Global populations of freshwater fish, reptiles, amphibians, and mammals have sharply declined, a new study finds. “This is a crisis of huge proportions that is not widely appreciated” says Zeb Hogan, who has studied the plight of freshwater megafish for two decades.

This is a story I wrote that was published by National Geographic on August 8, 2019

For the first time, researchers have quantified the global decline of freshwater megafauna—including fish, reptiles, amphibians, and mammals—and the results paint a grim picture. In four decades since 1970, the global populations of these freshwater giants have declined by almost 90 percent—twice as much as the loss of vertebrate populations on land or in the oceans.

Some have survived for hundreds of millions of years, but many of the world’s freshwater megafauna—including sumo-sized stingrays, colossal catfish, giant turtles, and gargantuan salamanders—may soon find themselves on the brink of extinction, according to a new study published.

Large fish species, such as sturgeons, salmons, and giant catfishes, are particularly threatened, with a 94 percent population decline. Most large freshwater reptile and many mammal species are also in trouble. The baiji, a Chinese river dolphin, is likely the first dolphin species driven to extinction by humans, and the Chinese paddlefish, which can grow 20 feet long, has not been seen in over a decade. Other species may be down to their last few individuals.

“This is a crisis of huge proportions that is not widely appreciated” says Zeb Hogan, a fish biologist at the University of Nevada, Reno, and a National Geographic Explorer who has studied the plight of freshwater megafish for two decades.

Hogan, who is a co-author on the study published today in the journal Global Change Biology, says the troubled story of giant fish specifically underscores the environmental crisis that many rivers and lakes around the world face today. “Once the largest animals go, it’s a warning that we need to do something quickly to improve the ecosystem health of our rivers and lakes,” he says.

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Underwater leviathans

Freshwater ecosystems are generally less studied than their marine counterparts, despite being home to a third of all vertebrate species and nearly half of all fish species worldwide. While population declines have been well-documented for both terrestrial and marine megafauna, few studies of large freshwater species have been conducted on a global scale. (See beautiful underwater photos of overlooked freshwater animals.)

For the study, a team of international researchers compiled population data on 126 out of 207 freshwater species weighing at least 30 kilograms (66 pounds) from 1970 to 2012, drawing in part on The Living Planet Index, a database managed by the Zoological Society of London in cooperation with the World Wildlife Fund. While that index shows that populations of all freshwater species declined by 83 percent during roughly the same period, the new study shows an even higher rate of decline in big freshwater animals, at 88 percent.

The study’s lead author, Fengzhi He, a freshwater ecologist at the Leibniz-Institute for Freshwater Ecology and Inland Fisheries (IGB) in Berlin, says public awareness of the freshwater biodiversity crisis is limited, with many people unaware that the giant creatures even exist. “They are not like tigers, pandas, lions, or whales—species that receive a lot of attention in the media and school education,” he says.

Among the biggest threats facing large freshwater species are overexploitation and habitat degradation, says He. Many of the these animals are targeted for meat, skin, and eggs. Megafish in particular tend to be more vulnerable than other fish to dams that block their migratory routes and limit access to spawning grounds. Large animals also tend to be slow to mature and have low reproduction rates, making them particularly vulnerable. (Read about how critically endangered giant fish are being poached to serve at restaurants.)

Trouble on the Mekong

According to the study, the biogeographic zones that have seen the greatest declines in freshwater megafauna are Indomalaya (99 percent) and the Palearctic, which encompasses Europe, northern Africa, and northern Asia (97 percent). Now, Hogan says, the most critical region may be Southeast Asia, and in particular the Mekong River, which runs through China, Myanmar, Thailand, Laos, Cambodia and Vietnam. More than a thousand species of freshwater fish live in the Mekong, including many of the world’s largest. The Mekong giant catfish, for example, is the current record-holder for the world’s largest freshwater fish ever caught, at 646 pounds.

Hogan says he has not seen a Mekong giant catfish in the wild since 2015. Existing and planned dams on the river may drive the species to extinction. He and other researchers are not sure what the ecological consequences of such fish disappearing will be, but in the case of the Mekong, it could threaten the food security and livelihoods of millions of people living along the river.

Amid the grim overall findings, the study did suggest that 13 freshwater megafauna species have seen their populations stabilize or even grow. Among them are the green sturgeon and the American beaver, both in the United States. In Europe, the Eurasian beaver has returned to many regions from where it had once disappeared, and in Cambodia, the population of Irrawaddy river dolphins has increased for the first time in 20 years.

“We don’t want this to be a situation of only doom and gloom,” says He, the freshwater ecologist. “We want to inform people about this biodiversity crisis but also show them that there is still hope to protect these giant freshwater species—that it can be done.”