title: Variable δ(15)N diet-tissue discrimination factors among sharks: implications for trophic position, diet and food web models.
abstract: The application of stable isotopes to characterize the complexities of a species foraging behavior and trophic relationships is dependent on assumptions of δ(15)N diet-tissue discrimination factors (∆(15)N). As ∆(15)N values have been experimentally shown to vary amongst consumers, tissues and diet composition, resolving appropriate species-specific ∆(15)N values can be complex. Given the logistical and ethical challenges of controlled feeding experiments for determining ∆(15)N values for large and/or endangered species, our objective was to conduct an assessment of a range of reported ∆(15)N 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 δ(15)N dietary values). Overall, the most suitable species-specific ∆(15)N values decreased with increasing dietary-δ(15)N 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 ∆(15)N value was not supported for this speciose group of marine predatory fishes. For example, the ∆(15)N value of 3.7‰ provided the highest percent overlap between prey and predator isotope ellipses for the bonnethead shark (mean diet δ(15)N = 9‰) whereas a ∆(15)N value < 2.3‰ provided the highest percent overlap between prey and predator isotope ellipses for the white shark (mean diet δ(15)N = 15‰). These data corroborate the previously reported inverse ∆(15)N-dietary δ(15)N 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 ∆(15)N values that reflect the predators' δ(15)N-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.
authors.affiliation: Great Lakes Institute for Environmental Research, University of Windsor, Windsor, Ontario, Canada.
authors.forename: Jill A
authors.initials: JA
authors.lastname: Olin
authors.forename: Nigel E
authors.initials: NE
authors.lastname: Hussey
authors.forename: Alice
authors.initials: A
authors.lastname: Grgicak-Mannion
authors.forename: Mark W
authors.initials: MW
authors.lastname: Fritts
authors.forename: Sabine P
authors.initials: SP
authors.lastname: Wintner
authors.forename: Aaron T
authors.initials: AT
authors.lastname: Fisk
mesh_terms: D000818:Animals; D002247:Carbon Isotopes; D005247:Feeding Behavior; D020387:Food Chain; D007553:Isotope Labeling; D008962:Models, Theoretical; D009587:Nitrogen Isotopes; D011235:Predatory Behavior; D012754:Sharks
chemical_list: D002247:Carbon Isotopes; D009587:Nitrogen Isotopes