Turnover and fractionation of nitrogen stable isotope in tissues of grass carp Ctenopharyngodon idellus

Determination of the isotopic turnover rate in the various tissues of an organism is one of the essential prerequisites for tracing the food sources of consumers and for elucidating trophic interactions in ecological studies using stable isotope analysis (SIA). Isotopic turnover and fractionation in the commercially important freshwater teleost grass carp Ctenopharyngodon idellus, however, are poorly understood so far. In the present study, we conducted a diet-switch experiment of C. idellus for 3 mo to assess nitrogen isotopic turnover and fractionation in different tissues of this species, including liver, muscle, and gill. The results revealed that turnover rates exhibited significant differences between tissues and increased in the sequence of gill < muscle < liver, owing to the differences in metabolic activities between the tissues. Contribution to long-term nitrogen isotopic turnover rates from metabolism was greatest for the liver (similar to 70 to 77% metabolic contribution) and greatest from growth for the gill (86 to 90% contribution of net tissue increase). Nitrogen half-lives estimated with time-or growth-based models were, respectively, 29.9 d and 1.18-fold mass increase for liver, 68.3 d and 1.45-fold mass increase for muscle, and 115.4 d and 1.81-fold mass increase for gill. The nitrogen isotopic fractionation of all tissues was enriched by 3 to 4% relative to the nitrogen isotopic signature of the diet, with significant differences between the 3 types of tissues, probably owing to the different biochemical pathways and constituents between the various tissues.