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 int roductions. This synthesis reviews this situation and perceived possib le consequences for Coregonus spp. and Eurasian perch (Perca fluviatil is), presents case histories from Lake Constance and Bassenthwaite Lak e, and examines natural distributions of ruffe, Coregonus spp., and pe rch in northern Europe. In Lake Constance, a large mesotrophic lake in central Europe supporting whitefish (Coregonus lavaretus) and perch f isheries, the ruffe was first recorded in 1987 and now occupies many a reas, feeding extensively on the eggs of inshore-spawning whitefish. I n contrast, Bassenthwaite Lake is a small eutrophic U.K. lake without commercial fisheries, but containing perch and vendace (Coregonus albu la), where the ruffe was first recorded in 1991 and is now widespread. Although predation by ruffe on vendace eggs was nor recorded during s pawning in late 1995, it was in late 1996. Vendace and perch populatio ns have shown no marked changes between 1991 and 1996 but the cyprinid roach (Rutilus rutilus) has declined. Among 705 lakes in Sweden, diff erences were found in the features of lakes in which ruffe occur allop atrically and sympatrically with vendace, whitefish, and perch, but ne vertheless such coexistence was found in 59, 37, and 210 lakes, respec tively. In Finland ruffe were found to inhabit 427 of 710 lakes, inclu ding 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.