Trophic ecology of Mexican Pacific harbor seal colonies using carbon and nitrogen stable isotopes

by Maricela Juárez-Rodríguez, Gisela Heckel, Juan Carlos Herguera-García, Fernando R. Elorriaga-Verplancken, Sharon Z. Herzka, Yolanda Schramm

There is limited information that provides a comprehensive understanding of the trophic ecology of Mexican Pacific harbor seal (Phoca vitulina richardii) colonies. While scat analysis has been used to determine the diet of some colonies, the integrative characterization of its feeding habits on broader temporal and spatial scales remains limited. We examined potential feeding grounds, trophic niche width, and overlap, and inferred the degree of dietary specialization using stable carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N) in this subspecies. We analyzed δ¹³C and δ¹⁵N on fur samples from pups collected at five sites along the western coast of the Baja California Peninsula, Mexico. Fur of natal coat of Pacific harbor seal pups begins to grow during the seventh month in utero until the last stage of gestation. Therefore pup fur is a good proxy for the mother’s feeding habits in winter (~December to March), based on the timing of gestation for the subspecies in this region. Our results indicated that the δ¹³C and δ¹⁵N values differed significantly among sampling sites, with the highest mean δ¹⁵N value occurring at the southernmost site, reflecting a well-characterized north to south latitudinal ¹⁵N-enrichment in the food web. The tendency identified in δ¹³C values, in which the northern colonies showed the most enriched values, suggests nearshore and benthic-demersal feeding habits. A low variance in δ¹³C and δ¹⁵N values for each colony (<1‰) and relatively small standard ellipse areas suggest a specialized foraging behavior in adult female Pacific harbor seals in Mexican waters.