Variable δ15N Diet-Tissue Discrimination Factors among Sharks: Implications for Trophic Position, Diet and Food Web Models

The application of stable isotopes to characterize the complexities of a species foraging behavior and trophic relationships is dependent on assumptions of delta N-15 diet-tissue discrimination factors (Delta N-15). As Delta N-15 values have been experimentally shown to vary amongst consumers, tissues and diet composition, resolving appropriate speciesspecific Delta N-15 values can be complex. Given the logistical and ethical challenges of controlled feeding experiments for determining Delta N-15 values for large and/or endangered species, our objective was to conduct an assessment of a range of reported Delta N-15 values that can hypothetically serve as surrogates for describing the predator-prey relationships of four shark species that feed on prey from different trophic levels (i.e., different mean delta N-15 dietary values). Overall, the most suitable species-specific Delta N-15 values decreased with increasing dietary-delta N-15 values based on stable isotope Bayesian ellipse overlap estimates of shark and the principal prey functional groups contributing to the diet determined from stomach content analyses. Thus, a single Delta N-15 value was not supported for this speciose group of marine predatory fishes. For example, the Delta N-15 value of 3.7% provided the highest percent overlap between prey and predator isotope ellipses for the bonnethead shark (mean diet delta N-15 = 9%) whereas a Delta N-15 value < 2.3% provided the highest percent overlap between prey and predator isotope ellipses for the white shark (mean diet delta N-15 = 15%). These data corroborate the previously reported inverse Delta N-15-dietary delta N-15 relationship when both isotope ellipses of principal prey functional groups and the broader identified diet of each species were considered supporting the adoption of different Delta N-15 values that reflect the predators' delta N-15-dietary value. These findings are critical for refining the application of stable isotope modeling approaches as inferences regarding a species' ecological role in their community will be influenced with consequences for conservation and management actions.