Mekong River Commission

Fisheries Research and Development in the Mekong Region

Volume 14, No.2, September 2008

Dams, fish and fisheries in the Mekong River Basin

By Chris Barlow*

Dams on the mainstream of the Mekong will affect fisheries. The MRC and its Fisheries Programme have started several activities to provide more information on likely impacts and possible mitigation measures.

Mainstream Dams

Hydropower development is today the most important issue facing fisheries in the Lower Mekong Basin, home to 60 million people and one of the most productive inland fisheries in the world. Many hydro-electric dams are either already operating, under construction or planned along various tributaries of the Mekong, mainly in the Lao and Vietnamese parts of the basin.

On the mainstream of the river, there are currently only three hydro-electric dams. These are all located on the Chinese stretch of the Upper Mekong Basin, which also includes northeast Myanmar. China will commission several more hydro-electric dams across the mainstream over the next eight years. In the Lower Mekong Basin, there are currently no mainstream dams. But plans for as many as ten hydro-electric projects across the mainstream are at various stages of consideration. Nine sites have been proposed for the Lao stretch of the river and two for the Cambodian stretch (Hill and Hill, 1994; www. poweringprogress.org).

Hydropower on the mainstream
The renewed interest in hydropower development on the mainstream of the Mekong is driven primarily by two factors – the increase in international crude oil and gas prices over the past two years, and increased dry season flows resulting from the operation of dams in China. A third factor is that climate change concerns have led to resurgence in interest in hydropower as a clean energy source.

Dry-season flows in the Mekong will increase significantly as more Chinese dams are commissioned. In the Mekong between Vientiane and the border with China, where plans for four dams are currently being considered for implementation, preliminary estimations have indicated average dry-season flows will increase by 40-60% (Adamson, 2001). The increased water in the dry season makes hydropower dams on the mainstream a more financially attractive proposition.

Impacts of dams on fisheries
Dams and reservoirs block natural fish migration. Many of the commercial species in the Mekong have highly-developed migratory patterns. They generally move upstream to spawn and then juvenile fish move back downstream to feed and grow on the floodplains and wetlands (Poulsen et al., 2002). Dams act as a barrier to fish migrating upstream; and returning fish migrating downstream past hydropower dams generally must go through turbines. This results in mortalities. As a result, spawning is greatly reduced and replenishment of fish stocks is diminished. Some species which currently undergo long-distance migrations may be able to adapt to new environments if their present migration patterns are facultative or opportunistic, rather than obligatory. However, whether this will occur or not can only be determined after migration routes are blocked (see box below, detailing an expert group meeting on dams as barriers to fish migration in the Mekong, and possibilities for mitigation).

Fisheries studies addressing dam development issues

The Fisheries Programme of the Mekong River Commission has a range of activities within its Fisheries Ecology, Valuation and Mitigation component that are designed to provide information relevant to basin planning processes. The recent focus on mainstream dams has resulted in the programme taking on a series of fast-tracked activities to provide information to regional policy makers so that they can make informed decisions about the costs and benefits associated with hydropower development. These fasttracked activities are listed below.

1. Identification of important spawning sites on the mainstream
Sampling of fish larvae and juveniles will be conducted along the length of the Mekong mainstream over a 12-month period to identify important spawning habitats. The study is being coordinated by the Thai Department of Fisheries because of the specialist expertise on larval fish taxonomy available in Thailand. Planning is currently underway for the quantitative survey work to be carried out in 2009, with preliminary results available at the end of that year. The results will also help improve the reliability of the predictions of the dam impact model described below.

2. Modelling the barrier effect of proposed mainstream dams
The Fisheries Programme and the World Fish Center (Phnom Penh office) are collaborating to model the barrier effect of proposed mainstream dams. The modellers will look at how the barriers will affect populations of highly migratory species of different sizes and with different life history strategies (for instance, early and late maturing fish, fish with many eggs or few eggs). A report on the work will be available by May 2009.

3. Expert meeting on dams as barriers to fish migration on the mainstream, and possibilities for mitigation
The MRC Fisheries Programme and the Hydropower Programme are convening a meeting of experts from around the world on fisheries ecology and hydropower development. The aim is to bring their experience and knowledge to assess the possible impacts of mainstream dams on the fisheries of the Mekong, and to see what mitigation activities could be relevant to this region. The meeting is scheduled to take place on 22-23 September, 2008 with a summary report to be made available to the Hydropower Consultation on 25-26 September. The full paper detailing the outcomes of the meeting will be published in the form of answers to frequently asked questions about dams, hydropower and fisheries.

