Issues Magazine

Fish Diversity and Migration in the Mekong River

By Chris Barlow and Lee Baumgartner

The diverse fish fauna of the Mekong provides food, employment and income for millions of people, but its sustainability is threatened by barriers that block fish migration. Fishways on low-level weirs can help to maintain the resource and improve the yield of fisheries.

The Mekong River is the world’s tenth-longest river, extending 4900 kilometres from the Tibetan Plateau in China to its mouth in southern Vietnam. Its physical diversity, tropical location and high productivity fostered the evolution of a diverse fish community comprised of an exceptionally large number of taxonomic families: the Mekong fish database lists a total of 87 families, with 65 being documented in Cambodia and 50 in Lao PDR. Species numbers are about 850 freshwater fishes and as many as 1100 indigenous species if coastal and marine species are considered.

Species richness in the Mekong is second only to the Amazon River. This diverse fish fauna supports the world’s largest inland fishery, with approximately 2.6 million tonnes harvested annually. To put that figure in perspective, it is 10–11 times the catch of all Australian fisheries, marine and freshwater combined, and equates to about 2% of the annual global fish catch.

Mekong fishes are often grouped according to their ecology and migration patterns. “White fishes” undertake long-distance migrations between floodplains and along major rivers, and often move hundreds of kilometres upstream to spawn. Perhaps the most famous is the Mekong giant catfish (Pangasianodon gigas), which grows to over three metres in length and 300 kilograms in weight.

“Black fishes” live in lakes and swamps on the floodplains. They are generally classed as non-migratory, although in the dry season they move to refuge pools in lakes and nearby rivers.

“Grey fish” are an intermediate group, living in lakes and swamps in the wet season and undertaking lateral migrations to tributaries and nearby river systems in the dry season.

One of the biggest threats to the fish fauna of the Mekong is dams and weirs, primarily because they block the passage of migrating fish, which interrupts life cycles and restricts access to large areas of aquatic habitat. There are 20 large dams in operation and more than 40 dams under construction or being planned in the tributary systems of the four lower Mekong countries.

An additional 150 potential dam sites have been identified. If all are constructed, these will have devastating effects on white species, which rely on longitudinal connectivity to complete essential life history processes such as feeding, spawning and colonising new habitat.

Additionally, throughout the catchment there are thousands of low-level structures (e.g. weirs, flood control gates) across waterways and on flood plains, which have been built primarily for irrigation and flood control. These obstruct movements of black and grey species and are contributing to population declines in some areas.

There are presently no effective mitigation measures that can eliminate the ecological impacts of high-level dams on migratory fishes, especially in tropical rivers with megadiverse fish communities and high migratory biomasses. However, fish passage solutions for low-level (less than approximately six metre) barriers are well-understood, and certain designs that have had success in Australia, North America and Europe could be directly applied to the Lower Mekong Basin.

To this end, the Australian Centre for International Agricultural Research is funding work to transfer Australian fish passage expertise to Lao PDR, one of the six countries bordering the Mekong River. In an initial scoping study in central Laos in 2007–09, fisheries scientists and local villagers constructed an experimental fishway to demonstrate the potential for this technology to rehabilitate floodplain fisheries. Monitoring over 44 days showed that the small vertical slot fishway enabled more than 15,000 fish from

108 species to gain access to aquatic habitat upstream of the weir, including threatened species such as Jullien’s golden barb (Probarbus jullieni). These results and other ecological investigations have clearly shown that fishways have potential for widespread application in the Mekong in terms of enabling migrating fish to pass low-level weirs and rehabilitating floodplain systems.

A five-year multi-organisational project is now underway to extend the technology to other areas of the basin, as well as to train Mekong fisheries scientists in all aspects of fishway design and operation. The ultimate outcome will be increased fisheries production and enhanced sustainability of the diverse fish fauna that inhabit floodplains of the Mekong River.