STUNG TRENG, Cambodia As the Mekong River descends into Cambodia from neighboring Laos, flowing languidly past sandbanks and forest-covered islands, it’s hard to imagine the explosion of life that occurs underneath its surface.
In this stretch of river, about 100 miles long, up to 200 billion fish are spawned every year, helping to make the 2,700-mile-long Mekong one of the most fish-rich rivers on the planet. Deep pools reaching down 260 feet serve as refuges for some of the world’s largest and most endangered freshwater fish species.
This biological richness usually remains hidden from human view. But last week it surfaced, literally, as fishers hauled a 13-foot-long giant freshwater stingray, weighing almost 400 pounds, out of the Mekong’s murky depths. The fish had been captured accidentally after it swallowed a smaller fish on a baited hook. Not wanting to kill the female ray, the fishers called a rescue team, which managed to unhook it, weigh and measure it, and release it back to the river unharmed.
For Zeb Hogan, who has long studied Mekong’s megafishes—which include critically endangered Mekong giant catfish and giant barb—the capture of the gargantuan ray is evidence of the outsized ecological and biological role that the deep pools of the Upper Cambodian Mekong River play—a role further revealed by Hogan’s recent explorations of the holes. The area is also home to rare Irrawaddy dolphins and giant softshell turtles.
“This is the last place on Earth where we find these creatures together,” says Hogan, who is a fish biologist at the University of Nevada, Reno, and leads the USAID-funded Wonders of the Mekong research project.
In the week prior to the ray capture, Hogan, who is also a National Geographic Explorer, led a scientific expedition to the area. On the international team of researchers taking part were two other Nat Geo Explorers: Kakani Katija, a deep sea researcher, and Kenny Broad, a cave diver and University of Miami environmental anthropologist.
In the first attempt of its kind to explore the deepest parts of the Mekong River, the team used as their eyes and ears unmanned submersibles equipped with lights and cameras, drop cameras suspended on long cables, and baited video cameras. The researchers also sampled for DNA to identify rare or previously undetected species in the river—which, despite its importance, remains acutely understudied.
Exploring the holes
The Mekong River, which starts in the Tibetan highlands and runs through six countries on its way to the South China Sea, is recognized as a global biodiversity hotspot, with close to 1,000 species of fish found throughout its system. It supports the largest inland fishery in the world, providing livelihoods for tens of millions of people living in the region.
The secret of the Mekong’s productivity is a seasonal flood that raises the river by as much as 40 feet and disperses juvenile fish downstream into the region’s floodplains, where the fish feed and grow. Many Mekong fish species are highly migratory and move up the river to spawn, often traveling long distances to places like the deep pools of Upper Cambodia.
Researchers have long known this area, with its braided channels and islands covered in seasonally flooded forest, to be a dry season refuge for many important Mekong species, including various megafauna. But its remoteness makes it a difficult area to study, and especially challenging is finding out what goes on at the very bottom of the river.
Katija, who leads the Bioinspiration Lab at the Monterey Bay Aquarium Research Institute, sees many similarities between the deep pools of the Mekong and the deep sea environment her team normally explores: deep water, little ambient light, and currents along the bottom. “What makes working in the Mekong River challenging, particularly from an imaging standpoint, is the high turbidity, or low visibility, even in the deepest depths,” she says.
This became clear during several days of Mekong exploration, as Katija and her team deployed a remotely operated vehicle along the river bottom. From a boat above, they watched on a computer screen as the vehicle moved through thick sediment and silt, which reduced visibility to just a foot or so.
But video from 250 feet below the surface still captured several species of fish, including migratory catfish known locally as trey chhwiet. This species once formed the basis of a communal fishery across the border in Laos that no longer exists because the fish’s migratory path was blocked in 2020 by the new Don Sahong Dam near the Lao-Cambodia border.
The trey chhwiet spotting was of particular interest to Hogan, who closely studied the communal fishery more than 20 years ago and has tried to establish where the fish has moved since the dam was constructed.
Broad, who has long experience diving in deep blue holes in places like the Bahamas, likened diving in the Mekong to “swimming in a bathtub of café con leche.” “Add strong currents, depths over 80 meters, and [various] debris and you have an extremely challenging environment to explore,” he says.
