Dietary specialization on elasmobranchs and seasonal foraging patterns of offshore killer whales Orcinus orca in the northeastern Pacific

Dietary specialization allows species with overlapping ranges and similar trophic levels to coexist by reducing competition. Offshore killer whales are sympatric with other populations of Orcinus orca in the northeastern Pacific, but their feeding preferences are poorly known. Using genetic, drone videography, photographic, and observational evidence collected during 84 predation events, we found that elasmobranchs comprised almost 90% of all prey sampled, revealing that the offshore killer whale ecotype has a specialized diet. Sharks are energetically profitable prey because they are predictably available, large-bodied, and have a large, lipid-rich liver. In addition to previously documented prey, we identified 4 new species: salmon shark, Pacific electric ray, albacore tuna, and broadnose sevengill shark. Targeted species differed depending on the time of year and region, matching the known seasonal availability and migratory behaviour of sharks. Pacific sleeper sharks (45.2% of identified prey) and salmon sharks (6.0%) were primarily taken in Alaska, USA, and northern British Columbia (BC), Canada, in spring and early summer, whereas blue sharks (17.9%) and Pacific spiny dogfish (16.7%) were typically caught in southerly locations along the continental shelf edge off BC in late summer and fall. Blue, broadnose sevengill, and shortfin mako sharks were hunted off California, USA, in winter. Teleosts comprised only 10.7% of predation events. We conclude that in addition to fish-eating and marine mammal-eating ecotypes, offshore killer whales represent another example of pronounced, likely culturally transmitted, dietary specialization among the killer whales of the northeastern Pacific.