Isotopic analysis of epidermal mucus in freshwater fishes can reveal short-time diet variations

Although stable isotope analysis is a powerful tool for determining animal diets, migration patterns, and the structure of food webs in aquatic systems, the slow response of isotopic ratios in the traditionally used muscle tissue of fishes often hampers this approach. By analyzing tissues with more rapid isotopic change, this technique can be used over a finer time scale than traditional isotopic analyses. We conducted a diet-switch experiment to compare changes in the stable carbon and nitrogen isotope ratio values (δ 13C and δ 15N, respectively) in epidermal mucus, fin and muscle tissues of three species of freshwater cyprinid fishes. The half-life of the isotopic change ranged from 62 to 144 days in mucus, 57 to 202 days in fin, and 196 to 680 days in the muscle tissue. The isotope ratios in mucus changed more rapidly than those in muscle tissues in the three species examined, similarly to the previously studied three species (catfish, steelhead, freshwater goby), whereas fin showed relatively variable trends in isotopic change. Interspecific variation in the trophic discrimination factor (TDF) was particularly large in the δ 13C values of fin tissue compared with those in the other two tissues. With generally rapid change and relatively invariable TDF, the use of epidermal mucus in isotope analysis instead of the widely used fin and muscle tissues is advantageous for studying short-term changes in food habits of fishes and food web structures.