Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 7497
Title: Stable isotope analysis elucidates sex and size effects on diet composition of Salish Sea harbor seals (Phoca vitulina)
Author: Rhema Bjorkland, Scott F. Pearson, Steven J. Jeffries, Monique M. Lance, Alejandro Acevedo-Gutierrez, E. J. Ward
Publication Year: 2015
Journal: Marine Ecology Progress Series
Keywords: stable isotope,seals,salmon,rockfish,ontogenetic shifts,diet,

 We apply a Bayesian stable isotope (SI) mixing model to estimate the contribution of prey sources to the diet of harbor seals, Phoca vitulina in the San Juan Islands in the Salish Sea. We  analyze stable isotopes of C and N  collected from 35 seals and 248  prey samples   the 18 most common prey items.  Our approach involved (1) determining ecologically important prey groups, (2) identifying significant covariates, (3) combining the results of the first two analyses into a Bayesian mixing model, to estimate the relative contribution of different prey items.SI analyses identified sex and size-based differences were significant in foraging habitat selection. Female harbor seals deriving more of their 13C signature from nearshore versus offshore food webs, however size effects on foraging habitat selection were somewhat independent of sex, with larger animals of both sexes displaying a greater offshore signal in their diet. In general, small females have a higher contribution of prey items that are 13C depleted.