Whales of the Wild West

In 1992, a lone killer whale carcass washed ashore in Alaska’s Prince William Sound. Inside its stomach, scientists discovered no less than 14 flipper tags that had come from Steller sea lions—a surprising find that prompted researchers to do some ruminating of their own: Could killer whales have caused or contributed to the alarming decline and lack of recovery in western Alaska’s Steller sea lions?

Even today, relatively little is known about the diet and behavior of killer whales in the northwestern Gulf of Alaska and the Aleutian archipelago, but a new study proposes to change this. Data collected by Craig Matkin (of the North Gulf Oceanic Society), Dr. Lance Barrett-Lennard and Harald Yurk (Vancouver Aquarium), David Ellifrit (Center for Whale Research) and Dr. Andrew Trites (University of British Columbia) presents a fascinating glimpse into the lives of western Alaska’s killer whales.

Exploring Ecotypes

“Most knowledge about killer whales in the North Pacific has been gathered between California and the northern Gulf of Alaska,” the authors write, “where three distinct lineages of killer whales have been identified: fish-eating ‘resident’ killer whales, which appear predictably in large groups from Washington to Alaska, marine mammal-eating ‘transient’ killer whales, which appear infrequently and in smaller groups, and ‘offshore’ killer whales whose feeding habits are poorly known, but are thought to eat fish including sharks.”

Although the three varieties of killer whales occur in overlapping ranges, they do not intermingle; each has its own peculiar diet, dialect and genetic lineage. For these reasons, scientists classify them as ecotypes, or behaviorally distinct populations of the same species.

From 2001 to 2004 the research team spent a total of 421 days on the water, with a view to determining whether these ecotypes extended into western Alaska. Each spring and summer they surveyed the waters of the Eastern Aleutians and the False Pass/Unimak Island area at the end of the Alaskan peninsula, covering some 22,491 nautical miles. They collected five types of data: dorsal-fin photographs to identify individuals and groups, acoustic recordings of underwater vocalizations, skin tissue samples for genetic analysis, prey samples and accounts of predation.

Applying these techniques over the course of 250 encounters with killer whales, the researchers positively identified at least 903 fish-eating ‘resident’ killer whales, although the rate at which they discovered new  individuals suggests this population is likely even larger. Notably, these killer whales appear to be distinct from populations in other regions of the North Pacific, such as those off the coast of British Columbia and Washington. The research team also identified 165 transient killer whales but only once encountered the rare offshore ecotype.

Dietary Dilemma

Through acoustic and genetic sampling, the researchers could unequivocally categorize each killer whale as resident, transient or offshore; demonstrating that the three known ecotypes did indeed extend into the eastern Aleutian Islands. As in other parts of the North Pacific, the individuals from each ecotype showed no signs of intermingling or interbreeding with other ecotypes. Not only did these results help to estimate a minimum population base for each ecotype in this area, they also provided some critical behavioral and dietary information.

“The majority of killer whales in the Eastern Aleutians are the resident ecotype, which do not consume marine mammals and should not impact Steller sea lion or other marine mammal populations except as competitors for fish,” the authors note. “The diet of transient killer whales in the Unimak Islands/False Pass region in spring was primarily gray whales while northern fur seals were primary prey west of Unimak Island in summer. Steller sea lions did not appear to be a preferred prey or major prey item during spring and summer.”

Based on this and other research, a curiously fragmented picture emerges of Alaska’s transient killer whales—one which defies any generalizations about their behavior or diet. In the Eastern Aleutian Islands, gray whales and northern fur seals are important seasonal prey. But further to the east, transients in Prince William Sound and Kenai Fjords hunt harbor seals and Dall’s porpoises, while those in the Glacier Bay/Icy Strait vicinity of southeast Alaska appear to favor harbor porpoises and harbor seals (the latter are also a favorite prey of British Columbia’s transient killer whales). Only in Kenai Fjords and along Kodiak Island have any transients demonstrated a preference for sea lions.

Was the killer whale that washed up in Prince William Sound with a belly full of flipper tags simply an anomaly, one of only a handful of sea lion-hunting specialists? Or do transient killer whales truly have the potential to impact populations of sea lions and other marine mammals? Much remains to be seen. But equipped with a growing understanding of western Alaska’s killer whales, scientists will continue their efforts to grasp the nature of these puzzling predators—from season to season and region to region.

Publication:
Ecotypic variation and predatory behavior among killer whales (Orcinus orca) off the eastern Aleutian Islands, Alaska. Matkin, C., L.G. Barrett-Lennard, H.Yurk, D. Ellifrit, and A.W. Trites. 2007. Fishery Bulletin 105:74-87. abstract From 2001 to 2004 in the eastern Aleutian Islands, Alaska, killer whales (Orcinus orca) were encountered 250 times during 421 days of surveys that covered a total of 22,491 miles. Three killer whale lineages (resident, transient, and offshore) were identified acoustically and genetically. Resident killer whales were found 12 times more frequently than transient killer whales, while offshore killer whales were only encountered once. A minimum of 901 photographically-identified resident whales used the region during our study. A total of 165 mammal-eating transient killer whales were identified, with the majority (70%) encountered during spring (May and June). The diet of transient killer whales in spring was primarily gray whales (Eschrichtius robustus), while northern fur seals (Callorhinus ursinus) were primary prey in summer. Steller sea lions (Eumetopias jubatus) did not appear to be a preferred prey or major prey item during spring and summer. The majority of killer whales in the eastern Aleutian Islands are the resident ecotype, which do not consume marine mammals.

29 May 2007