling in 1992 demonstrate
that the decrease in TSS in the downst ream sites has been caused by the entrapment of suspended sediment by the dam.

Figure: Suspended solids concentrations in the Mekong River at Chiang Saen, with linear regression lines fitted for the periods before and after the Manwan Dam commenced filling in 19927 The current report reviews the interactions between sediments and fish in tropical rivers and in the Mekong, and focuses more specifically on a reduction of sediment loads following dam
construction. Sediment loads increase due to reserv oir flushing and the impact of flushing on fishes are reviewed in another report (Reservoir sediment flushing and fish resources). The
second and third sections of the present report (respectively “Sediments” and “Mekong fish”)
introduce some core notions related to sediments and fish in the Mekong. In Section 4 (“Sediments and fish bioecology”) we review the interactions between sediments and i) fish biological functions (respiration, reproduction, etc.) and ii) fish habitat (bioto pe and water quality). In sections 5 the role of sediments in fish productivity is reviewed, before reviewing in the conclusion the main findings and knowledge gaps identified

.



### 2 SEDIMENTS: CORE NOTI ONS

Sediments in rivers are found in two main forms: suspended and bedded sediments (SABS). This distinction is based on the sediments’ size, nature and mode of transportation by the

water. Suspended sediments are made of “particulate organic and inorganic mat ter that suspend in or are carried by the water” while bedded sediments or bedload “accumulate in a loose, unconsolidated form on the bottom of natural water bodies” (USEPA, 2003) 2 . T hus, coarse material, gravel and pebbles are transported by the bed load at the bottom of the river bed, while fine materials, sand and silt, are transported in suspension (Fruchart 2009).

A major property of sediments is their ability to adsorb small organic or inorganic compounds ( i.e. these compounds stick to the sediment particles); thus the transportation of organic or inorganic matter in the hydrosystem is largely facilitated by sediments. This means that sediments are actually a combination of mineral, inorganic and organic compounds with
different properties. It also means that sediments largely facilitate the transportation of organic or inorganic matter in the hydrosystem. We summarize in Figure 2 the main components of sediments and detail these components b elow.

2 Castro and Reckendorf (1995) detail further the components of th e bedload: « Framework bedload refers to the larger particles that are moved only during large flow events. They create the structure of the bed. The matrix bedload refers to that part of the bed material that is small enough to be frequently entrained by low to moderate flows but is large enough to settle out of the water column in lower velocities. This also includes sediment deposited by intragravel flow. This wou ld incorporate the sand and silt size material. The matrix bedload is often referred to as "sediment" by fisheries biologists and is the size class that is of most interest and concern in fisheries studies”.8 Figure 2 : Main components of river sediment

2

. 1 Sediment particle size

A standard classification of mineral sediments based on particle size is detailed in Table 1.

Table 1 : Conventional grain size ranges for different sediment types

. Source: Ketelsen and Ward 20109 Welcomme (1985) gives a relationship between current speed and size of the sediment particles moved by the flow ( Figure 3 )

Figure 3 : Relationship between flow speed and size of particles moved. Source: Welcomme 1985.

For the purpose of this report, we adopt a simplified classification reduced to three categories that are significant in the case of fish bioecology (Rowe et al . 2003) :

• Coarse sediment , defined as particles larger than 0.95 or 1 mm. Being the main

componen t of the river bed, affect the morphology of the river, shape fish habitats or spawning sites, and constitute the main biotope of several macro invertebrate species that fish feed upon;

• Sand or fine sediment, consisting of particles smaller than 0.85 mm wh ich can be either bedded sediments or the suspended sediments depending on the river velocity ( Figure 4 ).

• Silt and clay , i.e. particles smaller than 0.063 mm suspended in flowing water (thus creating its turbidity) or deposited into mud beds.10 Figure 4 : Nature of sediments depending on location, slope and current speed

In the case of the Mekong, a review of sediment

- related information and data (Ketelsen and Ward 2010) led to the conclusion that “ considerable skepticism surrounds Mekong sediment data, which has been collected with inconsistent methods, measuring different parameters and has often yielded widely divergent estimates ”.

Yet several authors agree that t he dominant sediment categ ories in the Lower Mekong Basin are fine sands, silts and clays (Irvine et al. 2007, Sarkkula et al. 2009, Carling 2009). One of the few reliable sediment distribution curves available (cited in Ketelsen and Ward 2010, Figure 5 )
shows that in Pakse 99% of the sediment has a grain size smaller than fine gravel (>4.75 mm) and that 41% of the distribution is finer than coarse sand (>0.45 mm). In the Tonle Sap, silts and clays are the dominant grain size ( Sarkkula et al. 2010). The latter authors conclude that the Mekong probably transports proportionally less sand and more silt/clay than other large rivers,
and that clay and cohesive sediments seem to remain in suspension until the Mekong enters the Cambodian floodplain a nd the Mekong Delta

.11 Figure 5 : Sample grain size distribution curve for Pakse (cited in Ketelsen and Ward 2010)

In this context, sand is, by contrast and considering its limited abundance, an important component for the ecology of the Mekong . Sand and sandy islands are a typical component of the wetland zone stretching between Pakse and Kratie (MRC BDP 2 zone 4, including the Siphandone wetlands) and an essential habitat for the biodiversity of the Mekong, as detailed
for instan ce in Baird 2001, Bezuijen et al. 2008, Allen et al. 2008 or Meynell 2010. Sand transportation and dynamics (resuspension, movements of sand bars or in

- channel consolidation) are directly dependent upon hydrodynamic conditions ( Sarkkula et al. 2010)

, which implies that changes in these conditions may have a disproportionately large impact on the sandy habitats in the Mekong. More generally coarse materials are very important for the stability of the river bed as they constitute a carpet protecting th e bottom of the river from incision by water

- driven erosion; unfortunately coarse materials settle completely when entering dam reservoirs (see Figure 1 ), and cannot be flushed downstream (Fruchart 2009).

2

. 2 Sediments a nd organic matter

Sediment loads also include an organic component of great importance for the biological productivity of the river. This organic component is made of i) dissolved organic matter (e.g. degraded proteins from vegetal or animal origin, tanni ns) and ii) particulate organic material transported from the watershed slopes (pollen, leaves, branches, trees) and subject to biological degradation and size reduction ( Welcomme 1985). Dissolved Organic Matter (DOM) is part of the water flow but is not s ediment, whereas Particular Organic Matter (POM) is a part
of the sediment load of rivers.

Importantly, a fraction of the organic matter is adsorbed onto sediment particles , which facilitates the transport of such matter but also influences their chemical dynamics: this adsorbed organic matter may be stored within deposited sediments, or may be reincorporated into the flow through erosion. (Welcomme 1985).12 In the case of the Mekong, the organic content amounts to 6

- 8% of the total sediment load (Pantulu 1986), which is seen as a large percentage and which contributes to the considerable productivity of the river.

2

. 3 Sediments and inorganic matter

River sediments serve as a major carrier and storage agent for inorganic nutrients such as phosphorus, nitrogen or potassium, but also pesticides and metals ( Yang Xiaoqing 2003, Yi et al.
2008 ). As water contains low concentrations of dissolved nutrients, a river’s main nutrient load actually consists of nutrients associated with

— or ads orbed on

— sediments. Thus, measurements in North America and Europe showed that about 90% of the total phosphorus flux in rivers was associated with suspended sediment (Ongley 1996). The pattern seems to be similar in the Mekong ( Meynell 2010).

The most im portant sediments for the transportation of nutrients are the very small size sediments such as clay and colloids. The sediments that are chemically active are those that are smaller than 63 mm (i.e. silt and clay), in particular for phosphorus, a nutrient highly attracted to ionic exchange sites that are associated with clay particles and with the coating common on these small particles (Ongley, 1996). In South America, rivers with high sediment loads and high alkalinity have higher concentrations of nitro gen and phosphorus

— and thus are more productive

— than rivers with clear water (Forsberg et al. 1988). Unlike phosphorus, the transport and fate of organic sediments or organic matter attached to sediments is complicated by microbial degradation.

In the Me kong, the nutrients associated with sediments are responsible for the fertility of the floodplains and the delta (Meynell 2010). In particular, Sarkkula and Koponen (2010) showed that the high productivity of the Mekong floodplains is based on the nutrient loading associated with the sediments from upstream areas and that phosphorus is often the limiting nutrient. More specifically, 31

- 42% of the bioavailable phosphorus is transported into the floodplain with the sediment influx, which equates to 21,500 t/y r of bioavailable phosphorus (Sarkkula et al. 2010) . Suspended inorganic material from the mainstream channel is often seen as more important to productivity than dissolved and particulate organic material (Vannote et al. 1980,
Junk 1999 )

.

The Mekong River Commission has defined quality classes for the main water parameters (MRC 2007). These water quality classes are based on thresholds defined by French water

- basin
authorities and are detailed in13 Table 2 : Parameters and thre sholds used to classify the water quality in the Mekong. This table focuses on mineral and organic matter found in sediments. Source: MRC 2007

Class of water quality Very good Good Fair Bad Very bad Organic matter and other matter that can be oxidized DO (mg/l) ≥8 ≥6 ≥4 ≥3 <3 COD (mg/l O 2 ) ≤5 ≤7 ≤10 ≤12 >12
NH 4 - (mg/l NH 4 ) ≤0.5 ≤1.5 ≤2.8 ≤4 >4 Nitrogenous matter

NH 4 - (mg/l NH 4 ) ≤0.1 ≤0.5 ≤2 ≤5 >5 Phosphorous matter TP (mg/l) ≤0.05 ≤0.2 ≤0.5 ≤1 >1
PO 4 3

- (mg/l PO 4 ) ≤0.1 ≤0.5 ≤1 ≤2 >2 Suspended matter TSS (mg/l) ≤5 ≤25 ≤35 ≤50 >50

2

. 4 Sediment loads

The amount of sediment transported by a river is another important factor in relation to fish biology and productivity.

In the case of the Mekong there is limited knowledge and great uncertainty about the suspended sediment load and its variability. There is data

— whose quality is disputed

— on the suspended load, but there is no reliable data on bed load. This means that an ecologically important type of sediment transport that is affect ed by dams is not part of current analyses. (Kummu and Varis 2007, Ketelsen and Ward 2010).

It is now largely agreed that the total annual sediment discharge of the Mekong amounts to about 160 million tonnes per year (range 140

- 180 Mt/y), which is high i n comparison with other major rivers of the world (Meade 1996, Wolanski and Nguyen Huu Nhan 2005, Sarkkula
et al. 2010, Ketelsen and Ward 2010). The total load of the Mekong is about the same as the Mississippi, 85% of that of the Yangtze River and 12% lar ger than that of the Amazon (Carling 2009, Sarkkula et al. 2010,) 3 . Under natural conditions, 50% to 60% of the sediments originate from China, the ‘Three S’ catchments contributing about 17 Mt/yr (i.e. about 10% of the total load) and the remainder of tri butaries in the Lower Mekong Basin providing about 30% of the total load (Ketelsen and Ward 2010).

The Mekong’s sediment load is characterized by large intra

- annual and inter

- annual variability (Welcomme 1985, Ketelsen and Ward 2010 and Figure 6 ). On the seasonal level, water is heavily charged with silt during the rising flood, then deposits its suspended sediments

3 T hese figures are somehow disputed since Welco mme (1985) indicates 190 and 406 million to nnes of suspended load for the Mek ong and Amazon respectively, but 560, 812 and 1625 million ton ne s for the Yangtze, Brahmaputra and Ganges respectively. The Yangtze, Brahmaputra and Amazon are not part of Carling ’s comparison.14 over floodplains during the flood, and then becomes clear during the dry season (Welcomm e 1985). From an interannual

- perspective, LMB tributaries contribute a greater proportion of the annual load during wet years, while in dry years the amount of input from China is higher than normal (Ketelsen and Ward 2010).

The Mekong’s sediment load is also characterized by strong geographical variability, the total suspended load increasing farther downstream

. Thus, t he annual total suspended load has been estimated as 67, 109 and 132 million tonnes per year at Chiang Saen, Vientiane and Khone
Falls res pectively (Carling 2009). Overall the Cambodian floodplains and Mekong Delta receive 95% of the total suspended sediment flux from upstream (Kummu and Varis 2007) and 80% of the suspended sediments that enter the Tonle Sap Lake get deposited in its floodpl ain ( Lamberts 2006), which makes these intensive fishing grounds directly dependent on the flow conditions of the Upper Mekong (Baran et al. 2007 a ).

Figure 6 : Annual average sediment load (1997

- 2004) at Kratie. Source: Kummu et al. in Ketelsen and
Ward (20 10 )15 

### 3 MEKONG FISH: CORE NOTIONS

The exceptional features of the Mekong fish resources have been detailed in several publications (e.g. Lagler 1976 , Pantulu 1986, van Zalinge et al

. 2003
, Baran et al. 2007 b , Hortle
2009 ). We briefly review them here before focusing on the fish features related to sediments.

