Migratory freshwater fish are among the most threatened animals on the planet, a new report by a coalition of environmental organizations shows.
The global assessment, described as the first of its kind, found that populations of migratory freshwater fish have declined by 76 percent between 1970 and 2016—a higher rate of decline than both marine and terrestrial migratory species.
“We think migratory freshwater fish might be in even greater peril” than the dramatic drop the report indicates, says the report’s lead author, Stefanie Deinet of the Zoological Society of London (ZSL). “Adding currently missing information from tropical regions where threats of habitat loss and degradation, overexploitation, and climate change have been increasing, will surely bend the curve of loss downwards.”
Published Tuesday on the website of the World Fish Migration Foundation, a nonprofit conservation organization, the study draws upon The Living Planet Index, a database of global biodiversity managed by the ZSL and the World Wildlife Fund for Nature. It finds that Europe has seen the greatest decline in migratory freshwater fish—with populations there plummeting a staggering 93 percent in the past five decades—followed by Latin America and the Caribbean with an 84 percent decline. (It’s not just fish—read about how all freshwater animals face steep declines.)
Nearly half of the world’s more than 30,000 fish species live in fresh water, and many of them—perhaps most—migrate between habitats for breeding and feeding. Some, such as salmon, move from the sea into rivers to spawn; others, such as the European eel, mature in freshwater but spawn in the ocean. There are also many species of so-called potamodromous fish that migrate strictly within freshwater habitats. They include the dorado catfish which makes an epic journey from the Andes to the mouth of the Amazon and back, a distance of 7,200 miles.
Fish migrations serve a wide range of functions within the ecosystem, including the transportation of essential nutrients and larvae from one place to another. Many human populations depend on predictable migrations of fish for their subsistence and livelihoods.
“Migratory fish are extremely important to both economies and ecosystems, yet they’re often overlooked,” says Herman Wanningen, an aquatic ecologist and creative director of the World Fish Migration Foundation in Groningen, Holland.
The report points to habitat degradation, alteration, and loss as the largest threat to all migratory fish. Increasingly, dams and other river barriers block fish from reaching their mating or feeding grounds, thereby disrupting their life cycles. A study last year showed that only a third of the world’s large rivers remain free-flowing.
This may explain why Europe, which has few undammed major rivers, has seen such a large drop. One group of highly migratory fish found in Europe, sturgeon, declined by more than 90 percent since 1970—the biggest decline among the almost 250 species monitored for the report. Of the six sturgeon species historically swimming in the Danube River, at least one is believed to have gone extinct and most of the others are listed as critically endangered. (Read about another migratory fish, a close relative of the sturgeon called the Chinese paddlefish, that was declared extinct in early 2020.)
Invasive species, disease, pollution, and overfishing also are major threats to migratory fish.
Researchers also warn that climate change will inflict a heavy toll. In Australia, the wildfires of 2019-2020 led to ash washing into rivers, killing large numbers of fish. And last year, up to three million fish may have died when a severe drought caused the Darling River to dry up.
“You look at these river systems that are already so overdeveloped, and then you put climate change on top of that and you’ve got a big problem,” says Lee Baumgartner, a freshwater fish ecologist at Charles Sturt University in Albury, Australia, who contributed to the report.
In contrast to Europe, the study showed a less severe decline in North American migratory fish populations. The drop since 1970 was 26 percent—but researchers say that may be because many stocks of North American fish had already crashed before that time.
In large parts of the rest of the world—including Africa, South America, and Asia—a lack of information makes it difficult to accurately gauge the status of migratory fish. “Compared to migrating birds, for example, we know very little about migrating fish,” Wanningen says. “It’s hard to know what’s happening beneath the water’s surface.”
Zeb Hogan, a fish biologist at the University of Nevada, Reno, and a study co-author, suspects the decline in migratory fish in Asia, in particular, is far worse than the report outlines. He points to the Mekong River, which runs through six Southeast Asian countries and is home to some of the world’s largest freshwater fish species. Most of them are highly migratory and extremely vulnerable to dams and overfishing.
“Due to a lack of data, these fish were not included in the report, but their declines have been catastrophic,” says Hogan, who is also a National Geographic Explorer and leads a USAID-funded research project called Wonders of the Mekong.
A study by that project, published last month in the journal Water, showed that several of the giant species of migratory fish in the Mekong have almost disappeared. The Mekong giant catfish, which can grow to 660 pounds, is on the brink of extinction in the wild.
On a positive note, the report shows that when migratory fish don’t face threats, their populations often increase. Furthermore, species that have received some form of human conservation intervention—including fisheries restrictions, dam removals, or legal protections—declined far less than species that did not.
The report advocates for an emergency recovery plan that includes allowing rivers to flow more freely and naturally, improving the connectivity of rivers and waterways, reducing pollution and overfishing, and protecting wetlands. It also calls for better scientific monitoring of species, campaigns to inspire political and public will to protect freshwater animals, and investment in sustainable alternatives to hydropower dams.
“Saving migratory fish does not necessarily require big financial investments, but a change in current practices,” Baumgartner says. “What we are hoping is that this report will provide a wake-up call to governments and decisionmakers to take action before it’s too late.”