Following the Exxon Valdez oil spill, these studies were expanded and carried out on a more systematic basis. As a result of this research, approximately 250 animals in 15 pods have been identified in Prince william Sound. Because some areas of Alaska have never had any photoidentification research done, substantial numbers of killer whales may remain to be identified and counted. Killer whales are opportunistic feeders and have been observed to prey on virtually any large marine animal available. Resident pods appear to feed primarily on a wide variety of fish such as salmon, herring, halibut, and cod.
Transient pods feed primarily on any available species of marine mammal.
The stomach of a killer whale found stranded near Prince William Sound contained a harbor seal, a harbor porpoise, and a Steller sea lion. Killer whales also have been observed to prey on river otters, squid, and several species of birds. Killer whales may briefly leave the water to grab seals and sea lions from the shore. Animals within a pod often feed cooperatively. When preying on large animals such as gray or humpback whales, the killer whales may attack as a pack, tearing away at the prey animal from several angles.