The Mekong is a place of exceptional fish biodiversity . The Mekong River is the world’s second richest river after the Amazon. The Tonle Sap is the lake with the highest f ish biodiversity in the
world after the East

- African Great lakes. An analysis of lists of species and endangered species among 302 countries or territories worldwide shows that Lao PDR, Thailand and Viet Nam are among the top 5% for their number of freshwa ter fish species and number of threatened fish species.

Fish biodiversity increases downstream and the 3S system is a hotspot. Like the sediment load, the Mekong River mainstream is characterized by a gradient of increasing species richness
from the head waters down to the sea, with 24 species in Tibet and 484 species in the Mekong Delta. A recent study (Baran et al. 201 1 ) showed that within the Mekong Basin, the 3S system (Sekong, Sesan and Srepok watersheds) is home to 42% of the total Mekong fish biodiv ersity (i.e. 329 fish species out of 781), though the surface area of the 3S represents only 10% of the 800,000 km 2 of the Mekong watershed area. The 3S Rivers are also characterized by 17 endemic species found nowhere else in the world.

The Mekong produces about 18% of the world’s freshwater fish yield. The fish catch, estimated to be between 755,000 and 2.6 million tonnes (the most reliable assessment being 2.1 million tonnes), comprises about 18% of the global freshwater fish catch (range 6

- 22%), making the Mekong the largest inland fishery in the world.

Mekong fish resources are essential to food security in the basin : Cambodia, Lao PDR and Thailand are the top four countries in the world for their freshwater fish consumption: fish consumption r anges between 24.5 and 34.5 kg/person/year. Thus, fish contributes 81% of the population’s protein intake in Cambodia and 48% in Laos. The economic value of Mekong capture fish is up to USD 2.2

- 3.9 billion per year.

Fish migrations are an essential featu re of the Mekong . Of the 189 migratory fish species known, 165 are long

- distance migrants and these species represent more than 37% of the total yield, i.e. more than 770,000 tonnes per year. At the end of the dry season, more than 100 migratory species (Z iv et al. 2012 ) migrate up tributaries to lay eggs in a riverine environment characterized by clear water and its associated seasonal flora and benthofauna.16 Figure 7 : Fish migration patterns in the Mekong. Source: Baran and Un, in press

.

The exceptional abundance of Mekong fish resources is explained by a combination of high fish biodiversity, high productivity linked to high sediment load and flooding, and a high exploitation rate (Baran and Un, in press ).

Figure 8 : Factors explaining the size and importance of Mekong fisheries. Source: modified from Baran and Un (in press).17 

### 4 SEDIMENTS AND FISH B IOECOLOGY

Sediments constitute an important environmental factor in fish bioecology . Fish speci es inhabiting a river body are either adapted to specific sediment characteristics which are not

constants (e.g. seasonal and inter annual variations ) or tend to leave the area subject to excessive sediment load before returning after the event (flush floo d, seasonal flood). Thus

, from a fish ecology viewpoint

, the presence of sediments is normal ; it is variation outside of the
normal range that is detrimental to species , and there is no specific level at which all species of fish would be equally affected ( Sanderson 2009 )

.

Among rivers of the world there is a relationship between energy availability and sediment availability (Welcomme et al. 1989); in the Mekong the hydropower potential of the Lower Mekong Basin is estimated at 53,000 MW (ICEM 2010) , with 30,000 for the Lower Mekong Basin alone (MRC 2001). The impact of such power extraction on the river nutrient availability deserves being studies in more detail.

Figure 9 : relationship between energy availab ility and sediment availability. Source: Welcomme 1989

4

. 1 Sediments and fish respiration

Note: the issue of sediment overload and fish gill clogging is not detailed here since it is covered in the report “Reservoir sediment flushing and fish resources” also produced for the current project.

There are two main sources of oxygen in the aquatic environment: the atmosphere and photosynthesis by plants

. Oxygen from the atmosphere is diffused in the water through the air/water interface and this process is accelerated by wind and wave action; photosynthesis by18 aquatic plants, algae and phytoplankton is the most important source of dissolved oxygen in
aquat ic environment (Francis

- Floyd 2011)

.

The organic component of sediments has a direct negative impact on dissolved oxygen through its Oxygen Demand. Th ese organic compounds consume oxygen for ox i dation

processes , which reduces the amount of dissolved oxygen available in the water column (Welcomme 2001). WUP

- FIN studies mentioned this phenomenon in the case of the Tonle Sap floodplain where sediment carried by the rainy season freshwater pulse can dramatically
cha nge dissolved oxygen levels and subsequently impact fisheries ( Sarkkula and Koponen 2003, Koponen et al. 2005 ).

Suspended sediment s also influence the amount of dissolved oxygen through their
interference with photosynthesis

. Two opposite pathways can be noted ( UTAH State Water
Plan 2010) : i) turbidity reduces the amount of sunlight reaching the aquatic vegetation and thereby reduces its oxygen production (negative effect); ii) nutrients associated with suspended
sediments favor plant growth and thereby increase their oxygen production (positive effect).
Thus
, dam development reduc ing sediment concentration would have opposite effects by simultaneously promot ing and reducing plant

- driven oxygen production (the release of hypoxic
or anoxic reservoi r water being ignored here). The combination of these different factors makes it difficult to predict the overall impact of sediment load reduction on oxygen concentration in

water ( Figure 10 )

.

In the Mekong mainstream the average dissolved oxygen concentration in running waters ranges between 5.5 and 8.5 mg .l - 1 (1985

‐ 2005 data; MRC 2009). Dissolved oxygen concentrations below 5 mg .l - 1 may be harmful to fish and piping (gulping air at the surface) is observed when DO falls below 2 mg.l - 1 (Francis

- Floyd 2011)

.19 Figure 10 : Influence of suspended sediment on oxygen content in water

4

. 2 Sediment s and fish nutrition

In the Mekong, there is not enough information to establish a detailed connection between
sediment classes and the feeding of fish species. We reviewed for the present report the feeding of Mekong species in relation to sediments, as compiled in a literature review of 296 Tonle Sap species (period covered: 1938

- 2003; Baran et al. 2007 c ) and detailed in FishBase ( www.fishbase.org ) and the Mekong Fish Database (MRC 2003)

. The information on diet
contained in these two databases refers to the following food items and all their combinations: Detritus, Algae, Plants, Plankton, Zooplankton, Zoobenthos, Nekton, A nimals and Insects. Although some of these categories (e.g. detritus, zoobenthos) reflect to some extent a
sediment

- related feeding mode, they are not explicit about the type of sediment involved. Alternatively, Valbo

- Jorgensen et al. (2009) review, from u nspecified data, the food items of Mekong fish species, and find that mud feeders constitute 1% of Mekong fish species and that 69 species are detritivores ( Figure 11 ).20 Figure 11 : Number and percentage of Mekong fish species feeding on various food sources . Source: Valbo

- Jorgensen et al. 2009.

Evidence from other tropical rivers shows that sediment concentration influences i) the trophic
food chains that fish are part of or depend upon, ii) the predator

- prey relationship, and iii) the dominance of species in fish assemblages. We detail these aspects in the following paragraphs.

Fish nutrition and growth depends on the conce ntration and composition of sediments in the
river

. Fish sometimes feed directly on sediment (e.g. mullets), but more importantly sediments condition the presence and abundance of fish prey. The carbon component of sediments is the main food source of zooplankton and benthic invertebrates, and nutrients adsorbed on sediments are the main input for phytoplankton, algae and aquatic plant development. These first levels of the trophic pyramid constitute the diet of m any fish species and are essential to fish productivity. Although th e details of th ese very complex food webs are not well known, there is ample evidence that fish nutrition and productivity ultimately depend s on the concentration and composition of sedime nt s (in particular nutrients and organic particles )
availab le in the river ( Gray and Ward 1982, Ward 1992, Power et al. 1996, Ongley 1996, USEPA
2003 )

.

Although the overall positive role of sediments on fish nutrition is clear, detailed processes

are complex and sometimes in opposition : a decrease in sediment loads

— and attached
nutrients

— might result in a reduction of primary production ( UTAH State Water Plan 2010) but can also be favorable to primary production through increased irradiance (Lloyd et a l . 1987)

. The latter phenomenon was observed on the Nile River, where a significant reduction in water turbidity (from 3000 to 15

- 40 mg

- l - 1 ) following sediment trapping by the Aswan dam resulted in increased phytoplankton density (Dubowski 1997) ; yet , the Nile fishery gradually collapsed (Bebars et al. 1996)

. In the coastal zone
, maximal phytoplankton production is often found at
some distance from the mouth of a river, in a zone of lower nutrient concentration, and this is related to increased light a vailability (Parsons 1977, Randall and Day 1987). Yet in the Mekong, Wyatt and Baird (2007) note a paradoxical effect: the sediment retention by the Yali Falls dam21 on the Sesan River resulted in increased sediment load and turbidity downstream due to
incre ased riverbank erosion; this phenomenon in turn “ reduced the light available for algal growth, or smothered bottom

- growing algae, which are an important food source for some species ”.

In the Mekong t he link between upstream nutrient s and fish abundance was already noted in
1933 by Chevey : “A remarkable concentration of f ish can be noticed at the mouth of the Bassac and Mekong Rivers at the beginning of the dry season. The fish are attracted by the enormous concentration of nitrogenous material coming fro m the Mekong” ( Chevey 1933). In the South
China Sea f ish relying on Mekong river nutrients are found as far as 500 km from the river mouth ( MRC 2003).

Overall, permanently reduced s ediment loads modify food webs and result in a change in fish
communit ies and dominant species . Following the closure of the Tucurui Dam (Brazil) and the
downstream reduction in sediment load
, the biomass of piscivor ous fish es increased while that
of planktivores diminished, and some species change d their diet ( e.g. de Merona 2 001 ). More generally Welcomme ( 200 1), summarizing the impact of human interventions on fish, indicates that a decrease in suspended silt load results in changes in the fish community, with in particular a reduction in the number of non

- visual predators and carnivores, and that a lack of sediment downstream of dams changes the nutrient cycle and causes a loss of illiophages (mud feeders).

Permanently reduced sediment loads also alter the fish community structure and dominant species by altering fish predation patterns

. Turbidity determines underwater visibility
conditions , and it is now well established (review in Kerr 1995) that this in turn influences the predat ion efficiency of visually oriented fish species . Unlike species located at the bot tom of the trophic pyramid
, who usually filter water (planktivores) or forage at random (detritivores, bottom feeders), piscivorous predators mostly depend on their vision to catch prey. Turbidity subsequently influences the predator

- prey relationship and ultimately the progressive
dominance of specific predators and prey

. Thus
, de Melo et al. (2009) showed that in the Bananal floodplain (Mato Grosso, Brazil) changes to water transparency influence the composition of fish assemblages and dominant species am ong Cynodontidae

. In estuaries
B laber et al. (1985) also report that p redation, usually low due to high turbidity
, can increase under specific conditions of clear and deep water

. However, the relationship is not clear: reduced turbidity following retention of sediments by dams might favor top

- level predators, but also the survival rate of prey that would better perceive predators. Overall, the literature
review does not lead to clear conclusions about the impact of sediment retention on changes in trophic levels or the overall productivity of the fishery.22 4

. 3 Sediments and reproduction

The main relationship between fish reproduction and sediment is through spawning grounds

. The reproductive guilds 4 of fishes formalized by Balon (1975, 1981) and encompassing all the 30,000 fish taxa are based on three main criteria: forms and functions in early development stages, features of reproductive behavior, and preferred spawning grounds. The latter makes
extensive reference to the nature of the substratum where eggs are laid ( Table 3 ).

Table 3 : Main fish reproductive guilds. Guilds whose spawning is dependent on river sediment classes are highlighted in grey. S ource: Balon 1981.

Ethological section Ecological
group Guild Meaning Notes

NON

GUARDERS Open substratum

spawners Pelagophils Pelagic spawners Numerous buoyant eggs

Lithopelagophils Rock and gravel
spawners with pelagic larvae Embryos pelagic by positive buoyancy or active movement

Lithophils Rock and gravel spawners with benthic larvae Early hatched embryo hides under stones

Phytolithophils Nonobligatory plant spawners Adhesive eggs on submerged items

Phytophils Obligatory plant
spawners Adhesive egg envelope sticks to submerged live or dead plants

Psammophils Sand spawners Adhesive e ggs in running water on sand or roots over sand

Aerophils Terrestrial
spawners Small adhesive eggs scattered
out of water

Brood hiders Aeropsammophils Beach spawners Spawning above the waterline of high tides

Xerophils Annual fishes Eggs and embryos capable of surviva l for many months in dry mud

Lithophils Rock and gravel
spawners Eggs buried in gravel depressions called redds or in rock interstices
Speleophils Cave spawners

Ostracophils Spawners in live invertebrates

GUARDERS Substrate choosers Pelagophils Pelagic spawners Non

- adhesive
, positively buoyant
eggs
Aerophils Above water
spawners
Lithophils Rock spawners Strongly adhesive eggs, attached at one pole by fibers
Phytophils Plant spawners

4 “ A guild is defined as a group of species that exploit the same class of environmental resources in a similar way” (Root 1967)23 Ethological section Ecological
group Guild Meaning Notes

Nest
spawners Aphrophils Froth nesters

Polyphils Miscellaneous substrate and material nesters Adhesive eggs attached singly or in clusters on any available substratum

Lithophils Rock and gravel
nesters Eggs always adhesive, free embryos Ariadnophils Gluemaking nesters

Phytophils Plant material
nesters

Psammophils Sand nesters Thick adhesive chorion with sand grains gradually washed off or bouncing buoyant eggs
Speleophils Hole nesters Actiniariophils Anemone nesters

BEARERS External bearers 5 guilds

Internal bearers 4 guilds

This table illustrates the importance o f sediment s in river fish reproduction, and the very specific role of different sediment classes in fish spawning

.

