Some of the most astonishing creatures on Earth hide deep in rivers and lakes: giant catfish weighing over 600 pounds, stingrays the length of Volkswagen Beetles, six-foot-long trout that can swallow a mouse whole.

There are about 200 species of so-called freshwater megafauna, but compared to their terrestrial and marine counterparts, they are poorly studied by scientists and little known to the public. And they are quietly disappearing.

Following an exhaustive survey throughout the Yangtze River basin, researchers this month declared the Chinese paddlefish officially extinct. The paddlefish, last seen alive in 2003, could grow up to 23 feet long and once inhabited many of China’s rivers, but overfishing and dams decimated their populations.

The paddlefish may be a harbinger for many other giant fish. According to research published in August in Global Change Biology, freshwater megafauna have declined by 88 percent worldwide in recent years.