On June 13, Cambodian fisherman Moul Thun accidentally snagged the largest freshwater fish ever caught and recorded. The giant freshwater stingray (Urogymnus polylepis) is an elusive species that scientists know little about. This is exactly why Dr. Zeb Hogan and the Wonders of the Mekong research team are studying this and many other species of freshwater megafauna that inhabit the mighty Mekong River.
Researchers were in the area setting up underwater receivers to track migrating fish in the river when Thun latched onto the monster stingray. The team had been in communication with the local fishermen in case such a case occurred and had gone on site to weigh the fish.
The massive ray weighed 661 pounds and measured 13 feet from snout to tail. There are reports of ancient fish reaching up to 16.5 feet in length and weighing over 1,300 pounds, but these claims have not been verified. Anglers who accidentally snag these giant fish have capsized boats due to their strong pull. Although these behemoths of the deep generally do not pose much danger to humans, they are equipped with a poisonous beard at the end of their tail. Like a broad spike, the barb can easily penetrate skin and even bone, introducing toxin into the fresh wound. This particular specimen, however, had broken its beard.
Researchers in complete safety tagged and released the huge stingray, in hopes of capturing data and gaining knowledge about a species and ecosystem that has eluded scientific study for some time. This year alone, Dr. Hogan and his research team weighed and released two giant stingrays weighing more than 400 pounds, also taken nearby. The previous holder of the title of biggest freshwater fish ever caught, established in 2005, also came out of a Mekong river—a giant catfish caught in Thailand who weighed 646 pounds.
Dr. Hogan has been chasing the world’s largest freshwater fish for almost two decades now. His studies led him to the Mekong, where deep hollows are home to some of the largest freshwater species in the world.
“In 2020, one of the contenders for the world’s largest freshwater fish, known as the Chinese paddlefish, was declared extinct,” Dr Hogan said. “It was very sad news, and I felt like we were going to see more extinctions of these big fish, rather than breaking records.”
Although the giant freshwater stingray is listed on the IUCN Red List of Threatened Species, it is not illegal to fish for it in Cambodia. However, they are not considered good food on the table, so anglers do not often target them. As bottom-dwelling creatures that feed on shrimp, molluscs, and small fish, they are often bycatch of nets or hooks in deep water.
“The fact that the largest freshwater fish in the world was caught in the Mekong is remarkable,” Dr Hogan said. “It’s a very populated area and the river faces a ton of challenges.”
One of the most notable challenges is the construction of “megadams.” The massive structures prevent large fish from moving through the river and spawning, despite the presence of aids like fish ladders. Moreover, the giant dams also promise to uproot small villages, choke up the food supply in the region and increase the dramatic effects of flood and drought seasons.
The Cambodian government recently accepted the construction of a 1,400 megawatt hydroelectric dam on the Mekong just north of Stung Treng. “It will mean loss of fisheries, loss of biodiversity, loss of livelihoods,” Dr Hogan said. “It will alter this area forever.”
Thun captured this particularly impressive specimen in a deep area of the Mekong, where pools can reach up to 90 meters deep. The river is one of the few habitats in the world that can support freshwater megafauna of this caliber, but it’s unclear how long it will be able to do so with overfishing, pollution and dam construction.
“Big fish are endangered around the world. They are very valuable species. They take a long time to mature. So if they are caught before they are mature, they have no chance of reproducing.” , said Dr. Hogan. “Many of these large fish are migratory, so they need large areas to survive.”