MEPS 526:213-225 (2015)  -  DOI: https://doi.org/10.3354/meps11230

Stable isotope mixing models elucidate sex and size effects on the diet of a generalist marine predator

Rhema H. Bjorkland1,5,*, Scott F. Pearson2, Steve J. Jeffries2, Monique M. Lance2, Alejandro Acevedo-Gutiérrez3, Eric J. Ward

1Fisheries Resource Assessment and Monitoring Division, Northwest Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, 2725 Montlake Blvd. East, Seattle, WA 98112, USA
2Washington Department of Fish and Wildlife, Wildlife Science Division, 1111 Washington St. SE, Olympia, WA 98501, USA
3Department of Biology, Western Washington University, 516 High St. MS9160, Bellingham, WA 98225-9160, USA
4Conservation Biology Division, Northwest Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, 2725 Montlake Blvd. East, Seattle, WA 98112, USA
5Present address: US EPA Office of Pollution Prevent and Toxics, 1200 Pennsylvania Ave NW, Washington, DC 20460, USA
*Corresponding author:

ABSTRACT: We applied a 2-step clustering algorithm and Bayesian stable isotope mixing model to examine intraspecific differences in the contribution of prey sources to the diet and foraging habitat of harbor seals Phoca vitulina in the Salish Sea, USA. We analyzed stable isotopes of carbon and nitrogen collected from 32 seals and 248 prey samples representing 18 of 25 of the most common seal prey items identified in seal scat. Stable isotope analyses identified significant harbor seal sex- and size-based differences in diet and foraging habitat use. In comparison to males, female harbor seals had a higher contribution of prey items that were more 13C-enriched. This result may indicate that females derived more of their δ13C value from nearshore versus offshore food webs, an explanation supported by movement data on this population. However, large seals of both sexes displayed a greater offshore signal in their diet, indicating that seal mass effects on foraging habitat use were somewhat independent of sex. Our work contributes to understanding trophic linkages between these generalist consumers and their prey. The foraging differences that we detected between male and female harbor seals present complex challenges for fisheries management and for the design of marine reserves. Many marine reserves in the Pacific Northwest are located in close proximity to seal haul-out sites. By lowering the energetic costs of foraging of females, these reserves may ultimately have the unintended effect of increasing individual fitness, population growth rate, and influencing future predator-induced mortality on endangered species.


KEY WORDS: Stable isotopes · Bayesian mixing model · Harbor seal · Phoca vitulina · Pinniped · Salish Sea


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Cite this article as: Bjorkland RH, Pearson SF, Jeffries SJ, Lance MM, Acevedo-Gutiérrez A, Ward EJ (2015) Stable isotope mixing models elucidate sex and size effects on the diet of a generalist marine predator. Mar Ecol Prog Ser 526:213-225. https://doi.org/10.3354/meps11230

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