"Sixth mass extinction." Since the beginning of the year 2019, this expression is regularly repeated in the writings of scientists to denounce the acceleration of the rate of extinction of animal and plant species. And the fauna occupying the fresh waters is not left out. According to a study published Aug. 8 by the journal Global Change Biology, some species found in lakes and rivers have declined by 88% in the space of forty years.
Although rivers and lakes cover only 1% of the earth's surface, they are the cradles of more than a third of the world's vertebrate species and half of the world's fish. A study of some of the freshwater fauna, ie species weighing 30 kg or more, shows that, like their maritime counterparts, they are endangered.
Overfishing and overexploitation in question
A team of scientists has therefore examined the fate of 126 species of large freshwater animals for a period from 1970 to 2012. While on average the fauna has declined by 88% in the world, it is in indomestic ecozone (India and Southeast Asia) that these aquatic populations are the most threatened (-99%), as well as in the Palearctic, covering Europe, Asia and North Africa (-97% ).
The first victims are fish, such as sturgeon or giant catfish (-94%), followed by reptiles, such as crocodiles, turtles and aquatic snakes (-72%). "The results are alarming and confirm the fear of scientists involved in the study and protection of biodiversity in freshwater," said in a statement Sonja Jähnig, one of the authors of the study.
Two reasons are advanced by scientists to explain the phenomenon: overfishing and overexploitation of these bodies of water. Fengzhi He, ecologist and author of the study, explains that "despite fragmentation of rivers already around the world, 3,700 dams are planned or under construction [...], of which more than 800 are in areas of great diversity. freshwater fauna like Amazonia, Congo, Mekong and Ganges ".
Although 34 freshwater species are listed on the International Union for the Conservation of Nature's Red List of Threatened Species, they are less studied than those of terrestrial or marine fauna, and therefore less protected. Nevertheless, effective conservation measures are beginning to be put in place. Thirteen species - including American beaver or green sturgeon - have stabilized or increased in the United States, and the number of Irrawaddy dolphins in the Mekong River has increased for the first time in twenty years. Good news for this Asian region, where other aquatic animals such as giant Mekong catfish and aaptosyax grypus have almost disappeared.