On November 7, 2012, the government of Lao PDR held an official groundbreaking ceremony for the Xayaburi dam, the first mainstream dam on the Lower Mekong River.
The Xayaburi dam, the first of eleven dams planned for the mainstream of the lower Mekong River, will likely reduce ecosystem service values and undercut livelihoods of people living in Thailand, Lao PDR, Cambodia, and Vietnam. A recent Mekong River Commission study reports that the cumulative impacts of the planned dams in Lao PDR could disrupt the lifecycles of migratory fish, block access or destroy spawning grounds, and reduce catch by 270,000 to 600,000 metric tons.
This is especially significant because the Mekong is one of the most biodiverse and productive rivers on Earth. It is a global hotspot for freshwater fishes: over 1,000 species have been recorded there, second only to the Amazon. The Mekong River is also the most productive inland fishery in the world. The total harvest of approximately 2.5 million metric tons per year is valued at $3,600,000,000 to $6,500,000,000.
The Xayaburi dam also poses a serious threat to several of the largest, and rarest, freshwater fish in the world, including the critically endangered Mekong giant catfish (Pangasianodon gigas), the critically endangered giant pangasius (Pangasius sanitwongsei), and the endangered seven-striped barb (Probarbus jullieni).
Evidence suggests that these species, particularly the Mekong giant catfish and giant pangasius, are vulnerable to threats from Xayaburi dam because of their migratory behavior, requirements for specific water quality and flow, and complex life history, which is dependent on seasonal floods.
The official environmental impact report for the Xayaburi project does not assess the dam’s effects on these, and many other migratory and Red Listed, species. Depending on the scale of migrations and location of spawning sites, the Xayaburi could cause the extirpation of the Mekong’s giant fishes over a large (hundreds of kilometers) area and put basinwide populations on a steep trajectory of decline.
Several groups, including the Mekong River Commission, have called for a ten-year moratorium on mainstream dams to better assess the long-term social and environmental costs of such projects. Such large-scale assessments have become common on rivers where managers seek to rebuild migratory fish stocks but are urgently needed at the outset of projects to avoid unnecessary destruction of ecosystem services and costly restoration efforts. The long-term viability of vulnerable fish populations – and people who depend on fish for food – is dependent on the ability to minimize the impacts of any mainstream dams built on the lower Mekong River.
Megafish at Risk
At least five species of giant fish occur in the vicinity of the Xayaburi site and the three largest and most endangered, the Mekong giant catfish, giant pangasius, and seven-striped barb, will suffer the most serious consequences if the dam is built.
Mekong giant catfish (Pangasianodon gigas)
The critically endangered Mekong giant catfish, a Mekong endemic, reaches a maximum length of 300 centimeters (118 inches) and a total weight exceeding 300 kilograms (660 pounds). Based on catch data and genetic studies, it is likely that the Mekong giant catfish, though extremely rare, remains widespread throughout the basin. It also appears likely that the behemoth uses the stretch of river of the Xayaburi dam as a migration corridor.
If the Xayaburi dam is built, it could alter Mekong flows and disrupt spawning cues, block spawning migrations, and slow downstream dispersal. Some mortality may also occur if fish pass through dam turbines. Impacts from Mekong mainstream dams could conceivably cause the extinction of the species.
Giant pangasius (Pangasius sanitwongsei)
The critically endangered giant pangasius catfish grows to 3 meters (10 feet) and 300 kilograms (660 pounds). It once occurred in both the Chao Phraya and Mekong rivers, but wild self-sustaining populations are now limited to the Mekong. The giant pangasius catfish is a main river species. Adults seem to favor the deep pool areas of Chiang Saen, Chiang Khong, Loei, Xayaburi, Stung Treng, and Kratie, while the young are widespread in the main channel, especially along the Thai-Lao border and in Cambodia downstream of Kratie.
The giant pangasius, like many fish species in the Mekong River, migrates between habitats, requires specific water quality and flow, and has a complex life history dependent on migration and seasonal floods. Mature fish migrate up the Mekong River and spawn in April and May at unknown spawning grounds. Adult fish occur in both Chiang Rai and Loei Provinces in Thailand, and young fish occur along the Thai-Lao border from Nong Khai to Nakorn Phanom. This suggests not only that giant pangasius occur at the Xayaburi site, but that the Xayaburi site is within the migratory corridor and may be in the vicinity of a spawning area.
More information is needed about the distribution and behavior of giant pangasius, but it appears very likely that the Xayaburi dam site is critical habitat for the species. Construction of the dam could disrupt migratory behavior and spawning. Once the dam is built, it may alter water flows and cues to migration, block upstream spawning migrations, and slow downstream dispersal. Some mortality may also occur if fish pass through dam turbines. Impacts from the dam could conceivably cause the extirpation of the giant pangasius from the Mekong River.
Seven-striped barb (Probarbus jullieni)
The seven-striped barb occurred historically in Mekong, Chao Phraya, and Meklong basins in Southeast Asia and the Pahang and Perak basins of Malaysia. Adult seven-striped barb appear to prefer main river habitats, whereas juveniles will enter floodplain habitats during the rainy season. As recently as 1989, the seven-striped barb was reported as “extremely abundant” in the Mekong, but subsequent accounts indicate a significant drop.
The seven-striped barb occurs in the area that will be impacted by the Xayaburi dam. The fish is migratory: adults migrate upstream in the dry season and form spawning aggregations. Large fish remain in deep pools during low water. Young fish enter floodplain habitats during the rainy season. The Xayaburi dam could impact spawning sites, upstream migration of adults, and downstream dispersal of young.
Operation of the dam (variable flows) could also impact spawning triggers and dry season habitat. Depending on the exact location of the spawning sites and the distance the species migrates, the Xayaburi dam could impact seven-striped barb populations within several hundred kilometers of the dam site.