Ecotypic variation and predatory behavior among killer whales off the eastern Aleutian Islands, Alaska

Fishery Bulletin, Jan, 2007 by Craig O. Matkin, Lance G. Barrett-Lennard, Harald Yurk, David Ellifrit, Andrew W. Trites

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 groups (resident, transient, and offshore) were identified acoustically and genetically. Resident killer whales were found 12 times more frequently than transient killer whales, and offshore killer whales were encountered only 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, and the majority (70%) were encountered during spring (May and June). The diet of transient killer whales in spring was primarily gray whales (Eschrichtius robustus), and in summer primarily northern fur seals (Callorhinus ursinus). 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 does not consume marine mammals.

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In 1992, flipper tags from fourteen Steller sea lions (Eumetopias jubatus) were found in the stomach of a killer whale (Orcinus orca) that had died in Prince William Sound (Heise et al., 2003). This discovery prompted considerable interest and speculation about the role that killer whales may have played in the decline and lack of recovery of Steller sea lions in western Alaska (Barrett-Lennard et al., 1995). Since the late 1970s, Steller sea lions in the Gulf of Alaska and Aleutian Islands have declined by over 80% (Merrick et al., 1987; Trites and Larkin, 1996; Loughlin and York, 2000; Winship and Trites, 2006). Similar sharp declines have also occurred among some populations of harbor seals (Phoca vitulina), northern fur seals (Callorhinus ursinus), and sea otters (Enhydra lutris) (York, 1987; Pitcher, 1990; Trites, 1992; Estes et al., 1998). Whether or not these declines are related to killer whales is currently the subject of considerable scientific debate (Springer et al., 2003; Trites et al., 2006; DeMaster et al., 2006; Mizroch and Rice, 2006).

Most knowledge about killer whales in the North Pacific has been gathered between California and the northern Gulf of Alaska, 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 (Matkin et al., 1999a; Barrett-Lennard, 2000; Ford et al., 2000; Saulitis et al., 2000). These groups are genetically and behaviorally distinct, but have overlapping geographic ranges and are considered as ecotypes because of their differences in diet. However, prior to our study, it was not known whether these lineage and ecotype distinctions extended to the northwestern Gulf of Alaska and the Aleutian archipelago, nor was much known about the extent to which killer whales prey on Steller sea lions and other species of marine mammals in these regions.

The goals of our study were 1) to determine whether the eastern Aleutian Islands are also home to the three lineages and ecotypes of killer whales that have been identified elsewhere in the northeastern Pacific; 2) to derive estimates of killer whale numbers for this region; and 3) to document the behaviour of killer whales foraging on marine mammals. Obtaining such information about killer whale numbers, diets, and hunting behavior is critical for resolving the role that killer whales may have played in the decline and lack of recovery of Steller sea lions and other species of marine mammals in western Alaska.

Materials and methods

Following the research method used to collect information on killer whales in other regions (Matkin et al., 1999a; Ford et al., 2000), five types of data were gathered: photo-identification pictures of individuals and groups, acoustic recordings of killer whale calls, skin tissue samples for genetic analysis, prey samples, and accounts of predation. Additionally, we documented the presence of potential marine mammal prey. Analysis of these data allowed the determination of killer whale ecotypes and a description of killer whale feeding habits.

Field methods

Boat-based surveys over a wide geographic range occurred during June-September 2002-2004 from Unimak Pass to Samalga Pass, and surveys over a relatively small range occurred in May and early June 2003-2004 in the False Pass-Unimak Island region. Surveys in the broader region traversed 19,686 nautical miles and were focused in the Bering Sea within twenty miles of the shoreline between Unimak Pass and eastern Umnak Island (Fig. 1). The 2003-2004 False Pass-Unimak surveys traversed 1970 miles in Ikatan Bay and along the Pacific shore of Unimak Island. We operated in areas of suspected high killer whale density according to information provided by local fishermen and researchers from the National Marine Mammal Laboratory (NMML) (Dahlheim, 1997) during previous transect surveys that covered broader regions. We modified our surveys each season to cover the areas that were most productive in providing killer whale encounters.