According to the most comprehensive study to date, rivers in which fish populations have escaped severe damage caused by human activities account for only 14% of the world’s river basin area.
The scientists found that the biodiversity of more than half of the rivers had been profoundly affected, with large fish such as sturgeon replaced by invasive species such as catfish and Asian carp. They are also to blame for pollution, dams, overfishing, irrigation of farms and rising temperatures due to the climate crisis.
The most affected regions are Western Europe and North America, where abundant and rich populations mean that the impact of humans on rivers is higher, such as with the Thames in the United Kingdom and the Mississippi in the United States. Units.
Rivers and lakes are vital ecosystems. They cover less than 1% of the planet’s surface, but their 17,000 species of fish make up a quarter of all vertebrates, in addition to providing food for many millions of people. Healthy rivers are also needed to supply clean water.
Other recent research has shown that global migratory river fish populations have fallen by 76% “catastrophically” since 1970, with a 93% drop in Europe. Large river animals have gotten worse, with some like the giant Mekong catfish about to become extinct. According to a 2019 analysis, only a third of the world’s major rivers remained in free circulation due to the impact of dams.
Sébastien Brosse, of Paul Sabatier University in Toulouse, France, who led the new research, said the rivers of many rich nations were unrecognizable compared to what they were before the industrial revolution. “Then we had sturgeon over 2 feet in size, we had thousands of salmon and many other fish that are almost extinct today.”
“The Thames River is one of the hardest hit – it got the maximum 12 out of 12 in our study,” he said. “There has been an increase in water quality in the rivers of Western Europe and North America in recent decades, but I’m not sure the rate of change will be enough because there has been a very sharp decline. in fish populations “.
The highest river biodiversity is found in South America, but researchers found that only 6% of the most intact rivers were in this region. “We really need strong policy decisions to consider biodiversity as something important to humans,” Brosse said.
The research, published in the journal Science, examined about 2,500 rivers in all parts of the world except the polar regions and deserts. Previous work focused simply on the number of species, but this study included the ecological roles of the species as well as its intimate relationship between the different species. Researchers also took into account changes in biodiversity over the past 200 years.
An important change is the number of exotic species introduced into rivers. “In Western Europe, you’ll see American salmon, black ox head, which is an American catfish, carp, and goldfish that come from Asia and mosquitofish,” Brosse said.
All over the world, the common carp, sea bass and tilapia are among the most widespread exotic fish. They are adapted to calm waters and have prospered as the number of dams has grown. This makes it possible to homogenize fish populations in rivers, making them less able to cope with environmental changes, such as global warming.
The least affected rivers were found in remote areas with few people, especially in Africa and Australia, although the fish fauna of the Murray-Darling Basin has been damaged.
“But these less affected basins do not host enough species to maintain the global biodiversity of fish,” Grosse said. “They only house 22% of the world’s wildlife, so we also need to conserve biodiversity in basins heavily affected by humans.”
“Frankly, I’m surprised to find that only 53% of river basins have undergone marked changes,” said Zeb Hogan, at the University of Nevada, USA. “Almost every major river in the world has undergone significant changes. Where there used to be rivers full of salmon and sturgeon or puppies and suckers, now there are rivers with sea bass, cornflowers, carp and catfish. “
“The Amazon, Congo and Mekong have a greater impact than expected, a finding that may not be widely appreciated and could indicate that new dams and other pressures may have had large-scale impacts,” he said. he said. “Measures taken to protect and preserve terrestrial and marine wildlife often do not protect rivers.”
Brosse said the impact assessment in his study was probably an understatement, because more fish extinctions are likely to have occurred than officially recorded.