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Fisheries Research and Development in the Mekong Region
Volume 14, No.3, December 2008

Mainstream dams as barriers to fish migration:
international learning and implications for the Mekong

By Patrick Dugan*

Mainstream dams in the middle and lower reaches of the Lower Mekong Basin could affect more than 70 percent of the basin's catch. If such projects go ahead, experts agree that it would be better to build mainstream dams further upstream or relocate them to tributaries where they will have less impact on the dozens of fish species migrating over long distances.

Members of the Expert Group

Dr Patrick Dugan, Deputy Director General of WorldFish Center, Cairo, Egypt

Prof. Ian Cowx, Director of the University of Hull International
Fisheries Institute, United Kingdom

Dr Miguel Petrere, Universidade Estadual Paulista, Brazil

Dr Angelo Agostinho, Maringa State University, Brazil

Dr Eric Baran, WorldFish Center, Phnom Penh, Cambodia

Mr Roel Schouten, Environmental Engineer, Lao PDR

Dr Tuantong Jutagate, Ubonratchatani University, Thailand

Dr Sinthavong Viravong, Living Aquatic Resources Research Centre, Lao PDR

Dr Martin Mallen-Cooper, Fishway Consulting Services, Australia

Dr Glenn Cada, Oak Ridge National Laboratory, USA

Dr Gerd Marmulla, FAO, Rome, Italy

Dr John Ferguson, Northwest Fisheries Science Center, USA

Dr John Nestler, US Army Corps of Engineers, USA.

Dr Robert Davidson, US Army Corps of Engineers, USA

Prof. Chen Daqing, Yangtze River Fisheries Research Institute, PR China

Dr Pratak Tabtipawan, Kasesart University, Thailand

Mr Garry Thorncraft, fisheries consultant, Lao PDR

The MRC Secretariat convened a regional stakeholder consultation on hydropower from 25-27 September 2008 in Vientiane in light of the growing regional interest in the construction and use of hydroelectric power dams on the Mekong mainstream. To help prepare for this consultation, the MRCS also convened an international expert group from 22-23 September to review the impact of mainstream dams on fish migration. The 17 group members (see box) brought together extensive expertise in fish biology and ecology, and in efforts to design and operate hydro dams so as to reduce their impacts on fisheries. The members have worked on these issues in a wide range of countries and river systems in Asia, Africa, Australia, Latin America, North America and Europe, and so brought to the Mekong a wealth of experience and comparative ability. Together they were able to provide an authoritative assessment of the issues and their conclusions were conveyed to the stakeholder consultation on the first morning.

The group addressed 14 frequently asked questions (see box on page 13), which together covered five broad areas of concern:

  1. What is the importance and nature of fish migration in the Mekong?
  2. What will be the impact of barriers to migration on fish and fisheries in the Mekong?
  3. Can fish-passage facilities be used to provide effective passage for fish migrating upstream?
  4. Can fish-passage facilities be used to provide effective passage for fish migrating downstream?
  5. What can be done to compensate for losses in fisheries yield caused by dams?

The group's response to these areas of concern is summarised below.

What is the importance of fish migration in the Mekong?
The Mekong supports the world's largest inland fishery, with approximately 2.6 million tonnes harvested annually from the Lower Mekong Basin (LMB) (van Zalinge et al., 2004; Hortle, 2007). The full economic value of this fishery is still being assessed, but most recent estimates exceed US$2 billion at first-sale value (Lymer et al. in press; Mekong River Commission, 2005; van Zalinge et al. 2004). To this should be added the value generated through processing, transport and marketing of the product; in Cambodia for instance, the value of raw fish on retail markets represents 2.8 to 4.7 times the first-sale value (Yim & McKenney, 2003; Rab et al., 2004). Applying the same multiplier as a primary approximation, the total economic value for the Mekong fisheries is estimated at between US$5.6 and US$9.4 billion per annum. To this economic value should be added the many tens of thousands of enterprises that support the fishing communities, ranging from the shops and foods stalls that supply the fishing families, to boat builders and suppliers of fishing gear.

Underlying the dollar value of the Mekong fishery at the basin scale is the importance of fishing for household economies. In Lao PDR, more than 50% of people fish, and fishing provides 20% of household income; in the south of the country, fishing is even more important and here 80% of people fish. Of special importance is the linkage between fisheries and other sectors. For example, income from fisheries provides cash to buy rice seed at the end of the long dry season. In Cambodia, 80% of the 1.2 million people living around Tonle Sap use the lake and its rivers for fishing, and for 39% of these people fishing provides their primary income (Ahmed et al., 1998). This importance is mirrored downstream in Viet Nam's Mekong Delta, where capture fisheries are crucial to livelihoods. In An Giang province, 60% of people are part-time fishers, 7% full-time fishers, and 5.7% fish processors (Sjorlev, 2001). In Tay Ninh province, 88% of the "very poor", 84% of "low income", 58% of "medium income", and 44% of "high income" households depend on fisheries (Nho and Guttman, 1999).

