The diet of ruffe Gymnocephalus cernuus (L.) in northern lakes: new insights from stable isotope analyses

The diet of ruffe Gymnocephalus cernuus was studied in two native populations in lakes of different productivity in south-west Finland using both traditional stomach content analyses and stable isotopes. According to stomach content analyses, chironomids were the most important prey type in both lakes and the diversity of the other prey reflected the zoobenthos community in the lake. Stable isotope analyses of carbon and nitrogen showed distinctive lake-specific and total length ( L _T)-related patterns in both δ¹³C and δ¹⁵N values, which could not be explained solely with diet changes. In the large mesotrophic Pyhäjärvi, both ¹³C and ¹⁵N isotopes became slightly enriched with increasing ruffe L _T, although stomach contents analyses did not suggest any diet change in larger ruffe. In the hypereutrophic Köyliönjärvi, the carbon isotope signatures of ruffe especially showed wide variation (?33·5 to ?24·1‰), which was probably due to variable consumption of prey items with highly negative carbon isotope signatures. Overall, this study emphasizes that the interpretation of stable isotope results requires extensive background data of the system and that even then the diet composition of a consumer may be very difficult to define due to large variation in the signatures.