Lake fish hybridising themselves to extinction

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Fish species endemic to Alpine lakes are hybridising themselves to extinction as a result of pollution.

Coregonus whitefish living in Europe's Alpine lakes split into multiple species after the last ice age. Initially this speciation was driven by spawning behaviour as the fish took advantage of different environments within deep lakes, some spawning on the lake floor, with others favouring the surface layers.

This behaviour eventually led to each species developing differing appearances and habits.

The divergence continued until the mid-20th century, when fertiliser run-off from farmland surrounding the lakes increased nutrient levels dramatically, leading to eutrophication, causing algal blooms and an associated crash in oxygen levels in the lakes' deeper waters.

The scientific team behind this theory studied present day whitefish populations in 17 lakes and compared their results with pre-eutrophication studies carried out on the same lakes in the 1920's when deep lakes had more species.

The results were stark, showing a 38% fall in species in that period, as well as a convergence in shape in the remaining species.

Genetic studies of these survivors show that many carry genetic markers previously only found in the now extinct species, suggesting that these fish may have hybridised themselves to extinction.

These European fish are not the only victims of this process, with earlier studies of cichlids in Lake Victoria, east Africa showing a similar pattern occurring as a result of eutrophication.

Murky water made the different species' distinctive colouration redundant, leading to hybridisation.

Prof Dr. Ole Seehausen, head of Fish Ecology and Evolution at EAWAG Aquatic Research and co-author of the paper, says that hybridisation in endangered species is a particular risk as it can go unnoticed until it is too late. Endangered species assessments often concentrate on population size, but if hybridisation is not spotted it can appear that populations are stable while species diversity collapses. Seehausen notes that while evolution eventually makes hybridisation impossible, some fish species can interbreed 20 million years after splitting.

For more information see the paper: P. Vonlanthen, D. Bittner, A. G. Hudson, K. A. Young, Müller, B. Lundsgaard-Hansen, D. Roy, S. Di Piazza, C. R. Largiader and O. Seehausen 'Eutrophication causes speciation reversal in whitefish adaptive radiations'; Nature 482, 357-362 (16 February 2012) doi:10.1038/nature10824.

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