Isotope turnover rates and diet-tissue discrimination in skin ofex situBottlenose Dolphins (Tursiops truncatus)

Diet-tissue discrimination factors (Δ(15)N or Δ(13)C) and turnover times are thought to be influenced by a wide range of variables including metabolic rate, age, dietary quality, tissue sampled and the taxon being investigated. In the present study, skin samples were collected from ex situ dolphins that had consumed diets of known isotopic composition for a minimum of 8 weeks. Adult dolphins consuming a diet of low fat (5-6%) and high δ(15)N value had significantly lower Δ(15)N values than animals consuming a diet with high fat (13.9%) and low δ(15)N value. Juvenile dolphins consuming a diet with low fat and an intermediate δ(15)N value had significantly higher Δ(15)N values than adults consuming the same diet. Calculated half-lives for δ(15)N ranged from 14 to 23 days (17.2 ± 1.3 days). Half-lives for δ(13)C ranged from 11 to 23 days with a significant difference between low fat (13.9 ± 4.8 days) and high fat diets (22.0 ± 0.5 days). Overall, our results indicate that while assuming a Δ(13)C value of 1‰ may be appropriate for cetaceans, Δ(15)N values may be closer to 1.5‰ rather than the commonly assumed 3‰. Our data also suggest that understanding seasonal variability in prey composition is another significant consideration when applying discrimination factors or turnover times to field studies focused on feeding habits. Isotope retention times of only a few weeks suggest that, in addition, these isotope data could play an important role in interpreting recent fine-scale habitat utilization and residency patterns.