For their reproduction some river fish species are dependent upon bedload while others are dependent upon washload. A number of fish species, in particular in u pstream environments and in floodplains, spawn on the bottom and deposit their eggs in the empty spaces between gravel (e.g. croakers, lithophils) or on fine sediment and vegetation debris (e.g. some snakeheads). They may use their fins to create a microen vironment within the streambed with optimal water current and thus egg oxygenation (e.g. tilapias). Egg development requires a fine balance between discharge, velocity, and bed material for spawning to be successful. As opposed to this strategy, other spec ies release their eggs in the water column (e.g. freshwater clupeids), and in this case egg development requires a fine balance between egg buoyancy and water/sediment density.

There is little information in the literature about the relationship between t ropical fish reproduction and the impact of a reduced sediment load

. I f there is substantial information about sediments and fish in temperate rivers (in particular those with salmonids subject to sediment flushing), and about the spawning behaviors and pr eferences of East African lake fish species (in particular tilapias), overall the relationship between fish and sediment s in tropical
rivers is not well known, and reproduction as a physiological and behavioral process remains largely understudied in environments characterized by a high number of species, a diversity of life strategies and few fish biologists . Thus
, no substantial information has been generated at the crossing of two limited fields of research

.24 Furthermore
, a considerable body of information detail s the impact of an increased sediment
load (mainly due to erosion following deforestation, mining , etc

. ) on spawning, spawning sites
, egg development and overall reproduction success in clear temperate stream s (Kerr 1995,
Castro and Reckendorf 1995 5 ) , but very little information is available about the impact of a
reduced sediment load on fish reproduction (i.e. the main focus of this report).

F reshwater f ish at y ounger life stages are more vulnerable and more severely impacted by
sediment variations than those at adult life stages (Alexander and Hansen 1986

, Appleby and
Scarratt 1989
, Newcombe 1994 ). The most sensitive stage is the larval stage, more sensitive
tha n the egg or juvenile stages (Appleby and Scarratt 1989 , Isono et al. 1998 ).

In estuaries, high turbidity is favorable to the larvae of many coastal fish species. These species tend to spawn in the coastal zone, and then larvae drift or actively migrate towards river estuaries where they find turbidity caused by a high concentration of suspended organic matter decreases the efficiency of predators and increases their survival rate (Blaber and Blaber 1980, Cyrus and Blaber 1987a & b, Baran 1995, Lévê que and Paugy 1999).

A reduced sediment load tends to impact the reproduction success of species laying their eggs on river bed s . In particular
, when bedload transport is reduced, the riverbed’s sediments are not replaced by upstream sediments, which resu lts in “armoring” of the riverbed

. Th e resulting rock ier riverbed is less suitab le for the fish spec i es that require specific substrate compositions for reproduction (Bizer 2000 ). Fish eggs directly exposed to water friction are also generally more vulnerable than eggs buried in the substrate (Newcombe 1994, Kerr 1995).

A reduced sediment load may impact the reproduction success of some species spawning in
the water column (pelagophils)

. The eggs of these species must stay near the oxygenated

water surface, which implies that their buoyancy matches the water density in the river at the time of spawning , the latter being driven by the water’s silt content. When oocytes are released and fertilized, they get hydrated, which increases egg volume several times and reduces their specific weight, prompting flotation and drifting (Caro l sfeld et al. 2003). However
, in the case of
reduced silt content
, eggs lose their buoyan cy , sink and die. This phenomenon, relatively undocumented for river fish, is now well k nown for marine fish; e.g. Griffin et al. 2009
, Ospina

Álvarez et al. 2012; it was flagged by Baran et al. (20 07 ) as possibly having large

- scale implications for the Tonle Sap fish resources. This buoyancy issue can also be significant in the case of juvenile fish (Pavlov et al. 1995, 2008)

5 In their review, Castro and Reckendorf note that “ Researchers have consistently found that the introduction of

excess matrix bedload can have disastrous results for the spawning habitat of fish that require gravel substrate for spawning. ”25 In the Mekong, the spawning grounds and the nature of these grounds have not been
identified for the large majority of species (Chea Tharith et al

. 2005). However

, we reviewed for the present report the reproduction of Mekong species in relation to sediments, as compiled in a literature review of 296 Tonle Sap species (period covered: 1938

- 2003; Baran et al. 2007 c ) and detailed in FishBase ( www.fishbase.org ) and the Mekong Fish Database ( MFD 2003) . Findings are detailed below ( Table 4 )

Table 4 : Tonle Sap species whose reproduction is or is not dependent on sediments.

Species whose reproduction is dependent on sediments

Probarbus jullieni Falling water and atmospheric temperatures, together with decreased turbidity, may be important factors in controlling the
arrival time of Probarbus to its spawning grounds. The eggs are buoyant or semi

- buoyant, but slightly heavy and adhesive. Generally
i ntolerant of habitat alterations, it has disappeared from areas affected by impoundments. Large dams in Stung Treng and Kratie p rovinces would eliminate most of the rapids habitat that are important for the spawning of this species. Baird and Flaherty
2000 ; Ukkatawewat 1999; Amatyakul et al. 1995; Rainboth 1996;
Roberts 1992

Pangasius

macronema It spawns in rapids in the beginning of the rainy season. Schouten et al. 2000

Boesemania microlepis The identified spawning areas are characterized by being over 20 m deep, with hard rock or pebble and silt or sand substrate, and having slow
to moderate counter

- current eddies in the dry

- season, and with steep rock sides descending into the pools. Baird et al . 2001

Probarbus labeamajor Large dams on the mainstream in Stung Treng and Kratie p rovinces, would eliminate most of the rapids habitats that are important for its spawning. Roberts 1992

Puntioplites
proctozystron Spawning occurs in slow moving water areas with muddy bottom. The eggs are of the semi

- buoyant type. Watanadiroku and Murada 1985; Banyen
et al. 1989 a ; Banyen
et al. 1989 b ; Duangsawasdi et al.26 1988; Banyen 1988

Channa gachua Spawns in shallow water with a silt or gravel substrate

. Talwar and Jhingran
1992

Species whose dependence on sediments for reproduction is unclear
Datnioides pulcher Spawns in March, the eggs are buoyant and approximately 0.8 mm in
diameter. Rithcharung and Mahawong 1993

Labeo chrysopheka
dion It is reported to spawn in swamps, flooded areas, or just upstream from shallow sandbars that line long river bends (Rainboth 1996). The eggs are
semi

- buoyant

. Smith 1945; Poulsen
and Valbo

- Jørgensen 2000; Baird et al. 1999

Osteochilus melanopleur

us It is a pelagic spawner which produces
semi

- buoyant eggs. Warren 2000;
Watanadirokul et al. 1983;

Osteochilus
hasseltii Spawning occurs in floodplains in areas covered with submerged vegetation and gravel bottom. The eggs are buoyant or semi

- buoyant. Warren 2000; Pholprasith and Janesirisak 1972; Warren 2000; Pennapaporn et al.
1991
Pangasius conchophilus It seems likely that the species

spawn s at various times of year. An important spawning ground appears to be the Mekong mainstream somewhere between Kompong Cham and Khone Falls and in rapids and
riffles of the Mun River

. Baird and Phylavanh 1999; Poulsen and
Valbo

- Jørgensen 2000;
Schouten et al. 200027 Chitala blanci Spawning takes place within the main river channel in areas with submerged wood and rocks, and the female guards the fry. Poulsen and Valbo

Jørgensen 2000

Chitala
chitala Lays eggs usually on stake s or stump s of wood, male fans them with tail, keeps them aerated and silt

- free, guards them against small catfish and other predators. MFD 2003

Species whose reproduction is not dependent on sediment Clupeichthys
aesarnensis It spawns pelagically probably in the upper water column close to the water surface although earlier reported to spawn at stumps, plant roots and grasses. Scott and Crossman, 1973; Soemarwotto
and Costa

- Pierce 1988

Corica
laciniata It probably spawns at regular intervals throughout the year in the upper
water column close to the water surface. MFD 2003

Tenualosa
thibaudeaui It spawns pelagic ally in open water. Developed eggs occur in March

- June. In the Middle Mekong along the Thai

Lao border, young of 40 to 50 mm total length were first encountered in the middle of April, but the abundance of juveniles increases during the onset of the rising wat er levels when the suspended solids
increase. Poulsen and Valbo

Jørgensen 2000;
Rainboth et al. 1975;
Rainboth 1996

Amblyrhynchi chthys

truncatus Pelagic spawner, spawns in floodplains and mainstreams of large rivers during the wet season. Warren 2000

Barbonymus
altus It spawns pelagically in floodplains
and flooded forest s , or among flooded vegetation

, and has buoyant
or semi

- buoyant eggs, with an initial
diameter of 0.74 mm. Warren 2000; Baird et al. 1999; Duangsawasdi et al.
198 828 Cirrhinus microlepis It is a pelagic spawner and the eggs are buoyant or semi

- buoyant

. Warren 2000; Pinyoying 1970

Cirrhinus
molitorella It is a pelagic spawner, which produces buoyant or semi

- buoyant

eggs. Warren, 2000

Cyprinus carp io Open water/substratum egg scatterers with a fecundity of 281,936 eggs/kg

. Skelton 1993

Labeo dyocheilus Th is fish species spawns in August

September. It spawns pelagically in
the mainstream in the wet season. Baird et al. 1999;
Warren 2000

Puntioplites
falcifer The spawning season is protracted with eggs reported in the period from
March

- December (with most reports from May

- June), and 2 cm long juveniles reported all year (although in highest occurrence is from May to November). It is a pel agic spawner which lays buoyant or semi

- buoyant

eggs. MFD 2003

Monopterus
albus A bubble nest builder at the water surface near the shoreline during the rainy season. Rainboth 1996; Talwar and Jhingran 199229 4

. 4 S ediments and migration

Ch ange s in turbidity or water color are a migration trigger for at least nine of the Mekong migratory species. In the Mekong, 87% of the species whose migration status is known (i.e. 135 species) are migra tory species characterized by longitudinal or lateral migr ations; out of these, more than 100 species are long

- distance migrants (Ziv et al. 2012 ) . Change s in turbidity or water color at the beginning of the rainy season are a migration trigger for at least nine of these species , which makes these species possibl y sensitive to change s in sediment concentration. These species are: Pangasius polyuranodon, Pangasianodon gigas and Pangasius bocourti ( three catfishes, including a critically endangered species), Cyclocheilichthys enoplos and Paralaubuca typus (two cypri nids very dominant in catches), Bangana behri, Labeo chrysophekadion, Mekongina erythrospila and Tenualosa thibaudeaui (Baran 2006 )

.

Fish migrations can play a significant role in nutrient distribution in a river basin. In South America, some “black water” h ydrosystems are poor in nutrients (i.e. oligotrophic) with a limited in situ primary production, and yet they are characterized by a high fish biomass ( Goulding et al. 1988, Polis et al. 1997)

. In fact

, migratory fish es are the main vector of nutrient
transfer from nutrient

- rich turbid whitewater hydrosystems to nutrient

- poor clear blackwater systems in South America ( Kern and Darwich 1997, Winemiller and Jepsen 2004) and this system of "foodweb spatial subsidy" also c haracterizes mainstream

- floodplain exchanges (Junk 1999), temperate streams (Schuldt and Hershey 1995, Bilby et al. 1996) and estuaries (Deegan 1993)

In the Mekong a similar process of nutrient transfer from nutrient

- rich downstream floodplains towards more oligotrophic upstream tributaries via fish migrations is expected.
Thus at least 35% of the fish biomass is made of migra tory species ( Baran 2010 ) and the 3S system for instance is characterized by at least 89 migratory fish species representing 60% o f the fishermen’s total catch (Baran et al. 2011) ; i n the Tonle Sap River, more than 2000 tonnes of fish are harvested per day during the migration peak 6

. If this process is confirmed, then a
reduced sediment and nutrient load due to dam construction would result in a lower productivity of downstream floodplains, reduced fish migrations, and subsequently reduced nutrient transfer towards and productivity of the tributaries that are not directly subject to dams ( Figur e 12 ).

