Variable delta 15N Diet-Tissue Discrimination Factors among Sharks: Implications for Trophic Position, Diet and Food Web Models: e77567

• Published in: PloS one 2013-10, Vol.8 (10)
• Main Authors: Olin, Jill A, Hussey, Nigel E, Grgicak-Mannion, Alice, Fritts, Mark W, Wintner, Sabine P, Fisk, Aaron T
• Format: Article
• Language: English
• Subjects: Marine
• ISSN: 1932-6203
• DOI: 10.1371/journal.pone.0077567

The application of stable isotopes to characterize the complexities of a species foraging behavior and trophic relationships is dependent on assumptions of delta 15N diet-tissue discrimination factors ( Delta 15N). As Delta 15N values have been experimentally shown to vary amongst consumers, tissues and diet composition, resolving appropriate species-specific Delta 15N values can be complex. Given the logistical and ethical challenges of controlled feeding experiments for determining Delta 15N values for large and/or endangered species, our objective was to conduct an assessment of a range of reported Delta 15N 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 15N dietary values). Overall, the most suitable species-specific Delta 15N values decreased with increasing dietary- delta 15N 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 15N value was not supported for this speciose group of marine predatory fishes. For example, the Delta 15N value of 3.7ppt provided the highest percent overlap between prey and predator isotope ellipses for the bonnethead shark (mean diet delta 15N = 9ppt) whereas a Delta 15N value < 2.3ppt provided the highest percent overlap between prey and predator isotope ellipses for the white shark (mean diet delta 15N = 15ppt). These data corroborate the previously reported inverse Delta 15N-dietary delta 15N 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 15N values that reflect the predators' delta 15N-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.