The stable isotope composition of larval fish tissues may serve as a chemical tracer of recent settlement due to food web differences among planktonic and demersal habitats. We present the background for the utilization of δ13C and δ15N to trace settlement of red drum (Sciaenops ocellatus), an estuarine-dependent species. The effect of ontogeny and temperature on the relative contribution of growth and metabolic turnover to changes in isotopic composition was examined by simulating dietary shifts in the laboratory. Fractionation was examined as a function of size and the effect of food deprivation was evaluated. Published growth rates were used to estimate the time period within which the isotopic composition of a new food source should be reflected in larval tissues. In response to dietary shifts, larvae exhibited quick changes in δ13C and δ15N in a pattern closely resembling predictions based on growth alone. Fractionation values were about +1 for δ13C and +1.6 for δ15N. There was no effect of 4 days of food deprivation on δ13C and δ15N. Given the fast growth rates reported for newly settled red drum, their isotopic composition should exhibit a shift within 1-2 days and stabilize about 10 days following settlement.
There are very few studies reporting isotopic trophic discrimination factors and turnover rates for marine elasmobranchs. A controlled laboratory experiment was conducted to estimate carbon and nitrogen isotope trophic discrimination factors and isotope turnover rates for blood, liver, muscle, cartilage tissue, and fin samples of neonate to young-of-the-year leopard sharks ( Triakis semifasciata ). Trophic discrimination factors varied (0.13‰–1.98‰ for δ13C and 1.08‰–1.76‰ for δ15N). Tissues reached or were close to isotopic equilibrium to the new diet after about a threefold biomass gain and 192 days. Liver and blood exhibited faster isotope turnover than muscle, cartilage tissue, and fin samples, and carbon isotopes turned over faster than those of nitrogen. Metabolic turnover contributed substantially to isotopic turnover, which differs from most reports for young marine teleosts. We modeled the relationship between muscle turnover rates and shark size by coupling laboratory results with growth rate estimates for natural populations. Model predictions for small, medium, and large wild leopard sharks indicate the time to isotopic equilibrium is from one to several years.
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