6 Common catch rates during the days preceding the January full moon: 350 kg per haul, one haul every 15 mn for each “dai” net, 64 nets, 24 hours a day.30 Figure 12 : Reduction of nutrient transfers between river systems through reduced fish migrations.

4

. 5 Sediment s and fish habitat

Fish habitat in tropical river s should not be seen as a unique geographical area, but as group s of physically separated biotopes (feeding, spawning and nursery areas, dry season refuges) (Poulsen and Valbo

- Jorgensen, 2001, Hortle 2009), plus the water quality inherent to these
biota. Sediment plays a role in the characteristics of these habitats :

• geomorphology and bed substratum are determin ed by sediment loads;

• water turbidity is directly correlated with suspended sediment;

• pH of the water can be influenced by sediment.

Sediment load play s a major role in the geomorphology of river banks and river bed s , two drivers of aquatic organisms’ communities, in particular fish. The composition of aquatic organism assemblages basically reflects i) the nutrient inputs and ii) the physical form of the river system (Welcomme 1985). The reduction of the upstream sediment load results in large

scale changes in the downstream riverbed , the latter being a dynami c system in which sediments removed from the channel are replaced by the inflow of sediments from upstream.
This sediment reduction in turn makes river microhabitats less suitable for aquatic organisms that require specific substrate compositions for repro duction and/or foraging (Bizer 2000

, Osmundson et al. 2002 ). Thus, the l ong

- term ecological consequences of dams and other
stream disturbances are related to such geomorphological changes (Trush et al. 1995
, Pitlick
and Van Steeter 1998 ).31 A modification of the Mekong sediment concentration may affect deep pool fish habitat s . In the Mekong, deep pools constitu t e a critical habitat for many fish species, especially during the dry season (Poulsen et al. 2002 a , Baran et al. 2005, Viravong et al. 2006, Baird 2006 )

. H ydropower development on the Mekong would result in reduced flows in the rainy season and therefore less flushing of the sediments accumulat ed each year in the deep pools, i.e.
increased sedimentation of deep pools ( Conlan et al. 20 08 ) . However
, dam construction would
also reduce sediment concentration s and downstream bedload waves , resulting in a slower or reduced sedimentation process in pools . The balance of these two phenomena and the overall impact on deep pools as fish habitats is not well known.

Turbidity influence s the distribution and abundance of fish in the main habitats of tropical rivers including river stream s , floodplains, estuar ies and river plume s
. Several studies conducted in tropical floodplains have indicated that water transparency is a good predicator of fish assemblage composition and distribution (e.g. Tejerina

- Garro et al ., 1998; Pouilly and Rodríguez 2004). Turbidity is one of the parameters that strongly influence s the distribution
and abundance of communities in estuarine water s (K ennish 1990, Lévêque and Paugy 1999);
thus D iouf (1996) based on a review of 50 studies in estuarine areas, shows that in 75% o f the cases turbidity is one of the structuring factor s of fish communities

.

The c ommunity structure and abundance of fish species adapted to naturally turbid regimes can be affected by reduced sediment loads and flow regime regulation. Studies in S outh
A merican floodplain s showed seasonal changes in the relative abundance of visually and non

visually oriented fish groups following water turbidity fluctuations (Tejerina

- Garro et al . 1998).

In the Missouri R iver fish populations declined following the degradation of their habitats by river flow stabilization and sediment load reduction (Cross and Moss 1987, Pfleiger and Grace 1987, Hesse et al. 1993)

.

Sediment concentration influences water pH. In most tropical rivers
, the ionic composition of
the water derives primarily from the rain and the rock or sediments over which river flows. ( Welcomme 1985). Fine sediment particles adsorb and neutralize dissolved humic acids (Ertel et al. 1986) and help maintain pH at values compatible with fish survival. Modifications in water pH can affect reproduction and other biological processes of fish. Extreme pH values, i.e. those
below 4.5 or above 9.5
, alter aquatic organisms’ ionic regulation and are usually lethal (Gonzalez, 1996

, Val et al. 1999)

In the Tonle Sap the re are only 20 fish species whose ecological dependency on sediment is
known

. We reviewed for the present report the ecology of Mekong species in relation to sediments, as compiled in a literature review of 296 Tonle Sap species (period covere d: 1938

- 2003; Baran et al. 2007 c ) and detailed in FishBase ( www.fishbase.org ) and the Mekong Fish Database ( MFD 2003)

. Findings are detailed below ( Table 5 )

.32 Table 5 : Tonle Sap species that are ecolog ically dependent on sediments.
Species whose ecology is dependent on sediments
Hypsibarbus lagleri May migrate into flooded forest s immediately adjacent to rivers, but
does not occur over fine

- grained sediments, preferring rocks instead. Rainboth 1996

Homaloptera
smithi Adults are found in high gradient streams over fast bedrock, cobble runs, and rapids, and juveniles occur in slower stretches of gravel and exposed tree roots. Rainboth 1996; Krachangdara 1994;
Rainboth 1996

Crossocheilus
atrilimes Usually associated with clear, relatively fast waters with gravel or
boulders. Kottelat 2001

Opsarius
koratensis Found over gravel substrate in clear, swift, small streams on rapidly flowing stretches of large rivers, especially areas with many rocks. Rainboth, 1996; Baird
et al. 1999

Tor sinensis Occurs in pools and runs over gravel
and cobble in clear rivers in forest

areas. Rainboth 1996

Schistura pellegrini Inhabits shallow, clear, fast

- flowing water over rocky bottoms in upland

streams. Rainboth 1996

Garra
fasciacauda Inhabits rocky bottoms in fast flowing water of all sizes of rivers
and streams. MFD 2003

Garra cambodgiensis Inhabits rocky bottoms in swiftly moving water of small and medium

sized streams. MFD 200333 Setipinna
melanochir Becomes abundant in the Middle Mekong when the water levels rise and turbidity increases. Inhabits shallow, clear, fast

- flowing water over rocky bottoms in upland

streams. Rainboth 1996

Macrognathus
maculatus Found in lowland streams and peats.
Often occurs in clear water over rocky bottom s in flowing streams. Vidthayanon 2002

Mystacoleucus marginatus Found at bottom depths of rivers
and streams. Inhabits areas with sand or gravel from small streams to large river s

. Rainboth 1996

Hypsibarbus
malcolmi Found in large rivers in the dry
season and moves to medium

- sized rivers in the wet season. Usually
found over coarse substrate. MFD 2003

Glossogobius

aureus Found in rivers in clear to turbid water, usually over sand or gravel
bottoms and mud. Allen 1989; Allen et al.
2002

Mastacembelu
s favus Inhabits flowing waters. Most often found over gravel substrates where it buries itself during the day. MFD 2003

Tuberoschistur

a cambodgiensis Inhabits sandy

- bottomed streams. Previously found in a flowing stream with sandy bottom between Siem Reap and Kompong Thom near the Great Lake. Also found in sandy
bottomed streams south of Phnom
Penh. Rainboth 199634 Hemibagrus

nemurus This species occurs in most habitat types, but most frequent ly in large muddy rivers, with slow current s and
soft bottom s

. Kottelat 1998

Channa striata Survives the dry season by burrowing in to the bottom mud of lakes, canals and swamps as long as skin and breathing apparatus remain
moist. Davidson 1975

Pangio anguillaris Found near the bottom over sand or silt substrate in debris and decaying vegetation. Spends much of its time buried in the sand or slowly foraging
across the surface. Rai n both 1996 ; Kottelat 1998

Macrognathus
siamensis Spends much of its time buried in the silt, sand, or fine gravel with only a part of its head protruding from the bottom, but it emerges at dusk to forage for food. Rainboth 1996

Macrognathus taeniagaster During the daytime, it spends much of its time buried in silt, sand, or fine gravel with only its snout and eyes protruding from the bottom, but it emerges at night to forage. Rainboth 1996

Species whose dependence on sediments is unclear
Syncrossus
helodes Found in small upland streams with fast currents, as well as at bottom depths of the Great Lake Rainboth 1996

Cyprinus carp io Feeds mainly by grubbing in
sediments
, whereby it often uproots aquatic plants and stir s up mud. Scott and Crossman 1973; Rainboth 199635 Lepidocephal ichthys
hasselti Inhabits slow

- moving, shallow
waters of canals and inundated floodplains, usually on or in muddy substrates. Lives in swamps and rivers including peats. Also found in mountain streams, preferring sandy

or gravelly bottom. Rainb o th 1996; Kottelat 2001; Vidthayanon 2002

Pseudobagari
us filifer This species is present in the Mekong B asin, living in turbid, sandy

- bottom lakes with moderate

current. Ng and Rainboth 2005

Nemacheilus pallidus This species is known to congregate in areas receiving direct sunlight such as shallow depths of small streams and rivers with sandy to muddy bottoms. It is found over sandy substrates with slow current. R ainboth 1996 ; Kottletat 2001

Polynemus aquilonaris Found on sandy or muddy bottoms
of freshwater rivers and estuaries. Motomura 2003

Species whose ecology is not dependent on sediment

Parachela oxygastroides This species and P. siamensis seem to be more tolerant of high amounts of suspended solids than P. maculicauda

or P. williaminae and are more common in habitats disturbed by farming activities. Rainboth 199636 

### 5 SEDIMENTS AND FISH P RODUCTION

Nutrients transported by sediment loads are essential to fish productivity in tropical rivers. Nutrients are essential to vegetation growth (phytoplankton, algae and plants), which constitutes the primary production level of aquatic food webs and determine the produ ctivity of higher trophic levels. In tropical rivers, the majority of these nutrients are transported by the suspended sediment load. We detail in this section the linkages between sediments, flood pulse s and fish production, and the consequences of reduc e d sediment load (oligotrophication) on river productivity.

The exceptionally high productivity of the river and its associated flood plains is explained by
the river flood pulse

. The flood pulse concept developed by Junk (Junk et al. 1989, Junk and
Wantze n 2004) and based on the Amazon hydrosystem focuses on the lateral exchange of water, nutrients and organisms between a river or lake and the connected floodplains. This concept a rgues that the annual flood pulse is the most important aspect and the most
b iologically productive feature of a river ecosystem

. The flood pulse is typically characterized by three main components: amplitude, duration and timing. Dam development results in reduc ed
sediment concentration downstream , but also in reduced amplitude of the flood and in a delayed flood (change in timing due to the filling of the dam reservoirs at the beginning of the rainy season). This combination of sediment retention and reduced hydrodynamic pulse may have a synergetic and negative effect on the overall productivity of the river downstream.

Figure 13 : Flood pulse in the Tonle Sap L ake. Source: Osmose NGO, Cambodia.37 The flood pulse concept is widely accepted as an explanation for the exceptional productivity of the Mekong floodplains and , in particular , the Tonle Sap, yet the Mekong system has some unique characteristics

. Multiple authors have referred to the flood pulse as the driver of the productivity of the Mekong floodplains (e.g. Lamberts 2001 and 2006, Poulsen et al. 2002 b
, Sverdrup

- Jensen 2002, Sark kula et al. 2004, Davidson 2006, Baran et al. 2007 a , Kummu et al. 2008, TKK and SEA

- START RC 2009, Hortle 2009 )

. Campbell et al. (2009) are the only authors
who reject this explanation, without providing many details. The relationship between the flood pulse and fish production has been underlined in particular for the Tonle Sap L ake ( Sarkkula and Koponen 2003, Lamberts 2006, Halls et al. 2008, Lamberts and Koponen 2008,
Kummu et al. 2008, Sarkkula and Koponen 2010 ). However, the Mekong system is distinct from
South American hydrosystems; in particular
, the Mekong flood plains are covered with rice fields and are home to 11.3 million people, resulting in the release of local nutrients via artificial fertilizers, untreated sewage (60 million people, MRC 2003) and livestock (24 million heads,
Nesbitt et al. 2004). However

, it remains unclear whether these new sources of nutrient s can compensate for th ose lost due to sedimentation behind dams. Furthermore
, these expected additional nutrient inputs, in particular the phosphorus inputs, have to be analyzed in the
context of the upcoming supply crisis expected at the global scale in the medium term ( Roberts and Stewart 2002, Vaccari 2009, Cordell et al. 2009, http://phosphorusfutures.net )

If an abundant literature exists on eu trophication or naturally oligotrophic hydrosystems, very few studies have addressed dam

- driven oligo trophication. Oligotrophication is the process by which a hydrosystem becomes nutrient

- deficient and less productive, i.e. the opposite of the more common “ eutrophication”. While eutrophication was considered in the last decades as one of the major environmental issue s for hydrosystems under human influence, oligotrophication was perceived as an aesthetic improvement resulting in “clean” water, though from a fishery perspective, it also implies low productivity ( Stockner et al. 2000).

