Stable Isotope Analysis Reveals Group-Specific Reliance on Elasmobranch Prey by Killer Whales in the Gulf of California

Killer whales (Orcinus orca) globally exhibit a broad dietary spectrum, ranging from cartilaginous and teleost fish to large marine mammals. However, discrete populations often show prey specialization, which is commonly associated with the evolution of distinct foraging strategies and behavioral adaptations. In the Gulf of California, killer whales have been observed utilizing specialized hunting techniques on a wide range of prey, including different elasmobranch species. To quantify this dietary specialization, we analyzed δ13C and δ15N stable isotope ratios from skin biopsies of eight individuals from two different groups (n = 7 from the group designated as “Moctezuma pod” and n = 1 from a second distinct pod) of killer whales alongside isotopic profiles of six sympatric potential prey species (Mobula munkiana, Delphinus delphis bairdii, Balaenoptera physalus, Carcharodon carcharias, Isurus oxyrinchus, and Tursiops truncatus), as well as estimated trophic position, isotopic niche, and prey contribution by Bayesian mixing models. Killer whales exhibited mean (± SD) isotopic values of -17.04 ± 0.54‰ for δ13C and 19.59 ± 0.69‰ for δ15N, resulting in an estimated trophic position of 4.3. Elasmobranchs, specifically shortfin mako sharks (I. oxyrinchus) and white sharks (C. carcharias), collectively accounted for over 50% of the median estimated dietary contribution. Together with independent observations of elasmobranch predation in the Gulf of California, these results indicate substantial reliance on elasmobranch prey by the sampled killer whales during the dietary period integrated by the sampled tissue and suggest potential trophic specialization, possibly complemented by dietary plasticity. Additional temporal and individual sampling will be necessary to determine the persistence and broader prevalence of this dietary pattern.