Giants of the deep
For several years, Wonders of the Mekong scientists have conducted community and market surveys to gather information about aquatic biodiversity in the deep pools area. In collaboration with the Cambodian Fisheries Administration, the project has also established a network of fishers who report catches of giant and endangered fish.
So when the fishers on remote Koh Preah island, downstream from Stung Treng, hooked a giant stingray on May 4, they contacted project members, who traveled from Phnom Penh, a six-hour journey through the night and in the driving rain, to reach the island site.
Although the giant freshwater stingray (Urogymnus polylepis) is listed as endangered on the IUCN Red List of Threatened Species, it is not illegal to fish for it in Cambodia, though it’s not considered a good food fish in Southeast Asia. Instead, Hogan says, many fishers now recognize it as a species that deserves to be protected.
At the capture site, a large group of villagers congregated around the ray, which was kept wet as it was briefly moved out of the water onto a tarp so that it could be measured and weighed. Using several 100-kilo scales, the researchers confirmed its weight as 180 kilos (almost 400 pounds) and its length as 3.93 meters (almost 13 feet).
As enormous as the ray was, Hogan and others have conducted extensive interviews with fishers working in the region who say they’ve caught rays almost twice that size. Such accounts are very difficult to verify, but there is reason to believe the giant freshwater stingray is the largest freshwater fish species in the world. Other local leviathans, such as the Mekong giant catfish and giant barb, can reach 600 pounds and up to 10 feet in length.
Normally, the giant freshwater stingray packs a poisonous, serrated stinger up to 15 inches long, though this individual appeared to not have a stinger when it was caught. Hogan says it is not unusual for rays’ stingers to break off for a variety of reasons.
It was the first time many of the villagers had seen such a large fish. As handlers readied to return the ray to the river, a chief of the local commune, Long Tha, crouched next to a young boy who had come to watch the animal. “This is a giant stingray,” Tha explained to the boy. “When you grow up, you should protect it.”
According to Chea Seila, the Wonders of the Mekong program manager, who has long worked with the fishing communities here, attitudes among the fishers concerning the conservation of giant fish have changed as a result of the outreach efforts.
“In the beginning, they were afraid of getting arrested or put in jail if they reported catching megafauna,” she says. “But we appreciate them for doing so, and see them as role models in the conservation of endangered fish.”
“I believe this is not the last time we will release giant and endangered species back into their habitats,” she adds.
A multitude of threats
The research on this stretch of the Mekong has taken on urgency in recent years as the system faces increasing pressure from many directions. While they were videoing the deep river habitat, the scientists saw widespread evidence of plastic pollution, even in conservation areas, as well as “ghost nets,” nets that have been abandoned by fishers but can still snare fish.
The river as a whole is particularly threatened by climate change. In recent years, dry conditions, driven by the naturally occurring El Niño weather phenomenon but exacerbated by the warming climate, have caused seasonal water levels in the Mekong to fall to historically low levels. The situation is made worse by dam operators upstream regulating water flow for their own needs.
And although many community-run conservation reserves, where fishing is not allowed, have been established along the river, intense fishing pressure remains a major concern in the area, where small fishing camps are closely strung along the riverbanks. Illegal fishing methods, such as electrofishing, are also common, observers say.
Now, according to a report in the Phnom Penh Post, the Cambodian government has agreed in principle to the construction of a massive, 1,400-megawatt hydropower dam on the Mekong just north of Stung Treng, where the deep pools are found and inside an area designated as a wetlands site of international importance. It was previously thought the Cambodian government would refrain from building any dams on the main stem of the Mekong until at least 2030.
During their visits to the area, the researchers saw technical equipment stationed around the proposed building site, evidence that an initial geological study is underway. For Hogan, who has seen the nearby Don Sahong Dam in Laos wipe out the fisheries there, the proposed dam in Cambodia would wreak havoc on the river’s sensitive ecology.
“It will mean the loss of fisheries, loss of biodiversity, loss of livelihoods,” he says. “It will alter this area forever.”
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: In Search of the World’s Largest Freshwater Fish.