Oligotrophication is mainly related to p hosphorus deficiency, but the n itrogen/ p hosphorus
ratio is also an important factor

. Since p hosphorus is much less soluble than nitrogen and
leache s from the soil at a much slower rate, it is a more limiting nutrient of plankton and vegetation growth in aquatic systems than n itrogen. Consequently, oligotroph ication most often results from a decrease in p hosphorus, in particular in temperate systems. However , in some tropical aquatic systems n itrogen could also be the limiting factor (Moss, 1969, Guildford
et al. 2003
, Lehman and Branstrator 1993)

Oligotrophi cation results in changes to the food web structure (Stockner 1987

, Tonn 1990
, Mann 1993 in Stockner et al. 2000). Nutrient

- rich and productive hydrosystems (e.g. coastal upwellings, eutrophic lakes) are usually characterized by short food chains and productive fish resources (Cushing 1989
, Ware and Thomson 1991). In hydrosystems that are becoming
nutrient

- poor , primary production declines and food chains become less energy efficient and
become longer (Pomeroy 1974, Stockner and Porter 1988, Weisse and Stockner 1993).38 Oligotrophication results in declining fisheries resource s . Marked declines in yields from inl and
fisheries over the last 20

– 30 years in Europe and North America have largely been attributed to oligotrophication (Hartmann and Quoss 1993
, de Bernardi et al. 1995
, Ashley et al. 1997
, Hess
et al. 1982 ). Oligotrophication following dam construction also impacts fish production in the
coastal zone ( Marmulla 2001) . Thus
, in Cuba the reduction in sediment flow following the combination of river damming and reduced fertilizer use cause d a dramatic decrease in the catches of estuarine species (Baisre and Arboleya 2006). In Brazil

, a cascade of dams on the Sao
Francisco river trapped 95% of the suspended sediment

, reduced the nutrient concentration in the river plume by 90% and caused the estuary to become transparent, oligotrophic and

un productive (Knoppers et al. 2012 ) . In Egypt, oligotrophication due to sediment trapping by the Aswan Dam has had a catastrophic effect on coastal fisheries at the mouth of the Nile River (El

- Sayed and van Dijken 1995, Bebars et al. 1996)

In the Mekong the impact of oligotrophication on fish resources remains to be assessed. In particular
, the role of phosphorus as a possibly constrained and limiting nutrient has not been established in the Mekong ( Ongley 2009) . Overall dam

- driven oligotrophication is expected to significantly affect primary production and consequently the fish productivity of the Mekong;
this correlation has been clearly established in the case of the Tonle Sap L ake (Sarkkula and Koponen 2010). However f ish productivity decline is expected to be relatively more important in the Mekong Delta and in the sea area than in the Tonle Sap Lake (Sarkkula and Koponen 2010).

Overall, fish production in the Mekong is a multifactor phenomenon

. It is clear that dam development will reduce the downstream sediment concentration, but will also minimize the flood pulse, alter water quality and block fish migrations. Fishery productivity resulting from these four drivers will be affected in multiple ways that cannot be re lated to sediments only ( Figure 14 ) . Overall
, our literature review confirms the major role

— previously
under appreciated

— played by sediments and the need for new research to better assess the impact of oligotrophication on fish eries productivity. Ongoing hydrodynamic and physical modeling will clarify the retention rate of dams and the fate of sediments; however

, what has

not yet been addressed is the coupling between sediments and nutrients, and the amount of nutrients independ ent of sediment.

Figure 14 : Relationship between dams, environmental or biological impacts and fish productivity.39 

### 6 CONCLUSIONS

In the Mekong, the total annual sediment discharge is about 160 million tonnes per year on average, with a wide range of intra

- annual as well as inter

- annual variability. Sediments in rivers are found in two main forms (suspended and bedded sediments) and they consist of a variety of particle sizes (roughly categorized as gravel, sand, clay and silt)

. Most of the sediment load in the Mekong is suspended.

One property of sediments is that they adsorb and thus facilitate the transport of nutrients (in particular phosphorus and nitrogen) and organic particles. As dissolved nutrients in the water are generally low, the main nutrient load is associated with sediments. T he dominant sediment categories in the Lower Mekong Basin are fine sands, silts and clays ; the proportion of organic particles represents 6

- 8% of the total sediment load , which is considered high by global
standards

.

The Mekong is a place of exceptional fish biodiversity

. Fish biodiversity increases downstream and the 3S system is a hotspot. The Mekong produces about 18% of the world’s freshwater fish yield. Mekong fish resources are essential to food security in the basin

. Fish migrations are an essential feature of the Mekong

. Sediments influence fishes’ biological functions, partic ular ly respiration, nutrition, reproduction and migration, but also their habitat. Nutrient load is also a dominant factor explaining the overall productivity of tropical river ecosystems (including floodplains, estuaries and coastal areas).

Respiration. The organic component of sediments has a direct negative impact on dissolved oxygen through its Oxygen Demand. Suspended sediments and adsorbed nutrients also influence the amount of dissolved oxygen in the water through their interference with plant photosynthesis

.

Nutrition. Sediments are the basic input for the food chains and trophic webs that fish are part of at a higher level, and thus are an essential component of fish nutrition. Suspended sediments and the turbidity they cause reduce photosyn thesis and thus decrease primary production. Suspended sediments also play a role in predator

- prey relationship s

. In the Mekong, a reduction of sediment loads is likely to result in decreased primary production
, which might in

turn change the dominant fish species and drive down fish productivity. But the relative influence of nutrients loads and turbidity on primary production must be clarified for this potential impact to be assessed

.

Reproduction. The main relationship between fish reproduction and sed iment s is through spawning grounds. In estuaries high turbidity is favorable to the larvae of many coastal fish species. There is little information in the literature about the relationship between tropical fish reproduction and the impact of a reduced sed iment load.40 Migrations. Changes in turbidity or water color are a migration trigger for at least nine of the migratory species in the Mekong

. Fish migrations can play a significant role in the nutrient
distribution in the river basin. In the Mekong fish constitute a nutrient transfer from nutrient

rich downstream floodplains towards more oligotrophic upstream tributaries.

Habitat. Sediment load play s a major role in river geomorphology and in the riverbed and
ri verine habitats. Blocking sediment load will degrade the river bed downstream and will
therefore affect the aquatic organism communities adapted to these specific habitats. In the Mekong
, deep pools, an essential habitat for the survival of numerous fish species during the dry sea son, could be affected by modified water flow s that result in an infilling of the se pools, though this impact remains to be clarified. Water pH is also a characteristic of fish habitat s that is related to suspended sediment concentration s

.

The river flood pulse concept explains the exceptionally high productivity of river s and their associated floodplains. It has been widely accepted as describing the exceptional productivity of the Mekong. Dam development is expected to reduce `the flood pulse as well as downstream
sediment concentration s ; this combination may have a synergetic and negative effect on the overall productivity of the river downstream, leading to its oligotrophication

. This process of nutrient rarefaction, poorly studied, changes food web str ucture s and decreases fisheries

resource s . Oligotrophication is most commonly related to p hosphorus deficiency, but the

n itrogen/ p hosphorus ratio is also to be considered

. Ongoing hydrodynamic and physical modelling will also clarify the retention rate of dams and the fate of sediments, but the coupling between sediments and nutrients and the role of the amount of nutrient s that are independent of sediment s remain to be addressed.41 

### 7 BIBLIOGRAPHY

Alexander G.R., Hansen E.A. 1986. Sand bed load in a broo k trout stream. North American J. Fisheries Management. 6; 9

- 23.
Allen D., Darwal W., Dubois M., Kimsreng K., Lopez A., Mclvor A., Springate

- Baginski O., Thuon Try 2008. Intergrating people in conservation planning: an intergrated assessment of biodiversity, livelihood and economic implications of the proposed special management zones in the Stung Treng Ramsar Site, Cambodia. International Union for Conservation of Nature and Natural Resources. Cambodia Country Office, Phnom Penh. 111 pp.

Allen G .R. 1989. Freshwater fishes of Australia. T.F.H. Publications, Inc., Neptune City, New Jersey, USA. 240 pp.
Allen G.R., Midgley S.H., Allen M. 2002. Field guide to the freshwater fishes of Australia. Western Australian Museum, Perth, Western Australia. 410 pp. Amatyakul C., Kohanantakul K., Sansrimahachai C., Sumanochitraporn S., Sripatrprasite P., Wongsongsarn R., Wongsongsarn C., Juntana J., Rodrarung D., Yoorong N.N. 1995. Jullien Carp Probarbus jullieni Sauvage. Inland Fisheries Division, Department of Fisheries, Ministry of Agriculture and Cooperatives, Thailand. 51 pp.
Appleby J.P., Scarratt D.J. 1989. Physical effects of suspended solids on marine and estuarine fish and shellfish with special reference to ocean dumping: a literature review. Canadian T echnical Report of Fisheries and Aquatic Sciences. 1681; 66 pp.
Ashley K.A., Thompson L.C., Lasenby D.C., McEachern L., Smokorowski K.E., Sebastian D. 1997. Restoration of an interior lake ecosystem: the Kootenay Lake fertilization experiment. Wat. Qual.
R es. J. Can. 32; 192

- 212.
Baird I.G. 2001. Aquatic biodiversity in the Siphandone wetlands. Pp. 61

- 74. In Daconto G. (ed.): Siphandone Wetlands. CESVI, Bergam, Italy. 192 pp.
Baird I.G. 2006. Strength in diversity: fish sanctuaries and deep

- water pools in L ao PDR. Fisheries Management and Ecology. 13; 1

- 8.
Baird I.G., Flaherty M.S., 2000. Local waters, international markets, a review of the sustainability of a Mekong River probarbus fishery in Southern Laos. Technical Report of the CESVI Project for
Environm ental Protection and Community Development in the Siphandone Wetland, Champassak Province, Lao PDF. Cooperazione e sviluppo (CESVI), Bergamo, Italy. 25 pp.
Baird I.G., Inthaphaisy V., Kisouvannalath P., Phylavanh B., Mounsouphom B. 1999. The fishes of
Sout hern Laos. Lao Community Fisheries and Dolphin Protection Project. Ministry of Agriculture and Forestry, Lao PDR. 161 pp.
Baird I.G., Phylavanh B. 1999. Fishes and forests: fish foods and importance of seasonally flooded riverine habitats for Mekong River fish. Technical Report, Environmental Protection and Community Development in the Siphandone Wetland, Champasak Province, Lao PDR. Funded by European Union, implemented by CESVI, Bergamo, Italy. 46 pp.
Baird I.G., Phylavanh B., Vongsenesouk B., Xaiamanivon g K. 2001. The ecology and conservation of the smallscale croaker Boesemania microlepis (Bleeker 1858

- 59) in the mainstream Mekong River, Southern Laos. Nat. Hist. Bull. Siam Soc. 49; 161

- 176.
Baisre J.A., Arboleya Z. 2006. Going against the flow: effects of river damming in Cuban fisheries. Fisheries Research. 81; 283

- 292.42 Balon E.K. 1975. Reproductive guilds of fishes: a proposal and definition. J. Fish. Res. Board Can. 32 (6); 821

- 864.

Balon E.K. 1981. Additions and amendments to the classification of re productive styles in fishes. Env. Biol. Fish. 6 (314); 377

- 389.
Banyen S. 1988. Reproductive biology of Pla

- kamung Puntioplites proctozysron (Bleekers) in Ubolratana Reservoir. M.Sc. Thesis. Faculty of Graduates. Kasetsart University, Thailand. 66 pp.
Bany en S., Chomchaey C., Girdgomut B. 1989a. Study on length

- weight relationship of fishes in Kraseio Reservoir, Suphan Buri Province. Technical Paper No. 151, National Inland Fisheries Institute, Ministry of Agriculture and Cooperatives, Thailand. 34 pp.
Bany en S., Wattanadilogkul K., Ketkomut B., Keawcharoon P. 1989b. Biology of pla Kamung, Puntioplites proctozysron (Bleeker), in Ubolratana reservoir. Technical paper No. 111, National Inland Fisheries Institute, Department of Fisheries, Ministry of Agricultur e and Cooperatives, Thailand. 46 pp.
Baran E. 1995. Spatio

- temporal dynamics of estuarine fish assemblages in Guinea

- relationships
with abiotic factors

- . TDM vol. 142, Éditions ORSTOM, Paris. 236 pp.
Baran E. 2006. Fish migration triggers in the Lower Me kong Basin and other tropical freshwater system. Mekong River Commission, Vietiane, Lao PDR. MRC Technical Paper. 14; 56 pp.