In addition to these income figures, the Mekong's fisheries also play a central role in feeding the 60 million people who live in the LMB. For them, fish is the main source of animal protein and a critical source of micronutrients, particularly amino acids, vitamins and calcium (Baran et al., 2007). Consumption of fish and other aquatic resources in the basin ranges from 29-39 kg per person per annum (Hortle, 2007) with the upper rates of consumption being amongst the highest in the world.

What will be the impact of barriers to migration on fish and fisheries in the Mekong?
Fish migration in the Mekong is primarily for breeding and feeding. Because the river's annual hydrological cycle involves large floods (with a 30-fold difference between high and low season discharge), fish migrate upstream to breed, after which their eggs and larvae are carried downstream to the floodplains where they feed and grow. Fish also migrate to feed, normally moving from the main course of the river onto the highly productive floodplains at the beginning of the rainy season. In the Mekong, most species combine feeding and breeding migrations. But upstream migrations are dominated by larger adult fish moving up river to breed while downstream migrations are mainly feeding migrations undertaken by young fish and adults returning from the breeding areas. Movements also include lateral migrations between the mainstream or tributaries and floodplains. Poulsen et al. (2002) and Baran and Jutagate (in press) have used the approach of Lévêque and Paugy (1999) and Welcomme (1985, 2001) to characterise the fish groups (or "guilds") in the Mekong according to their ecology and migration patterns:

  • "Black fish" are those species with limited lateral migrations from the river onto the floodplains and no longitudinal migrations upstream or downstream. These fish do not leave floodplains and wetlands, and spend the dry season in pools in the rivers or floodplains. This group includes Channidae (snakeheads), Clariidae and Bagridae (catfishes) and Anabantidae (climbing perch).
  • "White fish" undertake long-distance migrations, in particular between lower floodplains and the Mekong mainstream. This group includes many cyprinids (e.g. Henicorhynchus spp and Cirrhinus spp.) but also most Pangasidae catfishes.
  • "Grey fish" do not spend the dry season in floodplain pools, but do not undertake long distance migrations either. When the flood recedes they leave the floodplain and tend to spend the dry season in local tributaries. This group includes for instance Mystus catfishes.

Dams are a barrier to fish migrations up and down rivers but the specific effect varies considerably depending on the groups concerned. The most important impacts will be on the long-distance migrants that move up the Mekong mainstream to breed, some as far as China. Critically, it has been estimated that more than 70% of the total fish catch in the Mekong Basin (i.e. more than 1.8 million tonnes worth US$1.4 billion at first sale) is dependent on these longdistance migrants. The effect of dams on fisheries production is also highly dependent on the location, design and operation of the dams (Baran et al., 2007). The mainstream Mekong is a corridor for most long-distance migrations and most of the production originates from floodplains in the middle and lower part of the Basin (see map). Thus dams built on the mainstream will have a much greater impact than dams built on tributaries, while those located in the middle and lower part of the LMB will have a greater impact on fish production than dams located in the upper part of the basin.

'The effect of dams on fisheries production is highly dependent on the location, design and operation of the dams'

Can fishways be used to provide effective passage for fish migrating upstream?
Given the projected impact of dams on fish migration in the Mekong, and the consequent economic and social losses, there is great interest in the potential of engineering solutions to provide effective fish passage upstream across dams. The most commonly-used approach is fish ladders that provide a step wise flow of water through a series of small basins and waterfalls up which fish may pass. Other alternatives are fish lifts that physically lift fish up the dam in the same way that elevators lift people in buildings, and fish locks, which operate in a manner similar to, or are integrated into the operation of, navigation locks. Finally, in some rivers part of the flow may be diverted through an artificial river bypassing the dam along a gentle gradient, so providing an alternative route along which fish may move. 

The three fish migration systems of the Lower Mekong Basin

Source: Baran & Jutagate (in press) after Poulsen et al. (2002)

Despite their diversity, the Expert Group concluded that there is currently no evidence that fish-passage facilities used in large tropical rivers in Latin America, Africa and Asia can cope with the massive fish migrations and high species biodiversity in the Mekong. Indeed, best evidence from South America (Oldani & Baigin, 2002) is that the success of fish ladders and lifts there is low even though the number of species and volume of migration there is lower than in the Mekong. Similarly, the group agreed that the technologies used on high dams in North America and Europe (mainly fish ladders and fish lifts) have been developed for a very limited range and number of fish species (generally about 5 to 8 species). Most of this experience has been with salmonid fish which have remarkable jumping abilities that enable them to scale waterfalls and fish ladders more successfully than any other group of fish. Biomass of fish involved is also relatively small, at around 3 million fish per year on the Columbia River in the USA. This experience from North America and Europe contrasts with the Mekong where there are at least 50 important migrant species, none of which are salmonids, and biomass is in the order of 100 times greater.