4. Development of guidelines for fisheries impact assessment, forecasting and mitigation
The development of these guidelines will be undertaken by a team of consultants contracted by the MRC Secretariat. The guidelines will draw upon lessons and experience from dam projects, environmental impact assessments, and related studies undertaken both within and beyond the Lower Mekong Basin to generate best practice guidelines for dam impact forecasting assessment and mitigation. The report will be completed by mid-2009.

5. Mitigating the impacts of dams on fisheries: a primer
The Fisheries Programme will prepare a report on measures that can assist in mitigating the impacts of dams on fisheries, before, during and after construction of a dam. The report will be written as a general source document for a wide audience. It will be published in the MRC Development Series in early 2009.

6. Review of fisheries in reservoirs in the Lower Mekong Basin
The programme also plans to publish a technical report on the development of fish communities and reservoir fisheries in the Lower Mekong Basin. This will look at what sort of fisheries we can expect in new reservoirs and will attempt to compare yields in reservoirs with those lost from the river fisheries. The report should be available by mid-2009.

Although the effect of blocking migrations on fisheries productivity has been observed in numerous rivers all around the world, it is extremely difficult to quantitatively predict the impact that blocking migrations has on overall fisheries productivity. What is obvious though, is that one dam’s impact on fish migration can be the same as the impact of ten dams. That is why, in fisheries terms, it is far better to have a cascade of say ten dams on one tributary than one dam on each of ten tributaries. And similarly, why one dam on the mainstream may have a severe impact on regional fisheries productivity if it blocks an important fish migration route.

Dams, particularly those with large storage capacity, also affect river hydrology. Changes to hydrology include modification of the extent, duration and timing of annual floods, which naturally occur between May and November in the Lower Mekong Basin. Smaller floods of shorter duration reduce the available habitat for fish, resulting in lower fish production (Halls et al., 2008). They also reduce the survival rate of eggs and juvenile fish, which in turn decreases the recruitment of younger fish into the natural population. Changes to the timing of the floods, and indeed permanent increases in dry season flows, can disrupt the spawning and migration cues that trigger changes in fish behaviour and which are required for reproduction and ultimately the survival of different species (Baran, 2006).

Fish cages on the Nam Ngum Reservoir, Lao PDR
Photo: Sommano Phounsavath

Scientists can roughly estimate the impact of smaller floods of shorter duration on the overall fisheries yield. However, without detailed experimental studies (which are currently not feasible in the Mekong region due to the experimental infrastructure required), it is very difficult to predict how spawning and migration cues will be affected by hydrological changes involving not only reduced wet-season flows, but also markedly increased dry-season flows.

Most of the dams proposed for the mainstream have only limited storage capacities. Consequently, they are unlikely to have any significant effect on seasonal hydrology patterns, although they may affect daily flow patterns in the dry season, especially if they are operated to supply peak-demand electricity.

New fisheries in reservoirs
Dams do create opportunities for new reservoir fisheries. Yields in new reservoirs are initially quite high as the nutrients are released from the recently inundated land. However, the yields usually decline after five years or so as the nutrient supply is used up, and then stabilise at a lower level. Fisheries biologists can predict the yields from reservoir fisheries. How it is difficult to determine the extent of the reduction in the river fisheries due to dam construction, because the impact is diffuse and widespread. Hence a comparison between production from new reservoir fisheries and diminished river fisheries is in most cases problematic (Jackson and Marmulla, 2000).

Reservoirs for the dams planned on the Mekong mainstream will be small, and generally confined within the river banks. Consequently, the fisheries which develop in them will be similarly limited.

Distribution of benefits
As the Lower Mekong Basin develops, active engagement between the fisheries and hydropower sectors is needed to address both the positive and negative impacts of dams on fisheries. The positive impacts of hydro-electric and irrigation dams are relatively easy to measure. They can raise food production, improve living standards, and create secondary industries as well as sources of government revenue. Income generated from such activities is part of the formal economy.

But fisheries production is also an important part of the agricultural sector in the four countries of the basin, accounting for 2-10% of each of the four countries’ gross domestic product between 2002 and 2006. Measuring the wider benefits of fisheries in terms of food security, livelihoods, biodiversity and ecological functions is much more difficult. Such benefits are not part of the formal economy. They do, however, form a crucial part of people’s livelihoods, especially for those who share the waters of the Lower Mekong Basin and their migrating fish species.

Fishing family on the Nam Houm Reservoir, Lao PDR
Photo: Sommano Phounsavath

The answer lies in comprehensive planning at both national and regional levels. Planning needs to encompass the region’s requirements for energy security, food security, economic development, environmental protection, and equitable distribution of the benefits of developments. In a region such as the Mekong, which has vast hydropower potential and important fisheries resources, with good planning it is entirely possible to have both ample hydropower and healthy river systems that support highly productive fisheries.

*Dr Barlow is Manager of the MRC Fisheries Programme