Catfish as big as grizzly bears and stingrays as large as paddling pools are becoming a thing of the past in the Mekong River, as human-created problems cause these freshwater behemoths to shrink.
A new study in the journal Biological Conservation has found that the length of the largest and most endangered aquatic giants in the Southeast Asian river has decreased by 40% in just seven years.
This includes the Mekong giant catfish which used to reach an average weight of 180kg, but which scientists found is now more likely to weigh just 80kg – a decline of 55% spanning 25 years.
Other shrinking species include the giant salmon carp and the giant barb, which is the world’s largest carp and Cambodia’s national fish.
Mekong's shrinking megafish
It’s thought the trend is driven by indiscriminate fishing and exacerbated by dam-building and habitat loss. When hunters target the biggest animals, as they so often do, species often evolve to become smaller. This has been seen in the oceans, where salmon, cod and grayling have all shrunk.
This can upset the delicate balance of ecosystems. Predators can become prey, and vital ecosystem services, such as nutrient cycling, can be disrupted.
In addition, smaller fish lay fewer eggs than bigger fish. A 300kg catfish, for example, can produce up to 20 times more offspring than a 50kg fish. So now scientists are concerned that in the Mekong, there will be fewer youngsters to replenish the already diminished populations.
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The scientists based their findings on seven years of monitoring data from 23 sites in the lower Mekong basin, spanning 257 species of fish. However, a similar trend has been noted in other parts of Southeast Asia.
"Certain stretches of the Mekong River remain among the world’s last biodiversity strongholds, harbouring numerous threatened fishes,” says Ngor Peng Bun from the Royal University of Agriculture, Cambodia, who studied the fish.
“The decline in large-bodied fish species that we observed suggests that the status of fish stocks in the basin is not in good condition, requiring urgent action for effective fisheries management and conservation.”
As well as being ecologically significant, the big fish are important in supporting the economic, sociocultural and food security of people inhabiting the basin. The Mekong giant catfish features in ancient rock art and folklore, and is revered a sacred creature that symbolises the health of a river, which is now suffering from the effects of human interference.
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Find out more about the study: Declining fish sizes across the Lower Mekong Basin highlights urgent conservation needs
Top image: Mekong giant catfish tag and release. Credit: Zeb Hogan
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