'Dams on the mainstream in the middle and lower part of the LMB will have a major impact on fisheries and serious economic and social implications'

On the basis of this analysis of available information, the meeting concluded that current fish-passage technology would not be effective in maintaining the migration of the large number and diverse fish species found in the Mekong. In view of this conclusion and the assessment of the value of the Mekong's fisheries, the group concluded that dams on the mainstream in the middle and lower part of the LMB will have a major impact on fisheries and serious economic and social implications. It went on to conclude that because there is less fish migration into the upper part of the LMB, dams built on the mainstream higher up in the basin would have less impact on fisheries production than dams built further downstream.

Can fish passes be used to provide effective passage for fish migrating downstream?

'Newly designed fish-friendly turbines are being tested in the USA. At the present moment, however, these are only at the experimental stage'

Dams are also a barrier to fish migrating downstream, together with their eggs and fry. They therefore need to find a way across, through or around the dam. There is currently little evidence regarding the performance of such fish-passage facilities on the Mekong or any other tropical rivers. Most experience in developing technologies that allow downstream migration has been in North America and Europe, where a range of approaches have been used. These include spillway passages, louvers and other behaviour-guidance technologies to channel fish away from turbines and through spillways or fish by-passes; and surface collectors and barges that physically capture and transport fish past the dam. These are all dependent on knowledge regarding the size and age of the specific fish species being targeted, their swimming abilities, and their distribution in the reservoir behind the dam, together with equally specific information on seasonal changes in conditions within the reservoir and water flow, and key elements of dam design and operation, such as the location of the turbines and spillway.

'On the basis of this evidence,compensation for loss in yield from river fisheries is impossible to achieve through development of reservoir fisheries'

There is also growing attention being given to the possibilities of reducing the mortality caused by fish passage through turbines. Studies have shown that survival through conventional (existing) turbines ranges from virtually zero in adult eels of Anguilla spp. (Montén, 1985) to very high levels of >95% (Eicher Associates Inc, 1987). This depends on numerous biological variables (fish species, size, and life stage) and physical variables (type of turbine, head, number of blades, and rotation speed). In general, survival is highest through horizontal, adjustable (bulb) turbines, followed by vertical axis, adjustable (Kaplan) turbines, then vertical axis, fixed (Francis) turbines, and finally impulse turbines (Pelton). Of the two most commonly-used turbines, survival is higher through Kaplan turbines than through Francis turbines because Francis turbines utilise higher head, more blades,  and rotate at higher speeds than Kaplan turbines, increasing the risk of mortality from blade strike, pressures, and hydraulic shear. However these studies have only been done on a limited number of species (usually salmonid fishes) and dams.

To address this problem, newly designed fish-friendly turbines are being tested in the USA. At the present moment, however, these are only at the experimental stage and none has been tested at full operational scale. There are therefore substantial uncertainties about their viability and benefits, including very limited information on their impact on different species and life stages, and on the indirect mortality they cause.

Questions Addressed by the Group

  1. What is the importance of fish migration in the Mekong?
  2. What is a barrier to fish migration?
  3. Would a barrier to migration have the same affect on all fish species?
  4. Can fish ladders be used to provide upstream passage for migrating fishes?
  5. Can other types of fish passes (such as fish locks and fish elevators) be used to enable fish to pass high-level dams?
  6. Should fish passes be built if the upstream habitat has been altered?
  7. Can fish survive downstream migration over spillways and through hydroelectric turbines?
  8. Do fish friendly turbines produce less electricity than traditional turbines?
  9. Are there mechanisms for guiding fish away from entrainment in turbines, and through an alternative route for downstream migration?
  10. How do survivals of fish passing hydroelectric dams via different routes (turbine, screening and bypass, spill, trap-and-transport) compare?
  11. What measures can be undertaken to maximise the yield from fisheries created in reservoirs used for hydropower production?
  12. What measures can be undertaken to maintain fisheries production in rivers downstream of dams?
  13. How does the yield from fisheries created in impoundments after dam construction compare with the yield lost from the former riverine fishery?
  14. What general conclusions can be made, based on experience, about the effects of dams on fisheries resources in tropical rivers?

