The size of megafish in the Mekong River has shrunk alarmingly in recent years due to overfishing, dams and the climate crisis, a study has found. The length of the largest and most endangered freshwater giants decreased by 40% in seven years.
Some species, like the Mekong giant catfish, have declined in weight by 55% over the past 25 years, dropping from an average of 180kg to 80kg. The giant barb, the world's largest carp and Cambodia's national fish, has shown a similar trajectory. Both species are critically endangered.
Size is crucial for survival because large fish produce exponentially more eggs. A 300kg catfish can produce 10 to 20 times more offspring than a 50kg fish. Many of the largest species are long-lived and slow to mature, meaning increased deaths can tip the balance towards population collapse.
Dr Zeb Hogan, a research associate professor at the University of Nevada, said the rapid shrinking was surprising and shocking. 'We've seen this pattern in the Atlantic cod fishery, where size-selective fishing removed the largest, most fertile fish, resulting in a dramatic collapse. As body size and reproductive capacity decline, these fish risk entering a death spiral,' he said.
The Mekong giants are crucial for the river's ecosystem and the livelihoods of millions who depend on fishing. The study, published in Biological Conservation, analysed seven years of monitoring data from 23 sites in the lower Mekong basin. As well as overfishing, the fish face dams that block migration, the climate crisis upsetting seasonal triggers, and destruction of flooded forests.
However, a sign of hope emerged in 2022 when a fisher caught and released a record-breaking 300kg stingray, the largest freshwater fish ever seen. Hogan said: 'That stingray showed us it's not too late. But new records will mean little unless we act now to protect these species and the river system they call home.'
