In Cambodia, researchers document the world's largest freshwater fish

Thanks to local fishers, a team of scientists on an expedition in Cambodia to tag Mekong River fish has discovered the largest freshwater fish ever documented--a 300-kilogram giant stingray that stretches nearly 4 meters from nose to tail.

"It's almost inconceivable that a fish this large still occurs in a river as heavily fished and developed as the Mekong," says Zeb Hogan, a fish biologist at the University of Nevada, Reno.

The team of scientists, part of an international collaboration called Wonders of the Mekong, tagged and released the record-breaking stingray ( Urogymnus polylepis ) thanks to having cooperative ties to local fishing communities and a bit of luck. The researchers were preparing to put acoustic tags on 200 fish and use arrays of receivers to track their movements through 300 kilometers along the heavily braided Mekong near the city of Stung Treng in northern Cambodia. The data could aid conservation efforts.

In early May, local fishers told researchers they had inadvertently caught a 180-kilogram female stingray. The Wonders of the Mekong team, in the area to deploy acoustic receivers, helped safely return the fish to the river.

Then, fishers from the same area reported catching a “much bigger” stingray during the night of 13 June. Scientists got to the site on 14 June and "luckily, they also had the supplies necessary for tagging" the fish, says Hogan, director of the Wonders of the Mekong project who was in Reno at the time. After measuring the fish, the team helped release it back into the wild. The stingray eclipsed the largest freshwater fish previously documented: a 293-kilogram endangered giant catfish caught on the Mekong in Thailand in 2005.

The Wonders of the Mekong researchers are not simply hunting for big fish. The project focuses on a remote stretch of the river that is home to close to 1,000 aquatic species and is thought to be a critically important dry season refuge for endangered freshwater fish. In addition to the biodiversity, the Mekong supports regional fisheries and is a vital food source for much of Southeast Asia.

This is the first time an acoustic array has been deployed in Cambodia to track tagged fish. The giant stingray also became the first of the 200 fish the Wonders' team plans to tag, getting the effort "off to an unexpected and incredible head start," Hogan says.

Finding such a rare specimen "signals there is still time to safeguard these freshwater megafish," says Solomon David, an aquatic ecologist at Nicholls State University in Louisiana who is not associated with the Wonders of the Mekong project. "Freshwaters are among the most imperiled ecosystems in the world,” he adds. “We need to act quickly to conserve these habitats and their uniquely charismatic biodiversity.”