Nutrient overload causes fish species loss

The presence of excess nutrients in Swiss lakes has led to a reduction of almost 40 per cent in the number of endemic white fish species, scientists have found.

Only in the deep alpine lakes least exposed to high nutrients – such as Lakes Thun, Brienz and Lucerne – have the original diversity of species been able to survive. But they have become less genetically distinctive.

These findings, in a study by the Swiss Federal Institute of Aquatic Science and Technology (Eawag) and Bern University, were published in the journal Nature on Thursday.

The reduction has largely been caused by the hybridisation of formerly distinct species. This in turn, the scientists say, can be explained by the eutrophication – run-off of excess nutrients into the waters, usually from agriculture – of the lakes in 1950-1980.

As the bottom and deep waters of many lakes became severely oxygen-deprived during this time, certain species had to move to shallower waters, where they interbred with related species. This led to a loss of their genetic and functional distinctiveness in a process known as speciation reversal.

“Speciation reversal appears to be more widespread than has previously been supposed; this process involves the rapid disappearance of species which evolved over thousands of years by adaptation to specific ecological conditions,” said evolutionary biologist Ole Seehausen, the lead author of the study, in an Eawag statement.

Biodiversity protection should therefore not only conserve existing species, but also protect the ecological and evolutionary processes which assure the maintenance of specialists and promote the generation of new species, added the statement.