Rivers in which fish populations have escaped serious damage caused by human activities account for only 14% of the world’s watershed, according to the most comprehensive study to date.
Scientists have found that the biodiversity of more than half of the rivers has been profoundly affected, with large fish, such as sturgeon, being replaced by invasive species such as catfish and Asian carp. Pollution, dams, overfishing, farm irrigation and rising temperatures are also to blame due to the climate crisis.
The most affected regions are Western Europe and North America, where large and rich populations mean that the impact of people on rivers is greatest, such as the Thames in the UK and Mississippi in the US.
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, and provide food for many millions of people. Healthy rivers are also needed for clean water supply.
Other recent research has shown that global migratory river fish populations have fallen by 76% “catastrophically” since 1970, by 93% in Europe. Large river animals were the worst, with some like the giant Mekong sleeping on the verge of extinction. A 2019 analysis found that only a third of the world’s major rivers remained free to flow due to the impact of dams.
Sébastien Brosse, of Paul Sabatier University in Toulouse, France, who led the new research, said rivers in many rich nations are unrecognizable compared to how they were before the industrial revolution. “Then we had sturgeons larger than 2 meters, we had thousands of salmon and many other fish that have disappeared today.”
“The Thames River is one of the hardest hit – it got a maximum of 12 out of 12 in our study,” he said. “There has been an increase in water quality in Western European and North American rivers in recent decades, but I am not sure that the rate of change is sufficient, as there has been a very sharp decline in fish populations.”
The greatest biodiversity of rivers is found in South America, but researchers found that only 6% of the most intact rivers were in this region. “We really need strong political decisions to see biodiversity as important to people,” Brosse said.
The research, published in the journal Science, examined nearly 2,500 rivers in all parts of the world except the polar regions and deserts. Previous work has simply focused on the number of species, but this study included the ecological roles of the species, as well as how closely related the different species were. Researchers have also considered changes in biodiversity over the last 200 years.
A major change is the number of alien species introduced into rivers. “In Western Europe, you’ll see North American salmon, black calf head, which is a North American catfish, carp and goldfish that come from Asia and moschitofish,” Brosse said.
Worldwide, common carp, sea bass and tilapia are among the most common exotic fish. They are adapted to calm waters and have prospered as the number of dams has increased. This homogenizes 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 in the Murray-Darling Basin was affected.
“But these least affected basins do not host enough species to maintain the global biodiversity of fish,” Grosse said. “They are home to only 22% of global wildlife, so we need to preserve biodiversity in basins that are heavily affected by humans.”
“Honestly, I’m surprised to find that only 53 percent of the watersheds have undergone significant changes,” said Zeb Hogan of the University of Nevada. “Almost all of the world’s largest rivers have undergone significant changes. Where there used to be rivers full of salmon and sturgeons or mushrooms and suckers, there are now rivers with bass, blue, carp and catfish. ”
“The Amazon, Congo and the Mekong are more affected than expected – a finding that may not be widely appreciated and could indicate that new dams and other pressures could already have large-scale impacts,” he said. “Measures taken to protect and conserve terrestrial and marine wildlife often fail to protect rivers.”
Brosse said the impact assessment in his study was probably an understatement, as more fish may have disappeared than were officially recorded.