Baran E. 2010. Mekong fisheries and mainstream dams. Contribution to the Strategic Environmental Assessment of hydropower on the Me kong mainstream for the Mekong River Commission. International Center for Environmental Management, Glen Iris, Victoria, Australia. 157 pp.
Baran E., Baird I.G., Cans G. 2005. Fisheries Bioecology at the Khone Falls (Mekong River, Southern Laos). WorldFish Center, Penang, Malaysia. 84 pp.
Baran E., Jantunen T., Chong C.K. 2007b. Values of inland fisheries in the Mekong River Basin. WorldFish Center, Phnom Penh, Cambodia. 76 pp. Also p. 227

- 290 in Neiland A.E. and Béné C. (eds) 2008. Tropical river fisheries valuation: background papers to a global synthesis. The WorldFish Center Studies and Reviews no1836. Penang, Malaysia. 290 pp.
Baran E., Saray Samadee, Teoh Shwu Jiau, Tran Thanh Cong. 2011. Fish and fisheries in the Sesan
River Basin

- Catchment baseline , fisheries section. Project report. Mekong Challenge Program project MK3 “Optimizing the management of a cascade of reservoirs at the catchment level”. WorldFish Center, Phnom Penh, Cambodia. 61 pp.
Baran E., So N., Arthur R., Leng S.V., Kura Y. 2007c. Bi oecology of 296 fish species of the Tonle Sap Great Lake (Cambodia). Report of the project “Study of the Influence of built structures on the fisheries of the Tonle Sap”. WorldFish, Cambodia National Mekong Committee and Asian Development Bank. 166 pp.

Bar an E., Starr P., Kura Y. 2007a Influence of built structures on Tonle Sap fisheries. Cambodia National Mekong Committee and WorldFish Center, Phnom Penh, Cambodia. 44 pp.
Baran E., Un Borin. In press. Mekong River: importance of the fish resource and examples of best practice. In Wanningen H. (ed.): “From sea to source

- guidance for the restoration of fish migration in rivers”. Hunze and Aa's Water Authority, the Netherlands.
Bebars M.I., Lasserre G., Lam Hoai T. 1996. Analyse des captures des pêcheri es marines et lagunaires d' Egypte en liaison avec la construction du haut barrage d' Assouan. Oceanologica Acta. 20 (2); 421

- 436.43 Bezuijen M.R., Timmins R., Seng T. 2008. Biological surveys of the Mekong River between Kratie and Stung Treng Towns, Northea st Cambodia, 2006

- 2007. WWF Greater Mekong, Cambodia Country Programme, Fisheries Administration and Forestry Administration, Ministry of Agriculture, Forestry and Fishery, Phnom Penh, Cambodia. 220 pp.
Bilby R.E., Fransen B.R., Bisson P.A. 1996. Incorpora tion of nitrogen and carbon from spawning coho salmon into the trophic system of small streams: Evidence from stable isotopes. Canadian Journal of Fisheries and Aquatic Sciences 53; 164

- 173.
Bizer J.R. 2000. International mechanisms for avoiding, mitigatin g and compensating the impacts of large dams on aquatic and related ecosystems and species. Review prepared for Thematic Review II.1 (Dams, ecosystem functions and environmental restoration) of the World Commission on Dams. Cape Town, South Africa. 92 pp.
Blaber S.J.M., Blaber T.G. 1980. Factors affecting the distribution of juvenile estuarine and inshore fish. J. Fish Biol. 17; 143

- 162.
Blaber S.J.M., Young J.W., Dunning M.C. 1985. Community structure and zoogeographic affinities of
the coastal fishes of t he Dampier region of north

- western Australia. Austral. J. Mar. Freshwat. Res. 36; 247

- 266.
Campbell I.C., Say S., Beardall J. 2009. Tonle Sap Lake, the heart of the Lower Mekong. Pp. 251

- 272. In Campbell (ed.): The Mekong biophysical environment of an inte rnational river basin; Elsevier, Amsterdam, Netherland. 432 pp.
Carling P.A. 2009. Geomorphology and sedimetology of the Lower Mekong River. Pp. 77

- 111. In Campbell (ed.): The Mekong biophysical environment of an international river basin. Elsevier,
Amster dam, Netherland. 432 pp.
Carolsfeld J., Harvey B., Ross C., Baer A. 2003. Migratory fishes of South America: biology, fisheries and conservation status. International Development Research Center, Ottawa, Canada, and the World Bank, Washington DC, USA. 272 pp. Castro J., Reckendorf F. 1995. Effects of Sediment on the Aquatic Environment: potential NRCS Actions to Improve Aquatic Habitat. Working Paper. 6; NRCS, United States Department of Agriculture, USA.
Chea Tharith, Hortle K.O., Thach Phanara, Em Samy 2 005. Drift of larval and juveniles fish in the Mekong main stream near Phnom Penh. Sustainable Development in the Mekong River Basin, 6th

- 7th October 2005. 77

- 79.
Chevey P. 1933. The Great Lake of Cambodia: the underlying causes of its richness in fish.

P roceedings of the Fifth Pacific Science Congress. 3809

- 3816.
Conlan I., Rutherfurd I., Finlayson B., Western A. 2008. The geomorphology of deep pools on the
lower

- Mekong River: controls on pool spacing and dimensions, processes of pool maintenance and pote ntial future changes to pool morphology. Final report submitted to the Mekong River Commission Secretariat, Vietiane, Lao PDR. 98 pp.
Cordell D., Drangert J.

- O., White S. 2009. The story of phosphorus: Global food security and food for thought. Global Envi ronmental Change 19; 292

- 305.
Cross F.B., Moss R.E. 1987. Historic changes in fish communities and aquatic habitats in plains streams of Kansas. Pp. 155

- 165. In Matthews W.J., Heins D.C. (eds.): Community and Evolutionary Ecology of North American Stream F ishes. University of Oklahoma Press. 384 pp.44 Cushing D.H. 1989. A difference in structure between ecosystems in strongly stratified waters and in those that are only weakly stratified. J. Plankton Res. 11; 1

- 13.
Cyrus D.P., Blaber S.J.M. 1987a. The influen ce of turbidity on juvenile marine fishes in estuaries. Part 1. Field studies at Lake St. Lucia on the southeastern coast of Africa. Exp. Mar. Biol. Ecol. 10953

- 70.
Cyrus D.P., Blaber S.J.M. 1987b. The influence of turbidity on juvenile marine fishes in es tuaries. Part 2. Laboratory studies, comparisons with field data and conclusions. Exp. Mar. Biol. Ecol. 10971

- 91.

Davidson A. 1975. Fish and fish dishes of Laos. Imprimerie Nationale Vientiane, Lao PDR. 202 pp.
Davidson P.J.A. (compiler) 2006. The biodiversity of the Tonle Sap biosphere reserve. 2005 status review. Wildlife Conservation Society Report. 76 pp.
de Bernardi R., Calderoni A., Mosello R. 1995. Environmental problems in Italian lakes, and lakes Maggiore and Orta as successful examples of correct management leading to restoration. Verh. Int. Ver. Limnol. 2; 145

- 160.
de Melo C.E., Lima J.D., da Silva E.F. 2009. Relationships between water transparency and abundance of Cynodontidae species in the Bananal floodplain, Mato Grosso, Brazil. Neotr opical Ichthyology. 7 (2); 251

- 256.

de Merona B., dos Santos G.M., de Almei da R.G. 2001. Short term effects of Tucuruí Dam (Amazonia, Brazil) on the trophic organization of fish communities. Environmental Biology of Fishes

. 60 (4 ); 375

- 392.
Deegan L.A. 1993. Nutrient and energy transport between estuaries and coastal marine ecosystems by fish migration. Canadian Journal of Fis heries and Aquatic Sciences 50; 74

- 79.
Diouf P.S. 1996. Les peuplements de poisons des milieux estuariens de l'Airique de l'Ouest l'exemple de l'estuaire hyperhalin du Siné

- Saloum. Paris, Orstom, Travaux et documents microfiches. 156; 267

pp. Duangsawasdi S. et al. 1988. Reproductive biology of some economic freshwater fishes in the Lower Chao Phraya River. Proceeding of the Seminar on Fisheries 1988, Department of Fisheries, Bangkok,
Thailand. 296

- 303.
Dubowski Y. 1997. The environmental effects of the hig h dam at Aswan. Quarterdeck (on line). 5 pp.

El

- Sayed S., van Dijken G.L. 1995. The southeastern Mediterranean ecosystem revisited: thirty years after the construction of the Aswan High Dam. Quarterdeck (on line). 3 (1); 4 pp.
Ertel J.R., Hedges J.I., Devol A.H., Richey J.E. 1986. Dissolved humic substances of the Amazon River system. Limnology and Oceanography. 31; 739

- 754.
Forsberg B.R., Devol A.H., Richey J.E., Martinelli L.A., Santos H. 1988. Factors controlling nutrient
concentrations in Amazon flo odplain lakes. Limnology and Oceanography. 33; 41

- 56.

Francis

- Floyd R. 2011. Dissolved oxygen for fish production. Institute of Food and Agricultural Sciences, University of Florida, USA. 3 pp.
Fruchart F. 2009. Impacts of main stem dams on the ecology and sediment balance of the Mekong River. WWF Greater Mekong Lao Country Programme. 38 pp.
Gonzalez R.J. 1996. Ion regulation in ion poor water of low pH. Pp. 111

- 122. In Val A.L., Almeida

- Val V.M., Randall D.J. (eds.): Physiology and Biochemistry of the Fish es of the Amazon. Instituto Nacional de Pesquisas da Amazoˆnia, Manaus, Brazil. 402 pp.
Goulding M., Carvalho M.L., Ferreira E.G. 1988. Río Negro: rich life in poor water. SPB Academic Publishing, The Hague, The Netherlands. 200 pp.45 Gray L.J., Ward J.V. 19 82. Effects of sediment releases from a reservoir on stream macroinvertebrates. Hydrobiologia. 96; 177

- 184.

Griffin http://www.biolbull.org/content/216/2/175.full

- aff

- 1 F.J., Smith E.H., Vines C.A., Cherr G.N. 2009. Impacts of suspended sediments on fertilization, embryonic development, and early larval life stages of the Pacific Herring, Clupea pallasi. Biol. Bull. ; 216 (2); 175

- 187.
Guildford S.J., Hecky R.E., Taylor W .D., Mugidde R., Bootsma H.A. 2003. Nutrient enrichment experiments in tropical great lakes Malawi/Nyasa and Victoria. Journal of Great Lakes Research 29 (Suppl. 2); 89

- 106.
Halls A.S., Lieng S., Ngor P., Tun P. 2008. New research reveals ecological insigh ts into dai fishery. Catch and Culture. 14 (1); 8

- 12. Hartmann J., Quoss H. 1993. Fecundity of whitefish (Coregonus lavaretus) during the eu and oligotrophication of Lake Constance. J. Fish Biol. 43; 81

- 87.
Hess L.W., Schlesinger A.B., Hergenrader G.L., Re etz S.D., Lewis H.S. 1982. The Missouri River Study

- Ecological Perspective. In: The Middle Missouri River. Missouri River Study Group (eds L.W. Hess, G.L. Hergenrader, H.S. Lewis, S.D. Reetz and A.B. Schlesinger). Pp. 287

- 301. Nebraska Game and
Parks Com mission, Lincoln, Nebraska (U.S.A.). 301pp.
Hesse L.W., Mestl G.E., Robinson J.W. 1993. Status of selected fishes in the Missouri River in Nebraska with recommendations for their recovery. Pp. 327

- 340. In Hesse L.W., Stalnaker C.B., Benson N.G., Zuboy J.R. (eds): Restoration Planning for the Rivers of the Mississippi River Ecosystem. Washington (DC): US Department of Interior, National Biological Survey. Biological Report 19.
Hortle K.G. 2009. Fisheries of the Mekong River Basin. Pp. 197

- 249 in Campbell (ed .): The Mekong biophysical environment of an international river basin; Elsevier, Amsterdam, Netherland. 432 pp.
ICEM (International Center for Environmental Management). 2010. Strategic environmental assessment of hydropower on the Mekong mainstream. Fina l report. Mekong River Commission, Hanoi, Viet Nam. 197 pp.
Irvine K.N., Droppo I.G., Sampson M., Sarkkula J. 2007. Water quality monitoring and non destructive suspended sediment size analysis in the Chaktomuk Junction

- Tonle Sap Lake Corridor,
Cambodia. O ral Presentation Proceedings, Fifth International Symposium on Southeast Asian Water Environment, 2007, Chiang Mai, Thailand. 32

- 39.
Isono R.S, Kita J., Setoguma T. 1998. Acute effects of kaolinite suspension on eggs and larvae of
some marine Teleosts. Co mparative Biochemistry and Physiology Part C: Pharmacology, Toxicology and Endocrinology. 120 (3); 449

- 455.
Junk W., Bayley P., Sparks R. 1989. The flood pulse concept in river

- floodplain systems. Canadian Special Publications in Fisheries and Aquatic Scie nces. 106; 110

- 127.
Junk W.J. 1999. The flood pulse concept of large rivers: learning from the tropics. Large River: Arch. Hydrobiol. 11 (Supp. 115/3); 261

- 280.
Junk W.J., Wantzen K.M. 2004. The flood pulse concept: new aspects, approaches and applications

