Epic river migrations of fish rapidly collapsing, UN report finds | Fish
“It’s very hard to imagine what’s happening underwater when you look at a river, but billions of fish are making these epic migrations, some of the largest animal migrations on Earth,” said Dr Zeb Hogan, from the University of Nevada in the US.
The longest migration of any freshwater fish species is that of the dorado catfish, which completes a 7,000-mile (11,000 km) migration, from spawning in the foothills of the Andes to feeding in the Amazon estuary and back. The silver and gold fish themselves were incredible, Hogan said: “They grow to about 2 meters long. »
Such fish migrations occur in rivers around the world – salmon and eels are more familiar examples – but many of them are rapidly collapsing, according to the most comprehensive assessment to date. The analysis, carried out by the United Nations Convention on the Conservation of Migratory Species (CMS) and led by Hogan, found that freshwater fish populations around the world have fallen by around 81% since 1970.
Freshwater species are particularly vulnerable to human damage, as pollution often flows into rivers and lakes, dams block vital waterways, and overfishing decimates populations. The climate crisis is making the damage worse by increasing water temperatures.
“Animal migration is one of the great wonders of nature,” said Amy Fraenkel, CMS Executive Secretary. “Their journeys, which can cross multiple national borders and even continents, are extraordinary feats of timing and endurance. But these species face increasing pressures at every stage of their life cycle. International cooperation is essential to ensure that these species survive and thrive.”
Migratory freshwater fish also support some of the world’s largest inland fisheries and support hundreds of millions of people, but they are among the most endangered wildlife species on the planet. Cambodia’s immense Tonle Sap Lake, part of the Mekong river system, is home to more than 100 migratory species. Hogan said: “They can catch several tonnes of fish in an hour. »
The 132 countries that are part of CMS are meeting this week in Brazil – the relatively undeveloped Amazon is one of the last major strongholds of migratory freshwater fish – and improving international cooperation to protect migratory fish is high on the agenda. Measures would include removing or preventing barriers in rivers, ensuring water flow, combating pollution and coordinating fisheries catches.
Half of the Earth’s surface lies in river basins shared by several countries, but the report reveals that the collapse of river migrations is a largely neglected biodiversity crisis. She assessed data on more than 15,000 freshwater fish species and found 325 that crossed borders and could meet the criteria for listing for protections. Only 24 have been recorded to date, most of them sturgeon, long targeted for caviar.
“Rivers know no borders – and neither do the fish that depend on them,” said Michele Thieme of WWF-US. “The crisis beneath our rivers is much bigger than most people realize, and we are running out of time. Rivers must be managed as connected systems, with coordination across borders and investment in basin-wide solutions before these migrations are lost forever.”
One of the 325 species identified is the piraiba, or goliath catfish, which can reach 225 kg (500 lbs) and migrates throughout the Amazon basin. “People think of the Amazon as a forest, but during the rainy season, large areas are flooded,” Hogan said. “So you actually have this aquatic, forested wonderland for fish.”
Brazil and other countries have already proposed a 10-year action plan for migratory catfish, which could serve as a model for other basins. “The action we are seeing in the Amazon is hopeful. Maintaining free flow and healthy rivers is extremely important,” Hogan said.
The Mekong Basin is particularly struggling, with all large migratory freshwater fish now threatened with extinction, including the colossal giant catfish, which has halved in size in recent years due to overfishing. However, none of the major Mekong countries – Thailand, Cambodia, Laos and Vietnam – are yet CMS treaty members. Brazil’s main political goal for the summit, Cop15, is to involve more countries.
Other priority river basins identified by the report are the Danube, the Nile and the Ganges-Brahmaputra.
Animals undertake gigantic migrations to capitalize on the food or habitats they need at different stages of their lives. For example, salmon use clean upstream waters with gravel beds to spawn, but then move downstream to take advantage of more abundant food. European eels do the opposite, spawning at sea and then returning to rivers – an 8,000-mile round trip – and the species is already listed as needing protection. “There is a big effort in Europe and the United States to improve connectivity, for example by removing dams,” Hogan said.
Some migratory fish have already disappeared, such as the Chinese paddlefish from the Yangtze River. “It’s the first of these large, iconic migratory species to go extinct,” Hogan said, attributing its disappearance to the construction of the Gezhouba hydroelectric dam in 1981.
