Argentina’s giant river stingray can grow up to 1.5 meters long and weigh more than 200 kilograms. But its massive size is no protection against fishermen, who are hunting freshwater stingrays at a worrisome pace, according to a new study. Scientists have long known that saltwater rays, sharks, and other cartilaginous fish face daunting challenges including overfishing and loss of coastal habitat. But this is the first look at the population status of river stingrays, which have evolved to live exclusively in freshwater. South America boasts the greatest diversity, with 32 species in the Amazon and other rivers. In the new study, researchers netted stingrays from six species in Argentina’s Paraná River from 2005 to 2016 and used those numbers to estimate their population. Their finding: Five species saw their numbers plummet up to 25% a year, they report in the current issue of Biological Conservation. To find out why, the team checked each stingray for a missing tail—a sure sign that a fisherman had once caught it. When fishermen hook stingrays in Argentina, they typically cut off the stingers to make them safer to handle before throwing them back into the river. The researchers discovered a higher proportion of healed tails in smaller populations, which suggests that fishing is taking a toll. Of greatest concern are the giant river stingray, which reproduces slowly, and the rare Paraná River stingray. But so far the population of one species—the ocellate river stingray (above)—has held steady, suggesting that it could be a sustainable resource for food or the aquarium trade.