World’s largest freshwater fish draws attention to delicate ecosystem | Science

Late one night last summer, Seila Chea got an urgent call from a fisherman on the Mekong River in northeastern Cambodia. He’d hooked an endangered giant freshwater stingray—and it was a monster. Chea, project manager for the Wonders of The Mekong initiative, quickly organized a posse that sped out to the river to bargain for the creature’s life. Nearly 4 meters from snout to tail, the female weighed in at a hair under 300 kilograms, making her the world’s largest known freshwater fish.

“It was a full moon that night,” Chea says, “so I named her Boramy,” the Khmer word for full moon. The scientists paid market price for her meat, about $600, implanted a radio tag at the base of her tail, and set her free.

Hydrophone tracking of Boramy over the past year has given scientists a new window into the behavior of the enigmatic giant freshwater stingray, or whipray (Urogymnus polylepis), Chea and her colleagues report in the current issue of the journal Water. In the months since Boramy’s release, the team, working with the Joint Environmental Monitoring Programme of the Mekong River Commission, has tagged nearly 300 more fish from 27 species in Cambodia and Laos.

“It’s vital research,” says Jake Brunner, head of the Lower Mekong Sub-Region for the International Union for Conservation of Nature, who was not involved in the study. “It leaves me more optimistic that the incredible concentration of fish diversity in the Mekong can be saved.”

The Mekong River and its 1000 or so known fish species support the world’s largest inland fishery. But dozens of hydropower dams in the Lower Mekong Basin have taken a toll, fragmenting habitat, reducing water quality, and crimping migrations. The endangered Irrawaddy River dolphin—an aquatic mammal—is among the species that have declined as a result. At least 123 more hydropower dams have been proposed, including 11 on the river’s main stem. These include the 980-megawatt Stung Treng Dam and the 2600-megawatt Sambor Dam, which would sandwich a stretch of the Mekong that includes Boramy’s habitat.

But there are signs of hope, says Zeb Hogan, leader of the stingray study and a biologist at the University of Nevada, Reno. Cambodia’s environment ministry in December 2022 nominated 180 kilometers of the Mekong stretching south from its border with Laos as a World Heritage Site. The country’s first biosphere reserve would encompass the planned sites for the Stung Treng and Sambor dams—and perhaps thwart their construction. Separately, the Cambodian government has halted dam building on the Mekong main stem until at least 2030.

Hogan’s team hopes to use that breather to fill in vital details about the Mekong’s lengthy list of leviathans. Globally, the plight of freshwater megafishes is dire, with 81 species having declined by 94% from 1970 to 2012, according to a 2019 report. In the Mekong, many are verging on extinction.

Perhaps most iconic is the critically endangered Mekong giant catfish (Pangasianodon gigas). A 293-kilogram specimen caught in Thai waters in 2005 was the previous record holder for the largest freshwater fish. The species migrates hundreds of kilometers, heightening the risk it will end up on a fisher’s hook. Cambodian fishermen caught and released one last year, but “that kind of story is just so rare these days,” says Dana Lee, a fisheries biologist with FishBio, a consulting firm. “We know the giant catfish is still out there, we pick up its DNA,” says Lee, whose firm is carrying out telemetry work for Wonders of the Mekong, an initiative of the U.S. Agency for International Development. “But it’s like a needle in a haystack.”

Most fishers realize that killing megafishes degrades the ecosystem, Hogan says. As a result, he says, “These fish tend to be caught at night. They’re butchered and sold under the cover of darkness.” Getting fishers to break that habit may be the only way to save endangered Mekong megafishes, including the giant salmon carp (Aaptosyax grypus). The carp hadn’t been spotted in Cambodian waters for more than 20 years before a fisher snared one last year—and sold it to a local market.

But years of outreach to fishing communities is starting to pay off. Through a shared Telegram channel, enlightened fishers now communicate with the Wonders of the Mekong’s office in the Inland Fisheries Research and Development Institute. Allies alert Chea and her colleagues when they snare a megafish. A community fish reserve near Sung Treng has offered a bonanza: In addition to Boramy, the team has tagged there seven-striped barb (Probarbus jullieni) and giant barb (Catlocarpio siamensis), two other critically endangered megafish. “These super-rare fish seem to be doing well in this one protected area,” Hogan says. “It’s like an underwater Shangri-la.”

The mystical aura of the Mekong is not lost on Chea, who recalls visiting a fishing village years ago and being warned not to venture out on the river at night. “They told me a water spirit lives there. It’s dark, it’s black. It can kill a water buffalo.” She believes villagers were referring to the giant freshwater stingray, which some locals venerate as a god of darkness.

Lee confesses he was a little daunted tagging Boramy—not because of its godlike status, but because it was the first time he’d done the surgical procedure on a giant stingray. “We were going in blind,” he says. The team has since learned that Boramy is something of a homebody, mostly hanging out in the community fish reserve, in stretches of the river that reach 70 meters or more in depth. “It has a tiny home range,” Lee says. “That really surprised us.”

Two other adult giant stingrays caught recently in the reserve suggest it provides critical habitat for the species, Hogan says. The proposed megadams would almost certainly wreck that habitat, he adds.

Later this year, Hogan aims to bring together experts from across the Mekong to hash out an action plan for saving the giant freshwater stingray. Keeping Cambodia’s upper Mekong mainstem dam-free will be a vital component, Hogan says. “It’s the most productive, healthy stretch of the river we have left.”