-- an update. RAP Publication 2004/16; 117

- 140.
Kennish M.J. 1990. Ecology of estuaries. Biological aspects. Boca Raton, CRC Press. 2; 391 pp.
Kern J., Darwich A. 1997. Nitrogen turnover in the Varzea. Ecological Studies. Pp. 119

- 135. In Junk W.J. (ed.): Th e central Amazon floodplain: ecology of a pulsing system. 126; 525 pp.46 Kerr S.J. 1995. Silt,turbidity and suspended sediments in the aquatic environment: an annotated bibliography and literature review. Ontario Ministry of Natural Resources, Southern Regio n Science & Technology Transfer Unit Technical Report TR

- 008. 277 pp.
Ketelsen T., Ward P. 2010. Hydrology and sediment baseline. In Baseline assessment report, Strategic Environmental Assessment of hydropower on the Mekong mainstream for the MRC. ICEM (In ternational Centre for Environmental Management), Hanoi, Vietnam. 86 pp.
Knoppers B., Medeiros P.R.P., de Souza W.F.L., Oliveira E.N., da S. Fontes L.C., do Carmo M.S., Carvalho I.S., Silva M.C., Brandini N., Carneiro M.E. 2012. A review on the patterns of river material fluxes, coastal plume dispersal, shelf sediment facies, and anthropogenic impacts of the Tropical
Land

- Sea Interface, Sergipe/Alagoas, Northeast Brazil. Geophysical Research Abstracts. 14 (EGU2012

- 14257); 1 pp.
Koponen J., Kummu M., Sarkkul a J. 2005. Modelling environmental change in Tonle Sap Lake, Cambodia. Verh. Internat. Verein. Limnol. 29; 1083

- 1086.
Kottelat M. 1998. Fishes of the Nam Theun and Xe Bangfai basins, Laos, with diagnoses of twenty

two new species (Teleostei: Cyprinidae, B altoridae, Cobitidae, Coiidae and Odontobutidae). Ichthyol. Explor. Freshwat. 9 (1); 1

- 128.
Kottelat M. 2001. Fishes of Laos. Wildlife Heritage Trust, Colombo, Sri Lanka. 198 pp.
Krachangdara T. 1994. Taxonomy and some biological aspects of fishes found in the impoundment of Rajjaprabha Dam, Surat Thani Province. M.Sc. Thesis. Faculty of Graduates, Kasetsart University, Thailand. 298 pp.
Kummu M., Keskinen M., Varis O. (eds) 2008. Modern myths of the Mekong: a critical review of water and development concep ts, principles, and policies. Water & Development Publications

Helsinki University of Technology. 187 pp.
Kummu M., Varis O. 2007. Sediment

- related impacts due to upstream reservoir trapping, the Lower Mekong River. Geomorphology. 85; 275

- 293.
Lagler K.F. 1976. Fisheries and Integrated Mekong River Basin Development. Field Investigations Appendix Volume 1. Annex E: Coastal Fishery Field Investigations, Terminal Report of the Mekong Basinwide Fishery Studies. University of Michigan School of Natur al Resourc es, Michigan, USA.
39 pp. Lamberts D. 2001. Tonle Sap fisheries: a case study on floodplain gillnet fisheries. Asia

- Pacific Fishery Commission. Food and Agriculture Organization of the United Nations, Bangkok, Thailand. RAP Publication 2001/11; 128 pp.
Lamberts D. 2006. The Tonle Sap Lake as a productive ecosystem. International Journal of Water Resources Development. 22 (3); 481

- 495.
Lamberts D., Koponen J. 2008. Flood pulse alterations and productivity of the Tonle Sap ecosystem: a model for impact ass essment.. Ambio; 37 (3); 173

- 184.
Lehman J.T., Branstrator D.K. 1993. Effects of nutrients and grazing on the phytoplankton of Lake Victoria. Verhandlungen der Internationale Vereinigung fur Theorctische und Angewandte Limnologie. 25; 850

- 855.
Lévêque C., Paugy D. (eds.) 1999. Les poissons des eaux continentales africaines: diversité, écologie, utilisation par l'homme. Institut de recherche pour le developpement. 534 pp.
Lloyd, D.S.; J.P. Koenings and J.D. LaPerriere. 1987. Effects of turbidity in fresh wat ers of Alaska. N. Amer. J. Fish. Mgmt. 7; 18

- 33.47 Lu X.X., Siew R.Y. 2005. Water discharge and sediment flux changes in the Lower Mekong River. Hydrol. Earth Syst. Sci. Discuss. 2; 2287

- 2325.
Mann K. H. 1993. Physical oceanography, food chains, and fish sto cks: a review. ICES J. Mar. Sci. 50;
105

- 119. Marmulla G. (ed.) 2001. Dams, fish and fisheries: opportunities, challenges and conflict resolution. Food and agriculture organization of the united nations Rome, Italy. FAO, Fisheries Technical Paper
419. 170 pp.
Meade R.H. 1996. River

- sediment inputs to major deltas. Pp. 63

- 85. In Milliman J.D., Haq B.U. (eds.):
Sea

- Level Rise and Coastal Subsidence. Kluwer Academic Publishers, Dordrecht. 369 pp.
Meador M.R., Carlisle D.M. 2007. Quantifying tolerance indicator values for common stream fish species of the United States. Ecological Indicators. 7; 329

- 338.
Meynell P.

- J. 2010. Aquatic systems baseline. In Baseline assessment report, Strategic Environmental Assessment of hydropower on the Mekong mainstream for the M RC. ICEM (International Centre for Environmental Management), Hanoi, Vietnam. 37 pp.
MFD (Mekong Fish Database) 2003. Mekong Fish Database. A taxonomic fish database for the Mekong Basin. Mekong River Commission, Vietiane, Lao PDR. CD

- Rom.

Moss B. 1969. Li mitation of algal growth in some central African waters. Limnology and Oceanography. 14; 591

- 601.
Motomura H. 2003. A new species of freshwater threadfin, Polynemus aquilonaris , from Indochina, and redescription of Polynemus dubius Bleeker, 1853 (Perciform es: Polynemidae). Ichthyol. Res. 50 (2); 154

- 163.

MRC (Mekong River Commission) 2001 MRC hydropower development strategy. MRC Water Resources and Hydrology Programme. 43 pp.
MRC (Mekong River Commission) 2003. State of the Basin Report 2003. Mekong River Commission, Phnom Penh, Cambodia. 300 pp.

MRC (Mekong River Commission) 2007. Diagnostic study of water quality in the Lower Mekong Basin. MRC Technical Paper No. 15. 69 pp.
MRC (Mekong River Commission) 2009. Preliminary design guidance for proposed mains tream dams in the Lower Mekong Basin. Mekong River Commission, Vientiance, Lao PDR. 38 pp.
Nesbitt H.J., Johnston R., Solieng M. 2004. Mekong River water: will river flows meet future agriculture needs in the Lower Mekong Basin. ACIAR Proceedings 116e. 86

- 104.
Newcombe C.P. 1994. Suspended sediment pollution: dose response characteristics of various fishes. Draft Report, Habitat Protection Branch, British Columbia Ministry of Environment, Lands and Parks, Victoria, British Columbia. 45 pp.
Ng H.H., Rainboth W.J. 2005. Four new species of Akysis (Teleostei: Siluriformes: Akysidae) from mainland Southeast Asia, with comments on A. similis . Raffles Bull. Zool. Supplement. 13; 33

- 42.
Ongley E.D. 1996. Control of water pollution from agriculture

- FAO irrigation and drainage paper 55. Food and Agriculture Organization of the United Nations, Rome, Italy. html.
Ongley E.D. 2009. Water quality of the Lower Mekong River. Pages 297

- 320 in Campbell (ed.): The Mekong biophysical environment of an international river basin; Elsevier, Amsterdam, Netherland. 432 pp.48 Osmundson D.B., Ryel R.J., Lamarra V.L., Pitlick J. 2002. Flow

– sediment

– biota relations: implications for river regulation effects on native fish abundance

. Ecological Applications . 12 (6); 1719

- 1739.
Ospina

- Álvarez A., Palomera I., Parada C. 2012. Changes in egg buoyancy during development and its effects on the vertical distribution of anchovy eggs. Fisheries Research

. 117

- 118 ; 86

- 95.
Palmieri A., Shah F., Annandale G.W., Dinar A. 2003. Reservoir conservation volume I: the RESCON approach economic and engineering evaluation of alternative strategies for managing sedimentation in storage reservoirs. A contribution to promote conservation of water storage assets
worldwi de, The International Bank for Reconstruction and Development /The World Bank, Washington, DC, USA. 101 pp.
Pantulu V.R. 1986. The Mekong river system. Pp. 695

- 720. In Davies B.R., Walker K.F. (eds.): The Ecology of River Systems. Dr. Junk Publishers, Dort recht, The Netherlands. 793 pp.
Parsons T.R., Takahashi M., Hargrave B. 1977. Biological oceanographic processes. Pergamon Press, Oxford. 332 pp.
Pavlov D., Mikheev V. N., Lupandin A. I., Skorobogatov M. A. 2008. Ecological and behavioural influences on ju venile fish migrations in regulated rivers: a review of experimental and field studies. Hydrobiologia. 609; 125

- 138.
Pavlov D.S., Nezdoliy V.K., Urteaga A.K., Sanches O.R. 1995. Downstream migration of juvenile fishes
in the rivers of Amazonian Peru. Journ al of Ichthylogy. 35 (9); 227

- 248.
Pennapaporn P., Nukua A., Ritthitham S., Gannaarong M. 1991. Breeding of Osteocheilus hasseltii (Cul and Val). Technical Paper No. 1, Khon Kaen Inland Fisheries Development Center, Department of Fisheries, Ministry of Agr iculture and Cooperatives, Thailand. 21 pp.
Pfleiger W.L., Grace T.B. 1987. Changes in the fish fauna of the lower Missouri River, 1940

- 1983. Pp.
166

- 177. In Matthews W.J., Heins D.C. (eds.) 1989.: Community and Evolutionary Ecology of North
American Strea m Fishes. University of Oklahoma Press. 320 pp.
Pholprasith S., Janesirisak S. 1972. The investigation of spawning ground of some species fishes in the Ubolratana Reservoir. Ubolratana Reservoir Fisheries Research Station. Inland Fisheries Division, Depart ment of Fisheries, Ministry of Agriculture and Cooperatives, Thailand. 15

- 21.
Pinyoying S. 1970. A preliminary observation on artificial spawning of Cirrhinus microlepis. Thai Fish. Gaz. 23 (1); 135

- 141.
Pitlick J., Van Steeter M.M. 1998. Geomorphology and endangered fish habitats of the upper Colorado River 2. Linking sediment transport to habitat maintenance

. Water Resources Research. 34 (2); 303

- 316.
Polis G.A., Anderson W.B., Holt R.D. 1997. Toward and inte gration of landscape and food web ecology: the dynamics of spatially subsidized food webs. Annual Review of Ecology and Systematics. 28; 289

- 316.
Pomeroy L.R. 1974. The ocean’s food web, a changing paradigm. BioScience. 24; 499

- 501.
Pouilly, M. & M. A. Rod ríguez. 2004. Determinism of fish assemblage structure in Neotropical floodplain lakes: influence of internal and landscape lake conditions. Pp. 243

- 265. In: Welcomme, R. and Petr T. (Eds.). Proceedings of the Second International Symposium on the Manageme nt of Large Rivers for Fisheries, vol. II. FAO, Bangkok, Thailand. 310 pp.
Poulsen A.F., Ouch P., Viravong S., Suntornratana U., Nguyen T.T. 2002b. Fish migrations of the Lower Mekong River Basin: implications for development planning and environmental man agement. Mekong River Commission, Vietiane, Lao PDR. MRC Technical Paper. 8; 1

- 62.49 Poulsen A.F., Ouch P., Viravong S., Suntornratana U., Nguyen T.T. 2002a. Deep pools as dry season fish habitats in the Mekong River basin. Mekong River Commission, Vietiane, Lao PDR. MRC Technical Paper. 4; 1

- 24.
Poulsen A.F., Valbo

- Jørgensen J. (eds.) 2000. Fish migrations and spawning habits in the Mekong
mainstream

– a survey using local knowledge. AMFC Technical Report, Mekong River Commission, Vietiane, Lao PDR. 149 pp.
Poulsen A.F., Valbo

- Jorgensen J. 2001. Deep pools in the Mekong river. Mekong Fisheries Network Newsletter. 7 (1); 3 pp.
Power M.E., Dietrich W.E., Finlay J.C. 1996. Dams and downstream aquatic biodiversity: potential food web consequences of hydrologic a nd geomorphic change. In Mattice J., Fraser M., Ragone S., Daugherty D., Wisniewski J.: Managing for biodiversity, Emerging ideas for the electric utility industry

