By the early 1980s it was clear that the coasts of British Columbia and Washington state is home to two populations that share the same waters but lead completely separate lives. Transients eat primarily marine mammals while residents prefer salmon. Hunting these types of prey requires vastly different techniques, which in turn has led to widely divergent social structures. The 170 known transient killer whales in the Pacific Northwest have a fluid social structure and their whereabouts are unpredictable. Pods typically have fewer than 10 individuals and may contain varying combinations of adults and young.
Adolescents often leave after the birth of a younger sibling, and adult males often travel alone.
Seen from the coastal waters of Mexico to the Bering Sea, transients roam bays, small coves, and channels in search of harbor seals, sea lions, and porpoises. In contrast, resident killer whales have a remarkably stable social structure and travel along predictable routes as they follow salmon runs. Males and females alike spend their entire lives in the pods in which they were born. Ranging in size from six to 50 or more, pods are matrilineal and can include mothers, grandmothers, and even great grandmothers. The 305 residents off British Columbia and Washington state are further divided into two populations that, like transients and residents as a whole, have never been seen to interact.