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¹ Instituto Politécnico Nacional, Centro Interdisciplinario de Ciencias Marinas

difficultto assess, because of large variations in morphology, habitat, feedinghabits, and social structure among areas, which may reflect phylogenetic segregationor ecological plasticity. In the Gulf of California, Mexico, two commonbottlenose dolphin ecotypes (inshore and offshore) have been reported. The offshoreecotype is frequently observed in association with sperm whales (Physetermacrocephalus) but the reason for this is still unknown. To explore the degreeof resource partitioning/overlap between these species and stocks, we used skinstable isotope values (d13C, d15N) to estimate quantitative metrics of isotopicniche width (Bayesian standard ellipse areas, SEAB) and estimated their dietcomposition using Bayesian isotopic mixing models. The inshore ecotype indifferent regions (north, central, and south) of the Gulf of California exhibiteddistinct d15N values and SEAB, suggesting a latitudinal gradient in nitrogensources of coastal localities. The SEAB of inshore and offshore bottlenose dolphinecotypes was completely distinct, indicating resource partitioning. Associatedoffshore ecotype and sperm whales had overlapping SEAB. The isotopicmixing model indicates that a considerable proportion of both species’ diet islarge Humbolt squid. Our results suggest that resource partitioning and speciesassociation are two strategies that bottlenose dolphin ecotypes use in thiszone.

Palabras clave: trophic relationships, isotopic niche width, bottl; truncatus, spermwhale, Physeter macrocephalus, Hum

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