Turnover and fractionation of carbon and nitrogen stable isotopes in tissues of a migratory coastal predator, summer flounder (Paralichthys dentatus)

Migratory and mobile fishes such as summer flounder (Paralichthys dentatus) often utilize dietary resources with stable isotope signatures that vary over time and space, potentially confounding diet analyses if tissues with slow turnover are sampled before reaching isotopic equilibrium. A laboratory diet-shift study was conducted using juvenile and young adult summer flounder to (i) determine isotopic turnover rates and fractionations of delta(13)C and delta(15)N in liver, whole blood, and white muscle and (ii) estimate the relative importance of growth and metabolic processes on isotopic turnover. Isotopic turnover rates were consistently ranked liver > blood > muscle owing to increased metabolic activities of liver and blood. Carbon and nitrogen half-lives ranged from 10 to 20 days (liver), 22 to 44 days (blood), and 49 to 107 days (muscle), indicating that liver and blood are more useful than muscle as shorter-term dietary indicators for summer flounder and other migratory fishes. Growth-based fractionation estimates of wild flounder tissues ranged from 0.71 parts per thousand to 3.27 parts per thousand for carbon and from 2.28 parts per thousand to 2.80 parts per thousand for nitrogen and included the first explicit estimates for isotopic fractionation in fish blood. A generalized model for predicting the time scale of isotopic turnover from growth-based turnover parameters was also developed to help evaluate isotopic equilibrium assumptions of fishes in the field.