Plans to build hydropower plants along South-East Asia's longest river could have devastating effects on the world's largest inland fishery and should be reconsidered, scientists say.
Plenty of attention has focused on plans to develop dams along the main stem of the 4600km Mekong River which passes through China, Burma, Laos, Thailand, Cambodia and Vietnam.
But the international study published in the Proceedings of the National Academy of Sciences examined the impact of building dams on dozens of the smaller branches, known as tributaries.
Since the area is home to many species of migratory fish, the analysis found that several dam projects could block fish from swimming upstream and cause massive losses to diversity and fish supply.
This could have devastating effects on the tens of millions of rural, poor residents in the region who depend on subsistence fishing for their main source of food, said scientists from Cambodia's Inland Fisheries Research and Development Institute and Stanford and Princeton University.
More than one million tonnes of freshwater fish are caught each year in the Cambodian and Vietnamese floodplains, and the entire Mekong River Basin is home to 65 million people, about two-thirds of whom rely on fishing to survive.
"We find that the completion of 78 dams on tributaries, which have not previously been subject to strategic analysis, would have catastrophic impacts on fish productivity and biodiversity," said the study.
"Our results argue for reassessment of several dams planned, and call for a new regional agreement on tributary development of the Mekong River Basin."
Specifically, four planned dams were found to create the largest fish biomass losses, including the Lower Se San 2 in Cambodia (9.3 per cent drop in fish biomass basin-wide), Se Kong 3d (2.3 per cent), Se Kong 3u (0.9 per cent), and Se Kong 4 (0.75 per cent) in Laos.
Those projects were among 27 dams focused on by the team because they have construction planned between 2015 and 2030 and their future remains up in the air.
In all, the researchers identified 877 fish species in the Mekong River Basin, 103 of which would be potentially blocked from making their upstream migrations by hydropower development.
Tributary dams fall under national laws and do not require international agreement, even though building these dams could have "potentially significant transboundary impacts" on fish in other countries' waters, said the study.
