Like many migratory fish, salmon are threatened by multiple threats: overfishing, habitat degradation, and dams that prevent it from migrating from the ocean to its upstream spawning grounds.
图源：CRISTINA MITTERMEIER，NAT GEO IMAGE COLLECTION
A new report from a coalition of environmental groups shows that migratory freshwater fish are among the most endangered animals on earth.
This global assessment ( a first-of-its-kind ) found that from 1970 to 2016 , migrating freshwater fish populations fell by 76 percent , a higher decline than marine and land migratory species.
"We think the dangers to migrating freshwater fish may be greater than the report suggests. If you add to that information that is currently missing in the tropics, the curve of migratory fish population declines is bound to bend further downwards. Threats to the tropics have been increasing, such as habitat loss and degradation, overexploitation and climate change," said Stefanie Deinet, lead author of the report and a member of the Zoological Society of London.
The critically endangered European eel, photographed at the bottom of a river in France, has probably the longest migration of any fish, from the Sargasso Sea to the rivers of Europe and back, a journey of 16,000 kilometers. European eels face threats from hydroelectric pumping stations, pollution and fish fishing.
Source: BRUNO GUENARD, MINDEN PICTURES
Published Tuesday on the website of the nonprofit conservation group, the World Fish Migration Foundation, the study draws on data from the Global Biodiversity Database, the Earth Vitality Index, managed by the Zoological Society of London and the World Wide Fund for Nature. The report found that Europe saw the largest decline in migratory freshwater fish populations, falling by 93 percent over the past 50 years, followed by Latin America and the Caribbean with a drop of as much as 84 percent.
An epic journey
There are more than 30,000 species of fish worldwide, nearly half of which live in freshwater, many of which, perhaps mostly, migrate between habitats for reproduction and forage. Some fish swim from the sea to the river to spawn, such as salmon; others mature in fresh water and then go to the ocean to spawn, such as the European eel. There are also many so-called pure freshwater migratory fish, which migrate only within freshwater habitats. These include the yellow-bodied catfish, whose epic migration journey stretches 11,600 kilometres from the Andes region to the mouth of the Amazon River.
Fish migration plays a wide range of roles in ecosystems, including the transport of essential nutrients and fry from one place to another. Large numbers of people depend on predictable fish migration for their livelihoods.
"Migratory fish are extremely important to both economies and ecosystems, but they are often overlooked," says Herman Wanningen, creative director and aquatic ecologist at the World Fish Migration Foundation in Groningen, the Netherlands.
The new report notes that habitat degradation, alteration and loss are the biggest threats to all migratory fish species. More and more fish are hampered by dams and other river banks from reaching mating and foraging places, which in turn disrupts their life cycles. A study last year showed that only one-third of the world's large rivers still flow freely.
This may explain why the number of migratory fish in Europe has decreased dramatically: almost all large rivers have dams. Sturgeon, a highly migratory fish in Europe, has fallen by more than 90 percent since 1970, the highest decline in the nearly 250 species of fish monitored in the report. At least one of the six species of sturgeon historically active in the Danube is considered extinct, and most of the others are classified as critically endangered.
Invasive species, disease, pollution and overfishing are also major threats to migratory fish.
The critically endangered flash sturgeon has disappeared from most of its native habitat in the Caucasus. Sturgeon's eggs are heavily caught and their migration routes are hampered by dams, making them the world's most endangered fish.
图源：JOEL SARTORE，NATIONAL GEOGRAPHIC PHOTO ARK
The researchers also warn that climate change will have serious consequences. In Australia, wildfires from 2019 to 2020 have caused tree ashes to flow into rivers, causing massive fish deaths. Last year, a severe drought dried up the Darling River, eventually killing as many as 3 million fish.
"River systems have been overexploited, and if you add in the influencing factors of climate change, you'll see that we face very serious problems," said Lee Baumgartner, a freshwater fish ecologist at Charles Stowe University in Australia, who was involved in the report.
Giant fish
Compared to Europe, studies have shown that the decline in migratory fish populations in North America is less severe. That's 26 percent since 1970, but researchers say that could be because many North American fish species have been drastically reduced before 1970.
In much of the rest of the world, including Africa, South America and Asia, the lack of information makes it difficult to accurately determine the status of migratory fish. "For example, we know very little about migratory fish compared to migratory birds, and it's hard to know what's going on below the surface of the water," Wanningen said.
Zeb Hogan, a fish biologist at the University of Nevada at Reno and a collaborator of the new study, believes that the decline in migratory fish populations in Asia may be much more severe than the report points out. He specifically proposed the Mekong River, which flows through six Southeast Asian countries and is home to some of the world's largest freshwater fish. Most of these fish are highly migratory and highly susceptible to dams and overfishing.
"The study did not include these fish because of a lack of data, but the reduction was devastating," said Hogan, a National Geographic explorer who led a U.S. Agency for International Development-funded research project, the Mekong Miracle.
A study by the Miracle of the Mekong Project, published last month in the journal Water, showed that several large migratory fish species in the Mekong Basin have all but disappeared. The 300-kilogram Mekong giant catfish is endangered in the wild.
Thankfully, the report shows that migratory fish tend to increase once they are not under threat. In addition, if fish are subject to some form of human conservation intervention, such as fishing restrictions, dam removal, or legal protection, the reduction will be much lower than for species that are not protected by humans.
The new report advocates an emergency recovery plan that includes allowing rivers to flow more freely and naturally, improving river and waterway connectivity, reducing pollution and overfishing, and protecting wetlands. The report also calls for better scientific monitoring of fish species, advocacy campaigns to inspire political and public will to protect freshwater animals, and investment in sustainable alternatives to hydroelectric dams.
"Saving migratory fish doesn't necessarily require significant financial investment, but rather a change in current practices. We hope this report will sound a wake-up call for governments and policymakers to act before it's too late," Baumgartner said.
(Translator: Stray Dog)