Summer distribution patterns of southern resident killer whales Orcinus orca: core areas and spatial segregation of social groups

A diverse range of marine wildlife species respond to dynamic environments and patchy resources with variations in their distribution pattern. For many cetacean species, highly structured social systems add to the complexity of modeling distribution patterns and understanding the effects on population viability. Southern resident killer whales Orcinus orca are comprised of 3 distinct, yet cohesive, matrilineally related social groups called pods, and recent population declines have prompted conservation actions in the US and Canada, including listings as an endangered species. Habitat protection is a major component of each listing, yet little is known about pod-specific core areas or patterns of space use for this population. Using 6 yr of sighting information within the inshore waters of Washington and British Columbia, we modeled pod-specific summer distribution and measured relative variation in the density of sightings on a uniform spatial grid. Our results indicate that the pods exhibit complex, non-uniform summer space-use patterns. Although some regions are used in common by all pods, some areas are used almost exclusively by individual pods or certain combinations of pods, indicating specialization to particular areas. Our results indicate that pod groups display variable patterns of space use; therefore, each pod may experience unique habitat conditions or exposure to potential risk factors. This result is highly relevant when considering pod-specific mortality rates and the relative contribution of each pod to overall population trends. Our analyses highlight important areas of conservation, and provide the necessary description for pod-focused conservation initiatives.