Late one night last summer, Seila Chea got an urgent call from a fisherman on the Mekong River in northeastern Cambodia. He’d hooked an endangered giant freshwater stingray—and it was a monster. Chea, project manager for the Wonders of The Mekong initiative, quickly organized a posse that sped out to the river to bargain for the creature’s life. Nearly 4 meters from snout to tail, the female weighed in at a hair under 300 kilograms, making her the world’s largest known freshwater fish.

“It was a full moon that night,” Chea says, “so I named her Boramy,” the Khmer word for full moon. The scientists paid market price for her meat, about $600, implanted a radio tag at the base of her tail, and set her free.

Hydrophone tracking of Boramy over the past year has given scientists a new window into the behavior of the enigmatic giant freshwater stingray, or whipray (Urogymnus polylepis), Chea and her colleagues report in the current issue of the journal Water. In the months since Boramy’s release, the team, working with the Joint Environmental Monitoring Programme of the Mekong River Commission, has tagged nearly 300 more fish from 27 species in Cambodia and Laos.

“It’s vital research,” says Jake Brunner, head of the Lower Mekong Sub-Region for the International Union for Conservation of Nature, who was not involved in the study. “It leaves me more optimistic that the incredible concentration of fish diversity in the Mekong can be saved.”

The Mekong River and its 1000 or so known fish species support the world’s largest inland fishery. But dozens of hydropower dams in the Lower Mekong Basin have taken a toll, fragmenting habitat, reducing water quality, and crimping migrations. The endangered Irrawaddy River dolphin—an aquatic mammal—is among the species that have declined as a result. At least 123 more hydropower dams have been proposed, including 11 on the river’s main stem. These include the 980-megawatt Stung Treng Dam and the 2600-megawatt Sambor Dam, which would sandwich a stretch of the Mekong that includes Boramy’s habitat.

But there are signs of hope, says Zeb Hogan, leader of the stingray study and a biologist at the University of Nevada, Reno. Cambodia’s environment ministry in December 2022 nominated 180 kilometers of the Mekong stretching south from its border with Laos as a World Heritage Site. The country’s first biosphere reserve would encompass the planned sites for the Stung Treng and Sambor dams—and perhaps thwart their construction. Separately, the Cambodian government has halted dam building on the Mekong main stem until at least 2030.

Hogan’s team hopes to use that breather to fill in vital details about the Mekong’s lengthy list of leviathans. Globally, the plight of freshwater megafishes is dire, with 81 species having declined by 94% from 1970 to 2012, according to a 2019 report. In the Mekong, many are verging on extinction.

Perhaps most iconic is the critically endangered Mekong giant catfish (Pangasianodon gigas). A 293-kilogram specimen caught in Thai waters in 2005 was the previous record holder for the largest freshwater fish. The species migrates hundreds of kilometers, heightening the risk it will end up on a fisher’s hook. Cambodian fishermen caught and released one last year, but “that kind of story is just so rare these days,” says Dana Lee, a fisheries biologist with FishBio, a consulting firm. “We know the giant catfish is still out there, we pick up its DNA,” says Lee, whose firm is carrying out telemetry work for Wonders of the Mekong, an initiative of the U.S. Agency for International Development. “But it’s like a needle in a haystack.”

Most fishers realize that killing megafishes degrades the ecosystem, Hogan says. As a result, he says, “These fish tend to be caught at night. They’re butchered and sold under the cover of darkness.” Getting fishers to break that habit may be the only way to save endangered Mekong megafishes, including the giant salmon carp (Aaptosyax grypus). The carp hadn’t been spotted in Cambodian waters for more than 20 years before a fisher snared one last year—and sold it to a local market.

But years of outreach to fishing communities is starting to pay off. Through a shared Telegram channel, enlightened fishers now communicate with the Wonders of the Mekong’s office in the Inland Fisheries Research and Development Institute. Allies alert Chea and her colleagues when they snare a megafish. A community fish reserve near Sung Treng has offered a bonanza: In addition to Boramy, the team has tagged there seven-striped barb (Probarbus jullieni) and giant barb (Catlocarpio siamensis), two other critically endangered megafish. “These super-rare fish seem to be doing well in this one protected area,” Hogan says. “It’s like an underwater Shangri-la.”

The mystical aura of the Mekong is not lost on Chea, who recalls visiting a fishing village years ago and being warned not to venture out on the river at night. “They told me a water spirit lives there. It’s dark, it’s black. It can kill a water buffalo.” She believes villagers were referring to the giant freshwater stingray, which some locals venerate as a god of darkness.

Lee confesses he was a little daunted tagging Boramy—not because of its godlike status, but because it was the first time he’d done the surgical procedure on a giant stingray. “We were going in blind,” he says. The team has since learned that Boramy is something of a homebody, mostly hanging out in the community fish reserve, in stretches of the river that reach 70 meters or more in depth. “It has a tiny home range,” Lee says. “That really surprised us.”

Two other adult giant stingrays caught recently in the reserve suggest it provides critical habitat for the species, Hogan says. The proposed megadams would almost certainly wreck that habitat, he adds.

Later this year, Hogan aims to bring together experts from across the Mekong to hash out an action plan for saving the giant freshwater stingray. Keeping Cambodia’s upper Mekong mainstem dam-free will be a vital component, Hogan says. “It’s the most productive, healthy stretch of the river we have left.”