Recent introductions of the ruffe (Gymnocephalus cernuus) to Coregonus and Perca lakes in Europe and an analysis of their natural distributions in Sweden and Finland

Although the ruffe (Gymnocephalus cernuus) is native to many parts of Europe, its distribution has recently increased through accidental introductions. This synthesis reviews this situation and perceived possible consequences for Coregonus spp. and Eurasian perch (Perca fluviatilis), presents case histories from Lake Constance and Bassenthwaite Lake, and examines natural distributions of ruffe, Coregonus spp., and perch in northern Europe. In Lake Constance, a large mesotrophic lake in central Europe supporting whitefish (Coregonus lavaretus) and perch fisheries, the ruffe was first recorded in 1987 and now occupies many areas, feeding extensively on the eggs of inshore-spawning whitefish. In contrast, Bassenthwaite Lake is a small eutrophic U.K. lake without commercial fisheries, but containing perch and vendace (Coregonus albula), where the ruffe was first recorded in 1991 and is now widespread. Although predation by ruffe on vendace eggs was nor recorded during spawning in late 1995, it was in late 1996. Vendace and perch populations have shown no marked changes between 1991 and 1996 but the cyprinid roach (Rutilus rutilus) has declined. Among 705 lakes in Sweden, differences were found in the features of lakes in which ruffe occur allopatrically and sympatrically with vendace, whitefish, and perch, but nevertheless such coexistence was found in 59, 37, and 210 lakes, respectively. In Finland ruffe were found to inhabit 427 of 710 lakes, including many in which they coexisted with vendace, whitefish, and perch. Factors influencing the distribution of ruffe in Finland included lake size, acidity, trophic status, hypolimnetic anoxia, and availability of spawning substratum.