On June 13, Cambodian fisherman Moul Thun accidentally hooked into the largest freshwater fish ever caught and recorded. The giant freshwater stingray (Urogymnus polylepis) is an elusive species that scientists don’t know much about. That’s exactly why Dr. Zeb Hogan and the Wonders of the Mekong research team are studying this and many more freshwater megafauna species that inhabit the mighty Mekong River.
Researchers were in the area installing underwater receivers to track migratory fish in the river when Thun hooked into the monster stingray. The team had been in communication with local fishermen in case such an instance occurred and got on location to weigh the fish.
The massive ray weighed in at 661 pounds and measured 13 feet from snout to tail. There are reports of the ancient fish reaching up to 16.5 feet long and weighing in over 1,300 pounds, but these claims haven’t been verified. Anglers accidentally hooking into these giant fish have capsized boats from their strong pull. Although these behemoths of the deep don’t typically pose much of a danger to humans, they are equipped with a venomous barb on the end of their tail. Like a broadhead, the barb can easily penetrate skin and even bone, introducing a toxin into the fresh wound. This particular specimen, however, had broken its barb off.
Researchers safely tagged and released the enormous stingray, hoping to capture data and gain knowledge on a species and ecosystem that has eluded scientific study for quite some time. This year alone, Dr. Hogan and his research team have weighed and released two giant stingrays weighing over 400 pounds, also caught nearby. The previous title holder for largest freshwater fish ever caught, set in 2005, also came out of a Mekong River—a giant catfish caught in Thailand that weighed in at 646 pounds.
Dr. Hogan has pursued the world’s largest freshwater fish for almost two decades now. His studies have led him to the Mekong, where deep troughs hold some of the world’s largest freshwater species.
“In 2020, one of the contenders for the world’s largest freshwater fish, known as the Chinese paddlefish, was declared extinct,” Dr. Hogan said. “That was very sad news, and it had me feeling like we were going to see more extinctions of these big fish, rather than records being broken.”
Although the giant freshwater stingray is listed as endangered on the IUCN Red List of Threatened Species, it's not illegal to fish for them in Cambodia. They aren’t considered great table fare however, so fishermen don’t often target them. As bottom-dwelling creatures that feed on shrimp, mollusk, and small fish, they are often by-catch of nets or deep water hooks.
“The fact that the world’s largest freshwater fish was caught in the Mekong is remarkable,” Dr. Hogan said. “This is a heavily populated region, and the river faces a ton of challenges.”
One of the most notable challenges is the construction of “megadams.” The massive structures keep big fish from moving through the river and spawning, despite the presence of aids like fish ladders. Additionally, the giant dams also promise to uproot small villages, bottleneck food supplies in the area, and increase the dramatic impacts of flood and drought seasons.
The Cambodian government recently agreed to the construction of a 1,400-megawatt hydropower dam on the Mekong just north of Stung Treng. “It will mean the loss of fisheries, loss of biodiversity, loss of livelihoods,” Dr. Hogan said. “It will alter this area forever.”
Thun caught this particularly impressive specimen in a deep area of the Mekong, where pools can get up to 90 meters deep. The river is one of the few habitats in the world that can sustain freshwater megafauna of this caliber, but it’s unknown how long it will be able to do so with overfishing, pollution, and dam construction.
"Big fish globally are endangered. They're high-value species. They take a long time to mature. So if they're fished before they mature, they don't have a chance to reproduce," Dr. Hogan said. "A lot of these big fish are migratory, so they need large areas to survive.”