- Su mmary Statement. 20 (6); 781

- 788.
Rainboth W.J. 1996. Fishes of the Cambodian Mekong. FAO Species Identification Field Guide for Fishery Purposes. FAO, Rome, Italy. 265 pp.
Rainboth W.J., Tucker H.G., Arden R.E., Drannon Buskirk Jr E., Hoagman W.J. 1975. P reliminary field guide to freshwater fishes of the Mekong River system. Mekong basinwide fishery studies, University of Michigan, USA. Working document. 6; 162 pp.
Randall J.M., Day Jr.J.W. 1987. Effects of river discharge and vertical circulation on aquat ic primary production in a turbid Louisiana (USA) estuary. Netherland Journal of Sea Research. 21 (3); 231

- 242.
Rithcharung S., Mahawong N. 1993. Study on natural breeding of Siamese tigerfish, Datnioides microlepis Bleeker. Thai Fisheries Gazette. 46 (6); 495

- 501.
Roberts T.E. 1992. Revision of the Southeast Asian cyprinid fish genus Probarbus, with two new species threatened by proposed construction of dams on the Mekong River. Ichthyol. Explor. Freshwat. 3 (1); 37

- 48.
Roberts T.L., Stewart W.M. 2002 Inor ganic Phosphorus and Potassium production and reserves Better Crops. 86; 2; 6

- 7. Root R. B. 1967. The niche exploitation pattern of the blue

- grey gnatcatcher. Ecol. Monogr. 37; 317

- 350.
Rowe M., Essig D., Jessup B. 2003. Guide to Selection of Sediment Targets for Use in Idaho TMDLs. Idaho Department of Environmental Quality, Idaho, the U.S. 84 pp.
Sanderson L. 2009. Changes in fish diversity due to hydrologic and suspended sediment variability in the Sandusky river, Ohio: a genetic programming approach. A Msc. Thesis Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of Master of Science. 81 pp.
Sarkkula J., Baran E., Chheng P., Keskinen M., Koponen J., Kummu M. 2004 Tonle Sap Pul sing System and fisheries productivity. Contribution to the XXIXe International Congress of Limnology (SIL 2004), Lahti, Finland, 8

- 14 August 2004.
Sarkkula J., Keskinen M., Koponen J., Kummu M., Richey J. E., Varis O. 2009. Hydropower in the Mekong region : what are the likely impacts upon fisheries? Pp. 227

- 249 in Molle F., Foran T., Käkönen M. (eds.): Contested waterscapes in the Mekong Region: Hydropower, livelihoods and governance. 426 pp.50 Sarkkula J., Koponen J. 2010. Final project. DMS

- Detailed Mode lling Support Project Contract #001

- 2009. Mekong River Commission/ Information and Knowledge Management Programme. Mekong River Commission, Vietiane, Lao PDR. 88 pp.
Sarkkula J., Koponen J., Keskinen M., Kummu M., Varis O. 2004. From Tonle Sap Lake to the Lower Mekong Basin. MRCS/WUP

- FIN Bridging Report. Mekong River Commission / Water Utilization Program Finnish Environment Consultancy Consortium April, 2004. 114 pp.
Sarkkula J., Koponen J., Lauri H., Virtanen M. 2010. Origin, fate and impacts of the Mekon g sediments. Mekong River Commission/ Information and Knowledge Management Programme, MRC, Vietiane, Lao PDR. 53 pp.
Sarkula J., Koponen J. 2003. Modelling Tonle Sap for environmental impact assessment and management support. MRC/WUP

- FIN

- Water utilizatio n program

- Modelling of the flow regime and water quality of the Tonle Sap

- October 2003, Phnom Penh, Cambodia. 107 pp.
Schouten R., Sripatrprasite P., Amornsakchai S., Vidthayanon C. 2000. Fish, and Fisheries Up

- and Downstream of the Pak Mun Dam. A Study of aquatic Resources, its Sustainable exploitation and the Impacts of the Construction of the Pak Mun Dam, World Commission on Dams. 52 pp
Schuldt J.A., Hershey A.E. 1995. Effect of salmon carcass decomposition on Lake Superior tributary
streams. Journal o f the North American Benthological Society. 14; 259

- 268.
Scott W.B., Crossman E.J. 1973. Freshwater fishes of Canada. Bull. Fish. Res. Board Can. 184; 1

- 966.
Skelton P.H. 1993. A complete guide to the freshwater fishes of southern Africa. Southern Book
Pub lishers (Pty) Ltd. 388 pp.

Smith H.M. 1945. The fresh

- water fishes of Siam, or Thailand. Bull. U.S. Natl. Mus. 188; 633 pp.
Soemarwotto, O. and B. Costa

- Pierce, 1988. Importation of pelagic freshwater sardine Corica goniognathus, to Indonesia reservoirs f or fisheries enhancement. Consultancy Report, IOE/ICLARM, Indonesia/Philippines. 30 pp.
Stemberger R.S., Miller E.K. 1998. A zooplankton

– N:P ratio indicator of lake ecosystem status. Environ. Monit. Assess. 51; 29

- 51.

Stockner J.G. 1987. Lake fertilization : the enrichment cycle and lake sockeye salmon (Oncorhynchus nerka) production. Pp. 198

- 215. In Smith H.D., Margolis L., Wood C.C. (eds.): Sockeye salmon (Oncorhynchus nerka) population biology and future management. Can. Spec. Pub. Fish. Aquat. Sci.
96.

S tockner J.G., MacIsaac E.A.. 1996. British Columbia lake enrichment programme: two decades of habitat enhancement for sockeye salmon. Reg. Rivers Res. Manage. 12; 547

- 561.
Stockner J.G., Porter K.G. 1988. Microbial food webs in fresh

- water planktonic ecosy stems. Pp. 69

- 83. In Carpenter S.R. (ed.): Complex interactions in lake communities. Springer

- Verlag, New York. 283 pp.
Stockner J.G., Rydina E., Hyenstranda P. 2000. Cultural Oligotrophication: causes and consequences
for fisheries resources. Fisheries. 2 5 (5); 7

- 14.
Sverdrup

- Jensen S. 2002. Fisheries in the Lower Mekong Basin: Status and Perspectives. Mekong River Commission, Vietiane, Lao PDR. MRC Technical Paper. 6; 95 pp.
Talwar P.K., Jhingran A.G. 1992. Inland fishes of India and adjacent countries. Volume 2. A.A. Balkema, Rotterdam. Oxford & IBH Publishing Co. Pvt. Ltd., New Delhi. 1158 pp.51 Tejerina

- Garro F.L., Fortin R., Rodríguez M.A. 1998. Fish community structure in relation to environmental variation in floodplain lakes of the Araguaia River, A mazon Basin. Environmental Biology of Fishes. 51; 399

- 410.
TKK and SEA START RC 2008. Water and climate change in the Lower Mekong Basin: diagnosis and recommendations for adaptation. Water and Development Research Group, Helsinki University of Technology, Finland (TKK) and Southeast Asia START Regional Center (SEA START RC), Chulalongkorn University, Thailand. Water & Development Publications, Helsinki University of Technology, Espoo, Finland. 75 pp.
Tonn W.M. 1990. Climate change and fish communities: a conceptual approach. Trans. Am. Fish. Soc. 119; 337

- 350.
Trush W., Ligon F., Dietrich W. 1995. Downstream ecological effects of dams

. BioScience. 45; 183

- 192.

Ukkatawewat S. 1999. The taxonomic characters and biology of some important freshwater fishes in Thailand. Manuscript. National Inland Fisheries Institute, Department of Fisheries, Ministry of Agriculture, Bangkok, Thailand. 55 pp.
USEPA (United States Environmental Protection Agency) 2003. What are Suspe nded and Bedded Sediments (SABS)? "Developing Water Quality Criteria for Suspended and Bedded Sediments (SABS): Potential Approaches (Draft, August Criteria for Suspended and Bedded Sediments (SABS): Potential Approaches (Draft, August 2003)." United State s Environmental Pr otection Agency, USA. 2003)."
6 pp. Utah State Water Plan 2010. Managing sediment in Utah's reservoirs. Utah Division of Water Resources, Utah State Water Plan. 172 pp.
Vaccari D.A. 2009 Phosphorus famine: the threat to our food supply. Scientific American Magazine,
June 2009.
Val A.L., Marcon J.L., Costa O.T.F., Barcelos J.F.M., Garcia J.T.M., Almeidaval V.M.F. 1999. Fish of the Amazon: surviving environmental changes. Pp. 389

- 402. In Sakesena D.N. (ed.): Ichthyology: Recent
Research Adv ances. Science Publishers, Inc., Enfield, New Hampshire. 453 pp.

Valbo

- Jorgensen J., Coates D., Hortle K. 2009. Fish diversity in the Mekong River Basin. Pages 161

- 196 in Campbell (ed.): The Mekong biophysical environment of an international river basin; E lsevier, Amsterdam, Netherland. 432 pp.
van Zalinge N., Deap Loeung, Ngor Pengbun, Sarkkula J., Koponen J. 2003. Mekong flood levels and Tonle Sap fish catches. Contribution to the second International Symposium on the Management of Large Rivers for Fisher ies

- 11

- 14 February 2003, Phnom Penh, Cambodia. 13 pp.
Vannote R. L., Minshall G.M., Cummins K.W., Sedell J.R., Cushing C.E. 1980. The river continuum concept. Can. J. Fish. Aquat. Sci. 37; 130

- 137.
Vidthayanon C. 2002. Peat swamp fishes of Thailand. Offi ce of Environmental Policy and Planning, Bangkok, Thailand, 136 pp.
Viravong S., Phounsavath S., Photitay C., Putrea S., Chan s., Kolding J., Valbo

- Jørgensen J., Phoutavong K. 2006. Hydro

- acoustic surveys of deep pools in Southern Lao PDR and Northern
Camb odia. Mekong River Commission, Vietiane, Lao PDR. MRC Technical Paper. 11; 76 pp.

Ward N. 1992. The problem of sediment in water for fish . Event London (Culbertson 1977). 1

- 8.
Ware D.M., Thomson R.E. 1991. Lin k between long

- term variability in upwelling and fish production in the northeast Pacific Ocean. Can. J. Fish. Aquat. Sci. 48; 2296

- 2306.52 Warren T.J. 2000. Indigenous Mekong Fish Species with Potential for Aquaculture. Stocking or Translocation. Management of Reservoir Fisheries in the Mekong Basin II. 92 pp.
Watanadirokul K., Kitisuwan P., Boonpeo L. 1983. Life history of Morulius chrysophekadion (Bleeker) in Ubolratana Reservoir. Annual Report 1983, Ubolratana Reservoir Fisheries Development Station,
Inla nd Fisheries Division, Department of Fisheries, Ministry of Agriculture and Cooperatives. 14 pp.
Watanadirokul K., Murada O. 1985. Fish yield from Ubolratana Reservoir 1985. Annual Report 1985,
Ubolratana Reservoir Fisheries Research Station. Inland Fisher ies Division, Department of Fisheries, Ministry of Agriculture and Cooperatives, Thailand. 32

- 42

. Weisse T., Stockner J.G. 1993. Eutrophication: the role of microbial food webs. Mem. Ist. Ital. Idrobiol. 52; 133

- 150.
Welcome R.L., Ryder R.A., Sedell J. A. 1989 Dynamics of fish assemblages in river systems

- a synthesis. Can. Spec. Publ. Fish. Aquat. Sci.106 569

- 577.
Welcomme R.L. 1985. River fisheries. FAO Fisheries Technical Paper. 262; 330 pp.
Welcomme R.L. 2001. Inland fisheries: ecology and management. Fo od and Agriculture Organization of the United Nations, Rome, Italy. 358 pp.
Winemiller K.O., Jepsen D.B. 2004. Migratory neotropical fish subsidize food webs of oligotrophic Blackwater Rivers. Pp. 115

- 132 In: G.A. Polis, M.E. Power and G.R. Huxel (eds.) Fo od Webs at the Landscape Level, University of Chicago Press, Chicago, USA. 548 pp.
Wolanski E., Nguyen Huu Nhan 2005. Oceanography of the Mekong River Estuary . Pp. 113

- 115 In Chen Z., Saito Y., Goodbred S.L.: Mega

- deltas of Asia

- Geological evolution and human impact. China Ocean Press, Beijing. 268 pp.
Wyatt A.B., Baird I.B. 2007. Transboundary impact assessment in the Sesan River Basin: the case of the Yali Falls Dam. 23 (3); 427

- 442.
Yang Xiaoqing 2003. Manaul on sediment management and measurement. Op erational hydrology report No. 47. The World Meteorological Organization, Geneva, Switzerland. 158 pp.
Yi Y., Wang Z., Zhang K., Yu G., Duan X. 2008. Sediment pollution and its effect on fish through food chain in the Yangtze River. International Journal O f Sediment Research. 23 (4); 338

- 347.
Ziv G., Baran E., So Nam, Rodríguez

- Iturbe I., Levin S. A. 2012 Trading

- off fish biodiversity, food security, and hydropower in the Mekong River Basin. Proceedings of the National Academy of Science; 109; 15; 5609

- 5614

.