Fish hybridise themselves extinct in Alpine lakes
http://www.newscientist.com/article/dn21482-fish-hybridise-themselves-extinct-in-alpine-lakes.html
When the going gets tough, species start merging. Lake-dwelling fish species that once lived separately began interbreeding when pollution forced them together. Ultimately most of the lakes' remarkable diversity has been lost – and the same could be happening in threatened habitats around the world.
After the last ice age, whitefish (Coregonus) in Europe's Alpine lakes split into several species, each with a specialised appearance and lifestyle. They first separated because they spawned in different places, some favouring the lake bottom and others the surface layers.
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.