What can be done to compensate for losses in fisheries yield caused by dams?

Dams create reservoirs and these provide potential for fish production (Bernacsek,1997). However, the ecological conditions in these reservoirs differ greatly from those in the undammed river and the fisheries production that can be achieved there is dependent on a wide range of factors including location, depth, surface area and management regime. Because fish production is generally higher in large rivers and on lowland floodplains, it will normally be more difficult to compensate for losses in production caused by dam construction in these areas than when dams are constructed on tributaries and further upstream. In large African reservoirs with moderate to heavy fishing effort, yields range from 27-65 kg/ha/year. In medium-sized reservoirs, this figure rises to 80 kg/ha/year. Highest yields in Africa are recorded in small reservoirs, and this is also true of shallow reservoirs in Lao PDR where yields are about 90 kg/ ha/year. However, these production figures compare unfavourably with estimated yields of 50-500 kg/ha/ year from natural Mekong floodplains (Hortle, 2007). Most mainstream dams proposed for the Mekong will create deep reservoirs confined to the river channel so fisheries production there is likely to be most directly comparable to the relatively low production recorded in large African reservoirs. Estimates of production from dams in South and Southeast Asia indicate that productivity levels are low and higher yields can be obtained only by fisheries enhancements such as stocking and fish cages, which necessitate significant additional cost in terms of both initial capital investment and recurrent annual costs. On the basis of this evidence, the Panel concluded that compensation for loss in yield from river fisheries is impossible to achieve through development of reservoir fisheries. Fisheries enhancements through stocking and some forms of aquaculture may be possible, but they will only be able to compensate for a small part of the production that is lost from the river fishery. They will also be costly, will not benefit the same people who currently benefit from the fishery, and can create substantial environmental problems.

What general lessons can be learned from mitigation efforts for dams in other regions?
The experience from river systems, dams and their fisheries in other regions is that each river fishery and every dam is unique. As a result, specific management measures need to be designed for each migrating species and each dam. In doing so, solutions for new dams cannot simply be copied from existing designs but can be developed from existing concepts. The approach to developing fish-passage devices designed to allow fish to migrate upstream or downstream past dams needs to be tailored to each species concerned and the location, design, and operation of the dam. Substantial biological knowledge for each fish species is required to design these measures. Where the number of species is high and the biological information is low, it is important to focus mitigation efforts on key target species, or design and apply multiple strategies for fish passage. For example, for downstream passage of multiple species of different sizes, high-flow volumes could be used in bypassing the turbines and/or multiple routes provided through and past the dams. This is likely to mean a reduction in the flow passing through the turbines and therefore a reduction in revenue generated.

An important lesson from the Columbia River is that successes in designing and operating fish passes have been realised because dams are managed for fish passage as first priority and power generation as second priority during the migration seasons. The chance of success with these approaches will be greatest where mitigation and flexibility are integrated into dam design at the start, rather than retrofitting which can be extremely expensive. Finally, the experience from other regions is that it takes many years of experimentation, and many hundreds of millions of dollars, to develop and apply effective mitigation measures. These costs and the time required to pursue effective mitigation need to be built into the planning and design of hydropower dams.

'It takes many years of experimentation, and many hundreds of millions of dollars, to develop and apply effective mitigation measures'

Bringing together the information presented in Vientiane, the group recognised that the Mekong's fisheries are of critical economic and social importance for the countries and people of the basin. It concluded that a large part of the benefit is dependent on mainstream fish migration and that mainstream dams will effectively stop much of this migration leading to reduced production, substantial economic cost and social deprivation. After reviewing the available evidence from dams in all other regions the group went on to conclude that on the basis of current knowledge:

  • existing mitigation technology cannot handle the scale of fish migration on the Mekong mainstream;
  • dams in the middle and lower LMB will have the largest impacts on fisheries and the largest economic and social costs;
  • dams higher in the basin and on tributaries will have relatively less impact on fisheries production;
  • if dams are built upstream and on tributaries, specific mitigation measures should be designed from the start and integrated into dam engineering and operation;
  • in considering the design of mitigation measures existing off-the-shelf designs cannot be used, but the basic concepts used in developing these can be drawn upon.

The meeting also recognised that the ability to provide the partial mitigation measures seen in North America and Europe has been dependent on substantive research and development over several decades and on teams of highly qualified biologists and fish passage engineers. Similar investments will be needed in the Mekong before any level of certainty on their effectiveness can be determined.

* Dr Dugan is Deputy-Director-General of the WorldFish Center. He chaired the Expert Group Meeting on the Impact of Dams as Barriers to Fish Migration in the Mekong in Vientiane on 22-23 September. 


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