Cambodia, Mekong RiverMoul Thun knew the giant stingray hooked on the end of his line was bigger than any fish he had ever seen. What the 42-year-old fisherman from Kaoh Preah, a remote island in the Mekong River, in northern Cambodia, didn’t know was that the ray would eventually be named the largest recorded freshwater fish in the world.
For Zeb Hogan, who’s been documenting large freshwater fishes for almost two decades, the discovery of the ray, which was released alive back into the river, filled him with hope.
“It proves these underwater leviathans, which are in critical danger, still exist,” says Hogan, a fish biologist at the University of Nevada, Reno, and a National Geographic Explorer.
Hogan’s quest for big fish, dubbed the Megafishes Project and supported by National Geographic Society, began in 2005 when fishermen in northern Thailand pulled a 646-pound catfish out of the Mekong River. Hogan, who had spent years in Southeast Asia studying the Mekong giant catfish, as the species is known, concluded it was the largest—that is, the heaviest-ever caught in the area. But then he started to wonder: Were there even larger river giants elsewhere?
Intent on finding out, Hogan began scouring watersheds around the world, often as the host of National Geographic television’s Monster Fish show. But the answer to his question proved more elusive than Hogan had expected. Not only were fishermen prone to telling tall tales, but he also faced logistical challenges, a lack of scientific information on freshwater fish, and evidence that was impossible to verify, including old drawings and photographs.
While encountering numerous giants—like the air-breathing arapaima in the Amazon, and the pigeon-eating wels catfish in Europe—he could not find any scientifically documented catches of a freshwater fish larger than the one that had sparked his search.
Until last week, that is, when the USAID-supported research team that Hogan leads in Cambodia, Wonders of the Mekong, got a call from Moul Thun. The fisherman told a team member he’d accidentally caught a giant freshwater stingray “much bigger” than any he had previously seen. It was so big, he said, that it might even be a different species.
Arriving at Kaoh Preah, the team determined that Thun’s fish, a female that appeared to be in good health, was the same ray species known to occur in the Mekong. But Thun had been right about her enormous size: more than 13 feet from snout to tail. After maneuvering the giant fish onto three scales placed next to each other, the researchers were shocked to see her weight: At 661 pounds, she set a new world record.
Guinness World Records certified the milestone on June 24, declaring the Kaoh Preah stingray to be the largest freshwater fish on record. It also announced that giant freshwater stingrays are now tied with Mekong giant catfish as the world’s largest freshwater fish species.
The original aim of the Megafishes Project was to find, study, and protect the world’s largest freshwater fishes. The project focused on species that could grow to at least the size of a human—at least six feet long or more than 200 pounds—and that lived exclusively in freshwater. (Fish like the Beluga sturgeon, which travels between fresh and saltwater, were not considered.) Hogan initially drew up a list of roughly 30 species to focus on.
The challenge, as Hogan soon learned, was that many of these fish are hard to find. They live in remote, inaccessible places, and often in murky waters. Sometimes, even people who have lived their entire lives near megafish habitat have never heard of the creatures, let alone seen them. Early on in the search, there were relatively few scientists studying them.
What was clear was that the river giants were shrinking in number, threatened by a host of factors including overfishing, invasive species that competed with them for food, water pollution, and the presence of dams, which block migrating fish from completing their life cycles.
Studies show that global populations of freshwater megafauna have declined twice as much as vertebrate populations on land or in the oceans, leaving many giant fish species critically endangered. One serious contender for the title of the world’s largest freshwater fish, the Chinese paddlefish, went extinct during the early stages of Hogan’s search.
As Hogan’s work progressed, its focus increasingly turned to conservation. “It was never about just finding the biggest fish,” Hogan says, “but looking for ways to protect these extraordinary animals that, in some cases, have been on Earth for hundreds of millions of years but were now drifting out of existence.”
Hogan has long suspected that the largest river giant could be found among the stingrays, of which there are dozens of freshwater species. On a Monster Fish expedition to Argentina, Hogan himself caught a short-tailed river ray weighing about 400 pounds. But he knew that the giant freshwater stingray (Urogymnus polylepis) of Southeast Asia could grow much larger than that.
Over several years, he and a team of Thai researchers studied stingrays in two rivers not far from Bangkok. During this period, the team caught several rays that appeared to rival 2005’s record Mekong giant catfish in size, but Hogan and his colleagues were never able to get a confirmed weight.
Then in 2016, a chemical spill on Thailand’s Mae Klong River killed at least 70 giant stingrays. When scientists returned to the river two years later, they found far fewer stingrays overall, and almost no large ones.
More recently, Hogan’s Wonders of the Mekong project has accelerated research in northern Cambodia, where the Mekong River is lined with seasonally flooded forest, contains high biodiversity, and is believed to spawn up to 200 billion fish a year. The region is also thought to be an important dry-season refuge for many of the Mekong’s megafish, including giant stingrays.
Working with local communities, the research team has built a network of fishermen who are encouraged to report catches of stingrays and other endangered fish before releasing them back into the river. Although it’s legal to fish for rays in Cambodia, fishermen rarely target them because they aren’t considered a good food fish. The rays, however, sometimes get hooked accidentally, as happened with the record-breaking female that was caught during the night of June 13.
After receiving Thun’s call, team members based in Phnom Penh, the Cambodian capital, raced to the riverbank. There, they were joined by another team of American scientists working in the area, who quickly implanted, at the base of the ray’s tail, an acoustic tag that researchers hope will help them learn more about the elusive species’ behavior, including where it feeds, travels, and gives birth to pups.
“The fact that [Thun] called us is testament to the importance of working with local communities,” Hogan says. “These fishers can actually be great allies in working to protect these animals.”
By the time the ray was returned to the river, a throng of people, including Cambodian fisheries officials and village well-wishers, had gathered. Many of them were photographing the giant creature, which the scientists had named “Boramy,” or “full moon” in the Khmer language, because the round-looking fish was released during a full moon. The name is also commonly used to describe a beautiful female.
Sign of hope
For Hogan, the stretch of the Mekong River where the ray was found is not only a potential hot spot for giant stingrays but also a source of hope for all threatened megafish species. For despite the dire situation for many of the world’s river giants, Hogan says, there remain reasons to be optimistic.
In North America, for example, conservation efforts have helped populations of several of the largest freshwater fish species, like the alligator gar and the lake sturgeon, make comebacks. The same holds true for the arapaimas in the Amazon, where indigenous communities have restricted fishing of the air-breathing giant.
“When people see that these animals exist, and begin to appreciate how incredible they are, they get inspired,” Hogan says. “I look at the fish that broke the record in 2005, and it was killed and sold for meat. Now we’re tracking the world’s largest freshwater fish. It’s such a contrast. It means that all is not lost.”