Life history traits of the fish community in lake Annecy: evidence from the stable isotope composition of otoliths

Stable isotope analysis (SIA) of fish otoliths shows great potential for exploring the ecology of fish, but this method has not been applied to the study of lacustrine fish communities. Both sequential and whole-otolith SIAs were performed on six fish species of Lake Annecy and their results compared to muscle SIA. The first purpose of these investigations was to test the use of delta O-18(oto) values for reconstructing fish thermal history and delimiting spatial distribution in a stratified lake. Comparison of species-specific fractionation equations and the general equation developed for freshwater fishes showed that the general one was the best suited for thermal estimation of Lake Annecy fishes and suggested that inter-specific differences or specific "vital effects" are not the only reason for apparent difference in fractionation. Thermal estimations based on SIA were consistent with descriptions of thermal habitats in the literature, except in the case of roach (Rutilus rutilus). Based on the current results, roach appears to live in a colder habitat than do perch (Perca fluviatilis). The high water transparency and thermal stratification of Lake Annecy could explain this distribution. Moreover, perch juveniles were found to live in two different thermal niches. This finding highlights the great plasticity of the species. Second, the potential use of delta C-13(oto) values to reconstruct variation in diet and metabolism was assessed. The proportion of metabolic carbon (M) contributing to otolith carbon varies markedly among species. Comparison with delta C-13(muscle) values shows no direct relationship between delta C-13(oto) and diet at either the intra-individual or the inter-specific level. A strong linear relationship between either M or the isotopic offset between otolith and muscle (Delta delta C-13(oto-muscle)) and delta O-18(oto) values was found; this relationship reveals the dependence of M on the ambient temperature at which the species occurs. This relationship might be general and could be used to explore variation in fish metabolism in the future