That all changed when the lakes became polluted in the mid-20th century, says Ole Seehausen of Eawag: Swiss Federal Institute of Aquatic Science and Technology in Kastanienbaum and the University of Bern, Switzerland. Fertiliser ran off farmland into the lakes, leaving them over-rich in nutrients – a phenomenon called eutrophication. This caused algal blooms in the lakes, which in turn caused oxygen levels to crash deep in the lakes.

Seehausen and colleagues now think that this oxygen crash forced species to merge. They studied the whitefish populations in 17 lakes, each of which was also studied in the 1920s, before the eutrophication began. Back then, the deeper lakes had more species.