Inside the plan to save some of the biggest freshwater fish

Captive-raised specimens of some of the world’s largest and most critically endangered freshwater fish—including a five-foot-long Mekong giant catfish—were released this month into Cambodia’s Tonle Sap, the largest lake in Southeast Asia. Scientists hope the released fish can survive and begin to rebuild wild populations decimated by decades of overfishing, dam building, and other human actions.

Those threats still exist, and the released species—the giant catfish, giant barb, and striped catfish—are considered bellwethers of the danger to a larger fishery that sustains millions of Cambodians. 

“This release is significant, but it is just the first step of many actions that will be needed for the long-term recovery of these giant fish,” says Zeb Hogan, a fish biologist at the University of Nevada, Reno, who coordinated the release effort for a U.S. Agency for International Development research project he leads, called Wonders of the Mekong.

The Tonle Sap is connected to the Mekong River, which flows through six countries. The Mekong Basin is a global biodiversity hotspot, home to almost 1,000 freshwater fish species, including the world’s largest.

Large freshwater fish are among the most threatened animals globally, studies show. Populations of Mekong giant catfish, which can reach the size of a grizzly bear—660 pounds— and giant barb, a type of carp that can grow to similarly huge proportions, have plummeted by more than 90 percent in recent decades, according to Hogan. Hogan, who has long studied giant fish in the region, has not seen a Mekong giant catfish in the wild since 2015.

Another species, the striped catfish, which can grow over four feet long and made up most of the fishes released this month, was once a staple food in the region. But it, too, has seen sharp declines and is now classified as endangered.

“These fish are the first to go as fishing pressure becomes unsustainable,” says Hogan, who is a National Geographic Explorer.

Many large fishes in the Mekong region are now bred in captivity. Introducing them into the wild is problematic, however, because a lack of genetic diversity may make them unable to reproduce successfully.

In contrast, the fish released this month were reared at an aquatic research center in southeastern Cambodia—but weren’t bred there.  They were collected in the wild when they were so tiny their species could not be distinguished.

The five-foot-long Mekong giant catfish was one of five donated to the research center by a local fish trader. This trader had collected the fish when they were little more than specks 13 years ago, not knowing they would turn into massive catfish too big for her ponds.

A long journey

The endangered fishes’ two-day trip back to the wild began around 8 p.m. on March 2, when two vans fitted with oxygenated water tanks departed the research center. One van carried hundreds of striped catfish, along with dozens of juvenile giant barbs, Cambodia’s national fish. In the other van were the two Mekong giant catfish.

Traveling through the night, the vans arrived in the northwestern city of Siem Reap at sunrise. There, the fish were held in ponds and tanks before being taken the following morning by boats into the open waters of the Tonle Sap, where they were released into a government-operated fish reserve. Of the more than 1,500 fish released, only one, a striped catfish, did not survive the ordeal.

For a reintroduction to succeed in the long term, the habitat must be healthy enough to support the fish. A study published in the journal Conservation Biology some years ago concluded that inadequately addressing the initial cause of decline in a fish population was the best predictor of “reintroduction failure.”

Such projects “cannot just stock a higher number of fish, even though this approach is the easiest,” says Jennifer Cochran-Biederman, a biology professor at Saint Mary’s University of Minnesota, and the lead author of that study. “They must tackle broader causes of habitat degradation, such as pollution, climate change, urban development, and dams, which is much more complicated.”

The Tonle Sap, which provides a fertile feeding ground for many fish, remains relatively intact, despite being disrupted by upstream dam operations and several years of drought exacerbated by climate change (though this year the lake has seen higher water levels). That makes it a good place to reintroduce the fish.

“We know that juveniles of these species historically occur in the lake at this stage in their life cycle,” says Hogan.

The main threat to fish in the lake, where communal fisheries employ hundreds of thousands of people, is overfishing. The use of illegal fishing nets and other gear is widespread—in fact, it’s not even hidden. Large ocean-going fishing vessels have increasingly been seen operating in the lake using huge trawling nets, in violation of fishing regulations.

But the Tonle Sap also has one of the world’s largest network of inland fish reserves, designated areas where fishing is not allowed. In addition to a string of government-operated ones established a decade ago, there are several hundred smaller community reserves. Such areas, conservationists say, have proved successful shelters for many fish species, especially large ones.

“Although these reserves aren’t as old as others in Southeast Asia, they’re showing comparable benefits, with more fish inside the reserves than in areas where fishing is allowed,” says Aaron Koning, a University of Nevada conservation ecologist (and National Geographic Explorer) who studies the efficacy of fish reserves.

But he and others stress that even if regulations are enforced—which most observers agree is spotty in the Tonle Sap, the world’s largest inland fishery—reserves alone cannot protect migratory fish that move large distances in the river system to complete their life cycles. “The likelihood of the fish getting caught once they move out of the reserves and even the lake is very high,” says Koning.

Community effort

The fish that were released were tagged so that researchers could track them and gather information if the fish are recaptured. Fishers will be rewarded for returning the tags with information about the capture.

“This information tells us the level of fishing intensity in the lake, which is critically useful to inform management interventions,” says Ngor Peng Bun, a fish ecologist and fisheries science dean at Cambodia’s Royal University of Agriculture.

Presiding at the fish release ceremony, which was attended by a large group of local officials, Poum Sotha, director-general of the Cambodian Fisheries Administration, urged fishers to help the research effort, while also suggesting that if anyone were caught who had killed tagged fish, they would face legal consequences.

There are early signs that fishers are willing to assist. In the days following the release, many fish captures were reported, including one of the giant barbs. It had been ensnared in a trap and was ultimately released into a different sanctuary.

More releases are planned, and researchers hope to fit the next batch of fish with tags that emit soundwaves, in order to track them more actively. They also want to release fish in other locations, such as the deep pools of the Mekong River in northern Cambodia, where many of the migratory fish are believed to spawn.

“This release is just the tip of the iceberg,” says Hogan. “It’s necessary to do more, to see what works and what doesn’t, and keep learning and improving if we want to save these iconic fish.”

He and his colleagues will closely monitor what happens to the two Mekong giant catfish before deciding whether to release those still being held in captivity. Hogan is confident that if the largest of the two, in particular, is caught, the catch will be reported and the fish will be re-released.

“It’s such a large and unique fish. People recognize it as something special,” he says. For extra protection, the researchers named it Samnang—the Cambodian word for “lucky.”

The National Geographic Society, committed to illuminating and protecting the wonder of our world, funded explorer Zeb Hogan’s work. Learn more about the Society’s support of explorers highlighting and protecting critical species. Hogan and Stefan Lovgren are co-authors of the forthcoming book Chasing Giants: The Search for the World’s Largest Freshwater Fish.