Rivers in which fish populations have escaped serious damage from human activities account for only 14% of the world’s watersheds, according to the most comprehensive study to date.
The scientists found that the biodiversity of more than half of the rivers had been deeply affected, with large fish such as sturgeon replaced by invasive species such as catfish and Asian carp. Pollution, dams, overfishing, agricultural irrigation, and rising temperatures due to the climate crisis are also to blame.
The worst affected regions are Western Europe and North America, where large and prosperous populations mean that the impact of humans on rivers is greatest, such as the Thames in the UK and the 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 represent 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 populations of migratory river fish have plummeted by a “catastrophic” 76% since 1970, with a 93% drop in Europe. Large river animals have fared worse, and some like the Mekong giant catfish are on the brink of extinction. A 2019 analysis found that only a third of the world’s great rivers continued to flow freely, due to the impact of dams.
Sébastien Brosse of the Université Paul Sabatier in Toulouse, France, who led the new research, said that rivers in many rich nations were unrecognizable compared to what they were before the industrial revolution. “Then we had sturgeons over 2 meters in size, thousands of salmon and many other fish that have almost disappeared today.”
“The River Thames is one of the hardest hit – it scored the highest of 12 out of 12 in our study,” he said. “There has been an increase in water quality in rivers in Western Europe and North America in recent decades, but I’m not sure the speed of change is sufficient because there has been a very steep decline in fish populations.” .
The greatest river biodiversity is found in South America, but the researchers found that only 6% of the most intact rivers were in this region. “We really need strong political decisions to consider biodiversity as important to humans,” Brosse said.
The investigation, published in Science magazine, examined nearly 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 how closely related the different species were. The researchers also took into account changes in biodiversity over the past 200 years.
An important change is the number of exotic species introduced into the rivers. “In Western Europe, you will see salmon from North America, black bullhead, which is a catfish from North America, carp and goldfish that come from Asia, and mosquitofish,” Brosse said.
Around the world, common carp, largemouth bass and tilapia are among the most widespread exotic fish. They are adapted to calm waters and have thrived as prey numbers have increased. This is homogenizing 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, uncrowded areas, particularly in Africa and Australia, although fish fauna in the Murray-Darling Basin has been damaged.
“But these least affected basins do not host enough species to maintain the world’s fish biodiversity,” Grosse said. “They only host 22% of the world’s fauna, so we also need to conserve biodiversity in basins heavily affected by humans.”
“Frankly, I am surprised that they found that only 53% of watersheds have undergone marked changes,” said Zeb Hogan, University of Nevada, USA. “Almost all the largest rivers in the world have undergone significant changes. . Where before there were rivers full of salmon and sturgeon or chubs and suckers, now there are rivers with sea bass, blue gills, carp and catfish. “
“The Amazon, the Congo and the Mekong are more affected than expected, a finding that may not be very appreciated and could indicate that the new dams and other pressures may have had large-scale impacts,” he said. “Measures taken to protect and preserve terrestrial and marine wildlife often fail to protect rivers.”
Brosse said the impact assessment in his study was probably an underestimate, because more fish extinctions have likely occurred than were officially recorded.
George is Digismak’s reported cum editor with 13 years of experience in Journalism