The size of megafish in the Mekong River has shrunk alarmingly in recent years owing to overfishing, a study has found. The length of the largest and most endangered freshwater giants, some as big as grizzly bears, decreased by 40% in seven years.

Some fish, like the Mekong giant catfish, have been studied for over a longer period and show a decline in weight of 55% in the past 25 years, dropping from an average of 180kg (397lb) to 80kg.

One of the study team, the biologist Ngor Peng Bun, helped catch a gigantic catfish in 2000 that weighed 270kg, before tagging and releasing it. “It was enormous – unforgettable,” he said, and nearly capsized his boat. The species, known in Khmer as trey reach, or royal fish, is now a shadow of its former self.

The 270kg Mekong giant catfish that the biologist Ngor Peng Bun helped catch in 2000, before tagging and releasing it. Photograph: Zeb Hogan

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 a matter of survival because large fish – which are more likely to be caught – produce exponentially more eggs, the scientists said. A 300kg catfish can produce 10 to 20 times more offspring than a 50kg fish, for example. Many of the largest species are also long-lived and slow to mature, meaning increases in deaths can tip the balance towards population collapse.

The Mekong giants are crucial for the river’s ecosystem but also central to the livelihoods of millions of people who depend on fishing. The scientists did find a sign of hope when in 2022 a fisher caught and released a record-breaking 300kg stingray. It was the largest freshwater fish ever seen and showed that some giants still remain.

A giant freshwater stingray, weighing 181kg, is caught and released in the Mekong in Cambodia. Photograph: Chhut Chheana/Wonders of the Mekong/AFP/Getty Images

Fishing and hunting often targets the largest animals and has led to the shrinking of many species including cod in the Baltic Sea, salmon in Alaska and sharks, and has led to smaller tusks in African elephants.

Dr Zeb Hogan, a research associate professor at the University of Nevada, US, and part of the study team, said the rapid shrinking of the Mekong megafish was surprising and shocking.

“We’ve seen this pattern in the Atlantic cod fishery, where decades of size-selective fishing removed the largest, most fertile fish, resulting in a dramatic collapse. As body size and reproductive capacity decline, [the Mekong] fish risk entering a ‘death spiral’, in which populations persist but are no longer viable,” Hogan said.

Sophorn Uy, at the Royal University of Agriculture in Phnom Penh and also part of the team, said: “Knowing these giants are shrinking, both in size and number, is like losing a small piece of Cambodia itself. Their existence carries deep cultural and ecological significance.”

The study, which is published in the journal Biological Conservation, analysed seven years of monitoring data from 23 sites in the lower Mekong basin and included 257 species of fish. Full data was available from 2007-14. Hogan said the shrinking trend was likely to have continued.

As well as overfishing, the Mekong megafish are struggling with dams, which block their migration to spawning grounds. Photograph: Andy Eames/AP

As well as overfishing, the Mekong megafish are struggling with dams, which block their migration to spawning grounds; the climate crisis, which is upsetting the seasonal triggers of temperature and rainfall; and the destruction of seasonally flooded forests for farming, which are an important habitat.

Hogan said: “That 300kg stingray showed us that it’s not too late. New records are still being set. But they will mean little unless we act now to protect these species and the river system they call home. The future of the Mekong’s giants hangs in the balance.”

The stingray was tagged and tracked for two years, which it spent in a protected area along a 3-mile (5km) stretch of river. “Connectivity [along rivers] and protected habitat – that’s the holy grail,” said Hogan. “If you have those, then these fish can survive.”

Catching the megafish is already illegal in some countries through which the Mekong flows, but enforcement remains a big challenge, the scientists said. “Local fishers were essential to gathering our data,” said Sophorn. “They are on the frontlines and must be part of the solution.”