World’s largest freshwater fish draws attention to delicate ecosystem

Scientists race to gain insights into the Mekong River’s disappearing giants—and how to save them.

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.

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