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 so-called 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 throughout the Yangtze River basin, researchers this month 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 August 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.”
To the relatively few scientists who focus on freshwater species, news that the largest are disappearing comes as no surprise. Since Hogan began studying giant fish 20 years ago, he has witnessed the decline of many species — and now, the extinction of at least one, the Chinese paddlefish.
“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,” he said.
In their paper, Hogan and his colleagues defined freshwater megafauna as any vertebrate animal that spends an essential part of its life in fresh or brackish water and can weigh over 66 pounds. They identified 207 such species and combed the scientific literature for at least two population measurements for each.
The researchers found data meeting those criteria for just 126 species. Their list mainly included fish, but also mammals like beavers, river dolphins and hippopotamuses, as well as coldblooded creatures like crocodiles, giant salamanders and alligator snapping turtles.
Had more data been available, “the picture probably would become even worse,” said Sonja Jähnig, an ecologist at the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin and senior author of the study.
According to the researchers’ 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 who wasn’t part of the study. “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.”
According to the World Wildlife Fund, 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.
According to research published in May, two-thirds of the world’s major rivers are no longer free flowing. Hundreds of dams are proposed or under construction in megafauna-rich river basins, including the Amazon, Congo and Mekong.
“We’re up against this challenge of how to balance species conservation with the human need for water,” Harrison said. “The effects of climate change will make this challenge even greater.”
The authors of the new study emphasize, however, that there are many strategies for ensuring freshwater giants survive — and that there are signs of positive change.
“We do not want to send a doom-and-gloom message to the public,” said Fengzhi He, an ecologist at the Leibniz Institute of Freshwater Ecology and Inland Fisheries, and lead author of the study.
Conservation initiatives can, and do, work. 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 because of overharvesting. But fishing villages in Brazil that sustainably manage the populations have seen arapaima numbers increase by as much as tenfold.
In the United States, protections afforded by the Endangered Species Act have helped stabilize declining 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 protections for freshwater bodies are generally rare. While about 13% of land in the United States is conserved, less than 0.25% of its rivers are.
According to John Zablocki, a conservation adviser for rivers at the Nature Conservancy, part of the problem is that people assume that rivers running through terrestrial protected areas are afforded the same protections by association. In fact, dams often are built within national parks.
“Rivers are basically the redheaded stepchild of protected areas,” he said. “If you look around the world, there are very few examples of rivers that are themselves protected in any sort of durable way.”
To change this, Zablocki, along with a growing group of scientists and advocates, is seeking a global policy framework to protect rivers, something that has long been in place for marine and terrestrial systems.
In the meantime, grassroots interventions sometimes force positive change in the absence of government commitment. Citizens in Bangladesh, New Zealand, Ecuador and other countries recently secured legal rights for rivers, meaning courts must treat those water bodies as living entities.
Huge dam projects in the Brazilian Amazon were suspended in 2018 after citizen protests and calls for a move toward renewable energy. In 2012, protests in Chile contributed to the decision not to dam the Pascua and Baker rivers, and instead to install solar and wind farms for energy production.
Indeed, as prices for renewables drop, solar and wind are becoming viable alternatives for hydropower, especially in developing countries that have yet to break up their rivers with major dams, said Michele Thieme, lead freshwater scientist at the World Wildlife Fund.
“We see a real opportunity in the developing world to leapfrog forward and avoid the mistakes that have been made in other parts of the world,” she said.
Cambodia, for example, recently greenlighted a 60-megawatt solar park, although the country is still considering a large dam on the Mekong River that would block migration of endangered fish and destroy critical habitat for endangered Irrawaddy dolphins.
While none of these strategies in isolation will save all of the world’s 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?”