Some of the most astonishing creatures on Earth hide deep in rivers and lakes: giant catfish weighing more than 600 pounds, stingrays the length of Volkswagen Beetles, 6-foot-long trout that can swallow a mouse whole.

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

After an exhaustive survey, researchers declared the Chinese paddlefish extinct. The paddlefish, last seen alive in 2003, could grow up to 23 feet long and once inhabited many of China's rivers. Overfishing and dams decimated their populations.

The paddlefish may be a harbinger. According to research published in Global Change Biology, freshwater megafauna have declined by 88% worldwide in recent years.

"This study is a first step," said Zeb Hogan, an aquatic ecologist at the University of Nevada, Reno, and a co-author of the study. "We want to go beyond just studying conservation status and look at ways to try to improve the situation for these animals."

In their paper, the researchers defined freshwater megafauna as any vertebrate animal that spends an essential part of its life in fresh or brackish water and can weigh more than 66 pounds. They identified 207 such species and combed the scientific literature for at least two population measurements for each.

Scientists say there is still time to save some of the world’s threatened megafauna, such as the giant stingray, above.

According to the analysis, freshwater megafauna populations underwent an 88% global decline from 1970 to 2012. Fish were hit hardest, with a 94% decline. Fish in Southern China and South and Southeast Asia experienced the greatest overall losses, at 99%.

"Freshwater megafauna are the equivalent of tigers or pandas," said Ian Harrison, a freshwater scientist at Conservation International. "There is a powerfulness to the message that these very charismatic species are extremely threatened, and that the threats they represent are incumbent on all species in freshwater systems."

The World Wildlife Fund said populations of freshwater animals in general are declining at rates more than double those observed among terrestrial and marine animals. A multitude of threats drive these declines, including overfishing, pollution, habitat degradation, and water diversion and extraction. Dams, however, inflict the deadliest toll on giant fish, many of which are migratory.

"The species that were rare when I started working on them are now critically endangered, and even some of the much more previously common ones have become rare," Hogan said.

Still, researchers emphasized that there are many strategies for ensuring freshwater giants survive — and that there are signs of positive change. People living around Wisconsin's Lake Winnebago, for example, have tracked the lake sturgeon population since the 1930s. The lake now holds one of the largest populations of that threatened species in North America.

Arapaima — a 10-foot-long South American fish that breathes air — have disappeared from much of the Amazon River basin. But fishing villages in Brazil that sustainably manage the populations have seen arapaima numbers increase by as much as tenfold.

In the U.S., protections afforded by the Endangered Species Act have helped stabilize populations of green sturgeon and Colorado pikeminnow. Policymakers have also used the Wild and Scenic Rivers Act to designate certain water bodies as pristine. Seven-foot-long green sturgeon in Oregon's Rogue River are protected this way, as are American paddlefish in the Missouri River in Montana.

River restoration and dam removal projects are gaining popularity: 1,500 dams have been dismantled in the United States.

Yet freshwater protections are generally rare. While about 13% of U.S. land is conserved, less than 0.25% of its rivers are.

While none of these strategies in isolation will save all freshwater megafauna, Hogan and his colleagues believe that, collectively, they can tip the scales for many species and help preserve freshwater biodiversity.

"These extraordinary fish make our life and experience on Earth richer and more worthwhile," Hogan said. "Do we want to live on a planet where we've killed all these amazing animals, or on one where we can find a way to coexist?"