Rivers in which fish populations have escaped serious damage as a result of human activities make up only 14% of the world’s catchment area, according to the most comprehensive study to date.
Scientists have found that the biodiversity of more than half of rivers is deeply affected, with large fish such as sturgeon being replaced by invasive species such as catfish and Asian carp. Pollution, dams, overfishing, irrigation on the farm and rising temperatures due to the climate crisis are also to blame.
Western Europe and North America have hit the hardest regions, where large and prosperous populations mean the impact of people on rivers is greatest, such as with the Thames in the United Kingdom and the Mississippi in the USA.
Rivers and lakes are vital ecosystems. They cover less than 1% of the planet’s surface, but their 17,000 fish species represent a quarter of all vertebrates, as well as food for many millions of people. Healthy rivers are also needed to provide clean water.
Other recent research has shown that world populations of migrating river fish have declined by a “catastrophic” 76% since 1970, with a 93% decline in Europe. Large river animals fared the worst, with some like the Mekong giant catfish on the verge of extinction. A 2019 analysis found that only a third of the world’s major rivers remain free-flowing due to the impact of dams.
Sébastien Brosse, at the Paul Sabatier University in Toulouse, France, who led the new research, said that rivers in many rich countries are unrecognizable compared to what they were like before the industrial revolution. “Then we had sturgeon of more than 2 meters in size, we had thousands of salmon and many other fish that almost disappeared today.”
“The River Thames is one of the most affected – it achieved 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’m not sure if the change is rapid, because the fish populations have been really declining.”
The highest biodiversity in the river occurs in South America, but the researchers found that only 6% of the most intact rivers were in this region. “We really need political decisions to consider biodiversity as something 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 merely focused on species numbers, 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.
A major change is the number of alien species introduced into rivers. “In Western Europe, you see North American salmon, black bullhead, which is a North American catfish, carp and goldfish that come to Asia, and mosquito fish,” Brosse said.
All over the world, common carp, beak and tilapia are one of the most common exotic fish. They were adapted for still waters and flourished as the number of dams increased. It is homogenizing the fish populations in rivers, making them less able to cope with environmental changes, such as global warming.
The least affected rivers have been found in remote areas with few people, especially in Africa and Australia, although fish fauna in the Murray-Darling basin have been damaged.
“But the least affected arrival does not house enough species to sustain the world’s biodiversity,” Grosse said. “They only house 22% of the global fauna, so we also need to preserve the biodiversity in sinks that are very much affected by humans.”
“Honestly, I’m surprised they found that only 53% of the river basins have undergone noticeable changes,” Zeb Hogan told the University of Nevada, USA. “Almost all of the world’s largest rivers have undergone significant changes. Where once there were rivers full of salmon and sturgeon or chops and suckers, there are now rivers with bark, eucalyptus, carp and catfish. ”
“The Amazon, Congo and Mekong are affected more than expected – a finding that may not be much appreciated and may indicate that new dams and other pressures have already had a large-scale impact,” he said. “Measures taken to protect and conserve terrestrial and marine wildlife often do not protect the rivers.”
Brosse said the assessment of the impact in his study is likely to be an underestimation, as there are likely to be more fish extinctions than have been officially recorded.