Carbon stable isotope turnover and fractionation in grass carp Ctenopharyngodon idella tissues

Carbon stable isotope signatures can effectively trace food sources, elucidate trophic interactions, and presumably reflect food web structure. The present study conducted a diet-switch experiment using freshwater teleost grass carp Ctenopharyngodon idella cultured for 3 mo with artificial feed that had a distinct carbon stable isotope composition. Turnover and fractionation of carbon stable isotopes were determined based on the proportional contributions of metabolism and growth to the isotopic turnover in different tissues. Carbon isotopic turnover rates showed significant differences among tissues and increased in the sequence of gill < muscle < liver due to the difference in the metabolic activities among the various tissues. Carbon half-lives based on the isotopic turnover models were 22.5 d or 1.13-fold mass increase for liver, 52.7 d or 1.35-fold mass increase for muscle and 101.1 d or 1.73-fold mass increase for gill tissue. Fractionation of carbon stable isotopes differed among the 3 types of tissues due to the differential biochemical constituents of the tissues. Based on the isotopic turnover models, the fractionation of carbon stable isotopes relative to the new diet approximately fitted the traditional carbon fractionation value of 1%. Metabolism in liver and muscle tissue accounted for approximately 75-85% and 50-60% of tissue turnover, respectively, whereas only 15-25% of tissue turnover might be attributable to metabolism in the gills.