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Carbon and nitrogen discrimination factors for elasmobranch soft tissues based on a long-term controlled feeding study

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Abstract

The foraging ecology of elasmobranchs (sharks, skates, and rays) is difficult to study because species have spatially and temporally diverse diets. Many diet and habitat preference studies for mammals, birds, and teleosts use stable isotope analysis, but interpretations are limited for elasmobranch studies because taxon-specific isotope discrimination factors from a controlled experiment are unavailable. Trophic discrimination factors for plasma, red blood cells, and muscle were determined from an experiment with leopard sharks (Triakis semifasciata) fed a constant diet of squid over 1000 days. The δ13C values for shark tissues at equilibrium with the squid diet did not vary significantly among individuals, but plasma and red blood cell δ15N values differed significantly among individuals and sampling day. Individual variation of muscle δ15N averages was observed and likely related to growth. Overall, carbon and nitrogen discrimination factors corresponded to previous studies featuring high-protein diets and carnivorous taxa. The muscle-to-diet discrimination factors from the controlled feeding study were applied to blue sharks (Prionace glauca) and smooth hammerhead sharks (Sphyrna zygaena) caught offshore from Baja California, Mexico. This case study demonstrates the potential of stable isotope analysis to illuminate differences in foraging patterns between elasmobranch species.

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Notes

  1. For ease of communication, we will refer to these as discrimination factors and omit the subscripts on Δ values. Δ values are most often used because they are trivial to calculate and accurate as long as the difference in δ values between diet and tissue is small (< 10‰, Cerling and Harris 1999).

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Acknowledgements

We thank J. Adams, A. Bennett, M. Gorey, L. Krol, S. Perry, S. Rumbolt, A. Sjostrom, A. Thell, and C. Spencer for their assistance maintaining and sampling the sharks throughout the duration of this study; the Marine Science Institute (Redwood City, California) that catches leopard sharks for educational demonstrations and donated specimens for this project; S. Carleton, K. Fox-Dobbs, and two anonymous reviewers for their constructive input on this manuscript. The Institutional Animal Care and Use Committee (IACUC) at UCSC approved sampling procedures according to the National Institutes of Health Policy. All squid fed to the sharks was donated from the Monterey Bay Aquarium. The infrastructure for the study was funded by NSF-OCE 0345943 and largely executed by N. Moore (Long Marine Lab, UCSC). An IGPP Mini-Grant provided the funding for analytical analysis.

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  1. Sora Lee Kim

    Present address: Department of Geology and Geophysics, University of Wyoming, 1000 University Ave. #3006, Laramie, WY, 82071, USA

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  1. Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High St, Santa Cruz, CA, 95064, USA

    Sora Lee Kim & Paul L. Koch

  2. Long Marine Laboratory, University of California, Santa Cruz, 100 Shaffer Rd., Santa Cruz, CA, 95060, USA

    Dave R. Casper

  3. Centro Interdisciplinario de Ciencias Marinas-IPN, Av. Instituto Politécnico Nacional S/N, Po Box 572, La Paz, Baja California Sur, Mexico, C.P. 23096

    Felipe Galván-Magaña, Ruth Ochoa-Díaz & Sandra Berenice Hernández-Aguilar

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Supplemental Material A

Growth rates for each individual throughout experiment. Individual symbols correspond to those used in Figure 1. Growth rates remain steady for two individuals, but the individual represented by “•” had low hematocrit values and a reduced growth rate. (DOCX 49 kb)

Appendix A

Appendix A

Table 5 Potential prey data for blue and hammerhead sharks used in MixSIR
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Kim, S.L., Casper, D.R., Galván-Magaña, F. et al. Carbon and nitrogen discrimination factors for elasmobranch soft tissues based on a long-term controlled feeding study. Environ Biol Fish 95, 37–52 (2012). https://doi.org/10.1007/s10641-011-9919-7

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