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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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The growth, survival and trypsin, lipase and amylase activities of red drum larvae were measured in two experiments. For the first trial, a group was fed live prey only (L) and another group was fed a combination of a microparticulate diet (MPD) and live food (L‐MP). For the second growth trial a group fed the MPD only (MP) and a starvation group (ST) were examined in addition to the L and L‐MP treatments. Enzyme activities of live prey were measured to estimate their possible contribution to larval digestion. No significant (P > 0.05) differences in final size and survival were observed between treatments L and L‐MP. Larvae subjected to starvation or fed the MPD diet alone were smaller than treatments fed live prey and did not survive past days 5 and 14, respectively. Trypsin, lipase and amylase activities were detectable at hatching. No significant differences (P > 0.05) in total enzyme activities among treatments were observed before day 14. Specific activity of trypsin, lipase and amylase peaked on day 3 (prior to first feeding) and subsequently decreased. For trypsin, the percentage of enzyme activity potentially attributable to ingested prey increased with age to a maximum of 17%. For lipase and amylase this fraction was less than 5% throughout the study, except on day 8 (12% and 24%, respectively). The lack of significant differences observed in the activity of digestive enzymes among treatments suggests that dietary regime, availability of prey and possible effects of exogenous enzymes did not significantly influence enzyme activity. Therefore, the lower growth rate observed in the L‐MP, MP and starved treatments cannot be attributed to low digestive enzyme production of the enzymes measured. It is more likely that the MPD failed to supply the required nutrients for adequate development.

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