The population biology and life history traits of Eurasian ruffe [Gymnocephalus cernuus (L.), Pisces: Percidae] introduced into eutrophic and oligotrophic lakes in northern Italy

We describe the population biology and life-history traits of two Eurasian ruffe [Gymnocephalus cernuus (L.), Pisces: Percidae] populations which have recently colonised two small lakes in the lake Maggiore catchment of Northern Italy, the eutrophic lake (l.) Ghirla (ruffe first recorded in the early 1990s) and the oligotrophic lake (l.) Mergozzo (ruffe first recorded during the present sampling in 2010). Survey gill net catches revealed that ruffe has become one of the most abundant fish species in both lakes. Relative weight and weight-length relationships indicated better growth of ruffe in l. Mergozzo. Thus, gonad weight, adjusted for body size, was higher in l. Mergozzo, indicating a stronger individual reproductive potential in the more recently colonised lake. The stomach contents of ruffe were dominated by chironomid larvae and other benthic organisms, while pelagic microcrustaceans only occurred in small amounts. The ratio of benthic vs pelagic prey in the diet increased with ruffe size. Ruffe intraspecific food niche overlap (based on prey numbers) between age classes was slightly higher in oligotrophic l. Mergozzo than in eutrophic l. Ghirla and decreased with age in both lakes.