Environmental DNA testing helps unlock mysteries of endangered giant catfish
The study, which was the first eDNA-based assessment of fish biodiversity in the Mekong, sought to better understand the habitat and distribution of this once abundant species that has declined by 90 percent in recent decades.
The study was released in advance of World Rivers Day, Sept. 27, an annual celebration of the world’s waterways and their vital importance in conserving biodiversity and sustaining communities.
The study identified the giant catfish at a single location in northern Thailand, locally recognised as a spawning ground. In total, it sampled 12 locations along the Mekong River in Laos, Thailand and Cambodia and detected 176 fish species, while also identifying other freshwater animals such as the Irrawaddy dolphin, several frog species and salamanders.
Every species sheds DNA into the environment, through skin cells, excreta and other organic matter. By extracting traces of eDNA from water samples and comparing them to genetic databases, scientists from WWF and French biotechnology company SPYGEN created a snapshot of life in the Mekong at the time and place of sampling.
“If we don’t know where the Mekong giant catfish is, we can’t save it. Good data leads to good conservation,” said Dr. Thomas Gray, Regional Species Lead for WWF-Greater Mekong. “Sampling environmental DNA is a pioneering research technique that helps us help species like the Mekong giant catfish survive the immense threats they face.”
The world’s largest freshwater fish can weigh as much as 300 kg and is only found in the Lower Mekong River, where its existence is threatened by dams and over-fishing.
The dramatic decline in Mekong giant catfish populations is due to overfishing, electro-fishing, dynamite fishing, destruction of spawning habitat, pollution and infrastructure development. To protect the species from extinction, scientists need to better understand its habits and distribution, but the few hundred individuals remaining have been nearly undetectable to traditional biodiversity surveys.
The giant catfish is believed to migrate north in the Mekong between the months of April and June and is generally found in deep river pools. While this single find underlines the rarity of the species, it is also an important step toward understanding and conserving this icon of the Mekong River.
Mekong giant catfish and Irrawaddy dolphins are seriously threatened by 11 proposed hydropower dams on the river’s mainstream.
One of these – the Xayaburi Dam in northern Laos – is almost complete, while another on the Don Sahong channel in southern Laos, is planned to break ground in the near future. The dams have the potential to irrevocably disrupt fish migration, endangering the world’s largest inland fishery and the main source of protein for the region’s 60 million people.
These dams could deal a crushing blow to Mekong giant catfish, as well as river dolphins and other species. The Don Sahong dam site is less than two kilometres downstream from a deep river pool that was the most biodiverse sampling location in the study, and which also contains Laos’ five remaining Irrawaddy dolphins.
“The Don Sahong Dam is an ecological time bomb that threatens the food security of millions and a population of critically endangered Irrawaddy dolphins,” said Sam Ath Chhith, Country Director, WWF-Cambodia. “The dam will have negative impacts on the entire Mekong River ecosystem all the way to the Delta in Vietnam. We ask the Laos government and the developer – Malaysia’s MegaFirst Corporation Berhad – to reconsider this ill-fated decision and wait until further studies are completed on the environmental and social impacts and all legal options and requirements under the Mekong Charter have been completed."
“These results help us understand fish presence and migration in the Mekong River in a new way, and offer important perspectives in the monitoring of fish and more generally biodiversity in the Mekong,” said Eva Bellemain from SPYGEN.
“eDNA can reveal all species present in an area, including the rarest and hardest to detect. It also washes away within days, so these samples only represent the species present at a given time, giving a clearer picture of which fish are in a specific location at a specific time," added Bellemain.
Funding for this study was provided by SPYGEN and the HSBC Water Programme, a five-year $100 million initiative to secure healthy flowing rivers in five priority areas around the world, including the Mekong River basin.