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Abstract

Transient (mammal-eating) killer whales inhabiting the California Current Ecosystem—from southern British Columbia to southern California—belong to the genetically distinct west coast population. Variations in their association patterns and habitat use suggest that they are not a single uniform group. I analysed 2,232 georeferenced encounters with individually identified transient killer whales from 2005 to 2021 using data from dedicated surveys and opportunistic sightings. By applying behavioural, social, and geospatial analyses, I identified two subpopulations of west coast transients: an inner coast subpopulation that frequents nearshore coastal areas and an outer coast subpopulation that inhabits deeper waters near the continental shelf-break. The inner coast subpopulation consisted of 345 identified whales and is primarily found within 5 km of shore where they fed on pinnipeds and small cetaceans in nearshore waters. The outer coast subpopulation, with 211 identified whales, tended to occur within 9 km of the continental shelf break and in offshore areas near submarine canyons where they preyed on pelagic pinnipeds, oceanic dolphins, and large cetaceans. A detailed analysis of the predatory behaviours of outer coast transients showed that they exploited deep submarine canyons and oceanic environments. They also demonstrated seasonal variations in occurrences, behaviours, and group sizes—with more encounters occurring off California and in the spring as grey whales migrated northward from their breeding and calving lagoons in Mexico. Groups of killer whales foraged exclusively in open water, with individuals typically following the contours of submarine canyons as they searched for prey. The outer coast transients that were encountered near submarine canyons spent 50% of their time searching for prey, and the remainder pursuing prey, feeding, travelling, socializing, and resting. I analyzed sightings of 49 killer whales of unknown ecotype encountered in the deep oceanic waters offshore California and Oregon (>65 km), and concluded that they may represent a distinct oceanic subpopulation of transient killer whales or an undescribed mammal-eating oceanic population. These findings underscore the complex ecological, behavioural, and social structure of transient killer whales, and highlight the need for conservation strategies that address the distinct habitat preferences, prey choices, and social dynamics of these subpopulations.