| Summary: | Ecogeochemistry has become a useful tool in studying foraging ecology of marine mammals and other consumers. Ratios of stable carbon (δ13C) and nitrogen (δ15N) isotopes in consumer tissues can reflect foraging history over longer time spans than traditional methods of dietary analysis. To better understand overall foraging patterns and potential stable isotope relationships between individuals from different age groups within the same population, I measured stable isotope values from unrelated northern fur seal (Callorhinus ursinus) pups and post-parturient adult females from San Miguel Island, California. Fur seals at San Miguel Island can be severely impacted by climate events such as the El Niño Southern Oscillation, increasing the need to further understand their foraging ecology. I analyzed sequential δ13C and δ15N values along single whiskers from five pups and five adult females, estimated pup and in utero whisker growth rates, and reconstructed stable isotope profiles over time for each vibrissa. I then performed correlation tests on the stable isotope profiles to compare mean δ13C and δ15N values from pups and adult females. Pups had significantly lower δ13C values and significantly higher δ15N values than adult females over an overlapping 50-day period prior to tissue collection. Stable isotope profiles for pups were significantly correlated, but there were no correlations among adult female stable isotope profiles. My results suggest large inter-individual variation among adult female foraging patterns, no significant relationship between stable isotope values from unrelated pups and adult females, and potentially distinct stable isotope minima indicating the birth event for pups.
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