Competition for Food Between Perch (Perca Fluviatilis L.) and Invasive Ruffe (Gymnocephalus Cernuus (L.)) in Reoligotrophic Lake

The central issue this doctoral thesis is the understanding of the mechanisms underlying the competitive relationship and the general performances of the two percids perch (Perca fluviatilis L.) and ruffe (Gymnocephalus cernuus (L.)) in oligotrophic lakes. Both species are potential competitors for benthic food resources. The visual predator perch is supposed to flourish best under mesotrophic conditions, whereas ruffe with its very sensitive lateral line organ performs best under meso- to eutrophic conditions. In oligotrophic lakes ruffe is far less abundant than perch or even absent. While a mechanistic explanation for higher ruffe abundances in lakes of high productivity is well established, it is less clear why ruffe are less abundant or even absent in lakes of low productivity. This thesis summarises laboratory, enclosure and field experiments on the competitive relationship between perch and ruffe in Lake Constance, a large, pre-alpine lake that has recently returned to oligotrophic conditions.