Inside the plan to save some of the biggest freshwater fish
Captive-bred specimens of some of the world’s largest and most endangered freshwater fish, including a 1.50-meter-long giant Mekong catfish, were released into the Tonle Sap this month. in Cambodia, 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.
These threats still exist, and the species released – giant catfish, giant barbel, striped catfish and others – are seen as indicators of danger to a larger fishery that sustains millions of Cambodians.
“This release is important, but it’s only the first step in 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 directs, called Wonders of the Mekong.
The Tonlé Sap is connected to the Mekong River, which crosses six countries. The Mekong basin is a global biodiversity hotspot, home to nearly 1,000 species of freshwater fish, including the largest in the world.
Large freshwater fish are among the most endangered animals in the world, according to studies. According to Hogan, populations of giant Mekong catfish, which can grow to the size of a grizzly bear (660 pounds) and giant barbs, a type of carp that can grow to equally huge proportions, have dropped by more than 90. % over the past few decades. 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 to over four feet long and made up most of the fish released this month, was once a staple in the region. But it too has seen steep declines and is now classified as endangered.
“These fish are the first to go because the fishing pressure becomes unsustainable,” says Hogan, who is a National Geographic explorer.
Many large fish in the Mekong region are now bred in captivity. Their introduction into the wild is problematic, however, as a lack of genetic diversity can prevent them from breeding successfully.
In contrast, the fish released this month were bred at an aquatic research center in southeast Cambodia, but were not bred there. They were collected from the wild when they were so small you couldn’t tell them apart.
The five-foot-long giant Mekong catfish was one of five donated to the research center by a local fishmonger. This trader had picked up the fish when they were little more than dots 13 years ago, unaware that they would turn into huge catfish too big for his ponds.
A long trip
The endangered fish’s two-day journey into the wild began around 8 p.m. one night in early March when two vans equipped with hydrogen peroxide tanks left the research facility. A pickup truck was carrying hundreds of striped catfish, along with dozens of juvenile giant barbs, Cambodia’s national fish. In the other van were the two giant Mekong catfish.
Traveling at night, the vans arrived in the northwest town of Siem Reap at sunrise. There the fish were held in ponds and tanks before being taken the following morning by boats to the open waters of the Tonlé Sap, where they were released into a government-run fish sanctuary. Of more than 1,500 fish released, only one, a striped catfish, did not survive the test.
For a reintroduction to be successful in the long term, the habitat must be healthy enough to support the fish. A study published in the journal Conservation Biology a few years ago concluded that an inadequate response to the initial cause of a fish population decline was the best indicator of “reintroduction failure”.
Such projects “can’t just stock more fish, even though this approach is the simplest,” says Jennifer Cochran-Biederman, professor of biology at Saint Mary’s University of Minnesota and lead author of the study. “They have to address the 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 disruption from upstream dam operations and several years of drought exacerbated by climate change (although this year the lake experienced higher water levels). This makes it a good place to reintroduce fish.
“We know that juveniles of these species historically occur in the lake at this point in their life cycle,” says Hogan.
The main threat to fish in the lake, where communal fishing employs 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 are increasingly seen operating in the lake using huge trawls, in violation of fishing regulations.
But the Tonlé Sap also has the world’s largest network of inland fish sanctuaries, designated areas where fishing is not permitted. In addition to a series of government-run reserves created a decade ago, there are several hundred smaller community reserves. These areas, say conservationists, have proven to be successful shelters for many species of fish, especially larger ones.
“Although these reserves are not as old as others in Southeast Asia, they have comparable advantages, with more fish inside the reserves than in areas where fishing is allowed,” says Aaron Koning, a conservation ecologist at the University of Nevada (and National Geographic Explorer) who studies the effectiveness of fish stocks.
But he and others point out that even if regulations are enforced – which most observers say is spotty in the Tonlé Sap, the world’s largest inland fishery – reserves alone cannot protect migratory fish. that move great distances in the river system to complete their life cycle. “The likelihood of fish being caught once they leave the reserves and even the lake is very high,” Koning says.
Fish that have been released have been tagged so researchers can track them and gather information if the fish are recaptured. Anglers will be rewarded for releasing fish into the lake and returning tags with information.
“This information tells us the level of fishing intensity in the lake, which is extremely useful in informing management interventions,” says Ngor Peng Bun, fish ecologist and dean of fisheries science at the Royal University of Agriculture. from Cambodia.
Presiding over the fish release ceremony, which was attended by a large group of local officials, Poum Sotha, director general of the Cambodian Fisheries Administration, urged fishermen to participate in the search effort, while also suggesting that if someone was caught while killing fish marked fish, they would face legal consequences.
There are warning signs that fishermen are ready to help. In the days following the release, several fish catches were reported, including one of the giant barbs. He had been caught in a trap and was eventually released into a different shrine.
More releases are planned and the researchers hope to fit the next batch of fish with tags emitting sound waves, in order to track them more actively. They also want to release fish in other places, such as the deep Mekong basins in northern Cambodia, where many migratory fish are thought to spawn.
“This release is just the tip of the iceberg,” Hogan says. “There is a need to do more, see what works and what doesn’t, and keep learning and improving if we are to save these iconic fish.”
He and his colleagues will closely monitor what happens to the two giant Mekong catfish before deciding to release those still held in captivity. Hogan is confident that if the larger of the two, in particular, is caught, the capture will be reported and the fish will be released.
“It’s such a big and unique fish. People recognize it as something special,” he says. For added protection, the researchers named it Samnang-the Cambodian word for “lucky”.
The National Geographic Society, committed to illuminating and protecting the wonders of our world, funded the work of explorer Zeb Hogan. Learn more about the Society’s support for 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.