The rivers in which fish populations have escaped serious damage from human activities make up just 14% of the world’s watershed area, 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, agricultural irrigation and rising temperatures due to the climate crisis are also responsible.
The most affected regions are Western Europe and North America, where large and wealthy populations mean that the impact of humans on rivers is greater, such as the Thames in the United Kingdom and Mississippi in the USA.
Rivers and lakes are vital ecosystems. They cover less than 1% of the planet’s surface, but its 17,000 species of fish represent a quarter of all vertebrates, in addition to providing food for millions of people. Healthy rivers are also needed to provide clean water.
Another recent survey showed that global migratory fish populations have fallen by 76% “catastrophic” since 1970, with a 93% drop in Europe. The big animals of the river had the worst performance, with some, like the giant Mekong catfish, on the verge of extinction. A 2019 analysis revealed that only a third of the world’s large rivers continued to flow freely, due to the impact of dams.
Sébastien Brosse, from Université Paul Sabatier in Toulouse, France, who led the new research, said that rivers in many wealthy nations were unrecognizable compared to what they were before the industrial revolution. “So we had the sturgeon over 2 meters in size, thousands of salmon and many other fish that almost disappeared today.”
“The River Thames is one of the most affected – it reached a maximum score 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 the past few decades, but I’m not sure if the speed of change is sufficient, because there has been a really sharp 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 that region. “We really need strong political decisions to consider biodiversity as important to humans,” said Brosse.
The research, published in the journal Science, examined almost 2,500 rivers in all parts of the world, except in 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 the proximity between the different species. 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 rivers. “In Western Europe, you will see North American salmon, black ox head, which is an American catfish, carp and goldfish that come from Asia, and mosquito fish,” said Brosse.
All over the world, common carp, largemouth bass and tilapia are among the most widespread exotic fish. They are adapted to standing water and have thrived with the increase in the number of dams. This is homogenizing 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, particularly in Africa and Australia, although the fish fauna in the Murray-Darling Basin has been damaged.
“But these less affected basins do not host enough species to maintain the global biodiversity of fish,” said Grosse. “They host only 22% of the global fauna, so we also need to conserve biodiversity in basins highly impacted by man.”
“Frankly, I am surprised that they found that only 53% of river basins have undergone marked changes,” said Zeb Hogan of the University of Nevada, USA. “Almost all of the world’s largest rivers have undergone significant changes. Where there used to be rivers full of salmon and sturgeon or chubs and suction cups, now there are rivers with sea bass, bluegill, carp and catfish. “
“The Amazon, Congo and Mekong are more impacted than expected – a finding that may not be widely appreciated and may indicate that new dams and other pressures may have already 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 underestimated, because more fish extinctions probably occurred than officially registered.