• ANIMALS

The 24-hour hotline helping save the world’s biggest freshwater fish

The ‘megafish’ living in southeast Asia’s Mekong River once faced extinction. A new rapid response system is helping reverse their dire outlook.

Kampong Cham, Cambodia — In early January, at the peak of Cambodia’s fishing season, Chea Seila was driving north along the Mekong River when her phone began to buzz. The call came from another part of the river’s vast watershed, hours away. A fisherman had just hauled in a 209-pound Mekong giant catfish.

Seila, a member of a national rapid-response team formed to rescue imperiled megafish, turned her car around. Within hours, she and her colleagues were on site—measuring, tagging, and eventually releasing the critically endangered fish back into the water. Then her phone rang again.

This time the call came from near Phnom Penh, where fishers had accidentally caught a 172-pound giant barb, known as Cambodia’s “King of Fish,” also critically endangered. Once again, the rapid-response team mobilized, and before long this giant, too, was back in the river. “There were seasons when we didn’t hear about a single giant fish,” Seila said. “Now the calls keep coming.”

For species long thought to be disappearing, the surge is striking. Once abundant across the Mekong basin, giant fish were killed by overfishing, dams, and disruption to the regular rise and fall of water levels along the river. The Mekong, spanning six countries and supporting the world’s largest inland fishery, holds more megafish species than any river on Earth, some of which can grow over 600 pounds.

In recent decades, their numbers have collapsed.

But in Cambodia, conservationists have persisted, working closely with fishing communities and local government officials strengthening cooperation along the river. In late 2025, a national hotline; a Mekong Fish App; and a dedicated rapid-response team transformed scattered rescues into a coordinated national effort. From mid-November to early February, the team returned 92 large fish that had been unintentionally caught—24 Mekong giant catfish, 23 giant barb, 32 seven-striped barb, and 13 striped catfish—back into the waters, alive.

To Zeb Hogan, a fish biologist at the University of Nevada, Reno, and a National Geographic Explorer who has studied Mekong fish for three decades, the season was a stunning shift. “After years of watching these species edge toward extinction, seeing numbers rise instead of fall feels almost unimaginable. But it's happening, and it's exciting,” he says.

Why megafish started to disappear

Giant freshwater fish are among the most threatened animals on Earth. In some regions, populations of freshwater megafish—which Hogan defines as species that can grow to at least 200 pounds or six feet in length—have declined by more than 95 percent since 1970. One such leviathan, the Chinese paddlefish, which could exceed 20 feet, was declared to have gone extinct in the early 2000s.

The reason the Mekong holds more megafish than any other river is likely because its immense size, productivity, and annual floods create the food, space, and connectivity that allow fish to grow extraordinarily large. In 2022, a giant freshwater stingray weighing a confirmed 661 pounds was caught and released in northern Cambodia, the largest freshwater fish ever recorded.

“If you’re looking for the planet’s freshwater giants, you look to the Mekong,” says Hogan.

In the early 1980s, the Mekong’s giants were commonplace in Kampong Cham, Cambodia. Huot Uong, now 73, recalls working on crews that hauled seine nets more than 2,000 feet long and 15 feet deep from the river. Each sweep could bring hundreds of striped catfish along with as many as 10 adult Mekong giant catfish.

Occasionally, truly enormous fish surfaced in the nets. One January day, the fishers landed a 660-pound giant catfish so oily that everyone who ate it fell ill. A month later they caught another, roughly 100 pounds heavier, so powerful it took nearly 100 men two hours to drag it ashore. No one would buy it, fearing it would make them sick. Looking back, Uong remembers thinking the boom could not last and that the river’s bounty would eventually fade.

His fears were not unfounded. The coming decline stemmed not only from overfishing but also from sweeping upstream changes. Beginning in the 1990s, China built a cascade of dams on the Upper Mekong, altering flows that migratory fish had followed for millennia. Laos expanded hydropower, sand mining intensified, and the river’s flood pulse was disrupted. “The Mekong is a system that has been engineered in ways that work against fish, and against the tens of millions of people who depend on them for food,” says Brian Eyler of the Stimson Center in Washington, D.C. and author of “Last Days of the Mekong.”

When Hogan began studying Mekong giant catfish in northern Thailand in the late 1990s, the species was already in steep decline. Records from the Chiang Khong Fishermen’s Club showed that as many as 80 large spawning fish were caught in a single season in the 1980s; by 1997, fewer than 10 remained. Within a decade, some seasons produced none at all. As the Thai fishery collapsed, Hogan shifted his focus to Cambodia, where deep pools and floodplains offered one of the last refuges for the giants.

In those early years, he worked with a small group of Cambodian colleagues, spending nights with fishing families along the river in a country with limited scientific capacity and few communication tools. Fishers had no phones; reports traveled slowly, if at all. “When you’re studying a critically endangered species and the catch goes to zero, you have to confront the possibility that it may be gone forever,” he says. “It felt like witnessing the end.”

How a hotline is saving fish

But the fish were still there. Through a research initiative called the Wonders of the Mekong project, Hogan and Cambodian colleagues spent years building trust with fishing communities, where these species are rarely targeted but sometimes caught by accident. As communication improved, reports became more consistent, among them the capture and release of the record-breaking freshwater stingray by local fishers.

In the 2024–25 season, more than a dozen Mekong giant catfish were reported, after years when, in some cases, none were reported.

To formalize the cooperation between fishing communities and conservationists, researchers developed the Mekong Fish App. In the past, fishers recorded catches by hand, if at all, and many hesitated to report protected species they had caught by accident. The app allows fishers to log catches in near real time—entering measurements, uploading photos, and automatically capturing GPS coordinates—with data shared directly with fisheries officials and Telegram alerts flagging threatened species.