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 fisheriesThe 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).
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.
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
Further Reading
Adamson, P. 2001. The potential impacts of hydropower developments in Yunnan on the hydrology of the lower Mekong. International water power and dam construction, 53(3): 16-21.
Baran, E., 2006. Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems. MRC Technical Paper No. 14, Mekong River Commission, Vientiane. 56 pages.
Halls, A., Lieng Sopha, Ngor Pengby and Tun Phall. 2008. New research reveals ecological insights into dai fishery. Catch and Culture 14(1): 8-12.
Hill, M. and S. Hill. 1994. Fisheries ecology and hydropower in the Mekong River: an evaluation of run-of-the-river projects. Mekong Secretariat, Bangkok. 106 pages.
Jackson D. and G. Marmulla. 2001. The influence of dams on river fisheries, pages 1-44 in Marmulla, G (ed) Dams, fish and fisheries: Opportunities, challenges and conflict resolution. FAO Technical Paper No 419, Rome. FAO. 166 pages
Poulsen, A. F., Ouch Poeu, Sintavong Viravong, Ubolratana Suntornratana & Nguyen Thanh Tung. 2002. Fish migrations of the Lower Mekong Basin: implications for development, planning and environmental management. MRC Technical Paper No. 8, Mekong River Commission, Phnom Penh. 62 pages.