Damming the mainstream of the lower Mekong River will represent a significant new threat to the survival of the Mekong giant catfish, one of the world’s largest and rarest freshwater fishes, according to a new study commissioned by the World Wide Fund for Nature.
The study sheds new light on the status of this elusive species, including data on its numbers, distribution, threats, and measures needed to prevent its disappearance.
While the exact population size is unknown, there could be as few as a couple of hundred adult Mekong giant catfish left.
According to the study, the Xayaburi dam in northern Laos will prove an impassable barrier for the migratory giant catfish — which reach up to three metres in length and weigh as much as 300kg – and risks sending the species to extinction.
“A fish the size of a Mekong giant catfish simply will not be able to swim across a large barrier like a dam to reach its spawning grounds upstream,” said the study’s author and associate research professor at the University of Nevada in the US, Dr Zeb Hogan.
“These river titans need large, uninterrupted stretches of water to migrate, and specific water quality and flow conditions to move through their life cycles of spawning, eating, and breeding.”
Numbers are already in steep decline due to overfishing, habitat destruction, and dams along the Mekong’s tributaries.
In the Mun River, the largest tributary, a dam already blocks the migration of the creature and has isolated the Mun River from the remainder of the Mekong river basin.
The study claims that the controversial Xayaburi dam could disrupt and even block spawning, and increase mortality if the fish pass through dam turbines.
“It is likely the Mekong giant catfish use the stretch of river of the Xayaburi dam as a migration corridor, with adult fish likely passing through this area on their migration from floodplain rearing areas to upstream spawning sites,” Hogan said.
Regional countries agreed at a Mekong River Commission meeting in 2011 to delay a decision on building the Xayaburi dam pending further studies on its environmental impacts.
Puyry, the Finnish firm advising Laos on the US$3.5-billion dam construction, argues that “fish passages” can be built to enable fish to get past the dam’s turbines and swim up and down the river. But this claim has never been successfully put into practice.
“You cannot expect fish ladders to work without understanding your target species, their swimming capabilities, and the water current that will attract these fish toward the pass entrance,” Dr Eric Baran of the World Fish Centre said.
“Research is still needed to ensure mitigation efforts will work.”
Mekong giant catfish were once widely distributed through the Mekong river basin, possibly as far as Myanmar and south-western China, and were relatively abundant until the early 1900s.
Their numbers have since plummeted and the species is now limited to the Mekong and its tributaries in Cambodia, Laos and Thailand.
Catch figures also offer sobering evidence of the decline, with numbers dropping from thousands of fish in the late 1880s, to dozens in the 1990s, and only a few in recent times.
Despite laws being in place in Thailand, Laos, and Cambodia to regulate its fishing — with a ban in Thailand and Cambodia — the species is still fished illegally and caught accidentally in fisheries targeting other species.
“Catches should be monitored to ensure that Mekong giant catfish are not being illegally targeted by fishers,” Hogan said.
“Incidental catch should also be monitored since it is the only source of information about the distribution, life history, and abundance of this river giant.”
The study identifies key measures to prevent the fish’s disappearance, like immediate efforts to safeguard migratory corridors and critical habitat, and increased international cooperation since the species occurs in an international river and crosses country borders to complete its life cycle.
“The Mekong giant catfish symbolises the ecological integrity of the Mekong River because the species is so vulnerable to fishing pressure and changes in the river environment,” Dr. Lifeng Li, director of the WWF’s Global Freshwater Programme, said.