84 



Fishery Bulletin 105(1) 



als in the spring, indicates that a smaller percentage 

 of the summer whales have been identified. A survey 

 of nearshore waters from the Gulf of Alaska to the 

 Aleutian Islands revealed that the highest densities of 

 transient killer whales were from the Shumagin Islands 

 through the eastern Aleutian Islands, and an estimated 

 abundance of 226 (CV=0.45) transient killer whales 

 were present west of the Shumagin Islands in summer 

 (Zerbini et al., 2006). 



Transient killer whales near False Pass in May were 

 concentrated on the Pacific Ocean side of Unimak Is- 

 land and in Ikatan Bay, where gray whales pass along 

 a shallow shelf and water depth rarely exceeds 70 me- 

 ters (Fig. 1). Reports from mariners and pilots have 

 indicated that other areas around Unimak Island and 

 along the Bering Sea coast (e.g.. Cape Lutke and the 

 coastline near Nelson Lagoon) may also be points of 

 interception of gray whales by killer whales. Our own 

 surveys (Fig. 2B) indicate that transient killer whales 

 inhabit a wide area around Unimak Island and the 

 tip of the Alaska Peninsula, where we recorded kills 

 of gray whales at Deer Island (110 km northeast of 

 False Pass) and Cape Lutke (140 km southwest of False 

 Pass). 



Despite uncertainties regarding the range of tran- 

 sient killer whales, it is evident that they are numerous, 

 concentrated, and consistently present in the spring 

 from Unimak Pass eastward. Gray whales have been 

 previously reported as killer whale prey (Matkin and 

 Saulitis, 1994); however, the extent to which transient 

 killer whales were focused on gray whale predation 

 during May-June around Unimak Island has not been 

 previously described. Although subsequent surveys in 

 these areas during summer (C. O. Matkin, unpubl. 

 data; Durban'-.) have identified some of the same whales 

 as those identified in the spring, most of the whales do 

 not remain in these nearshore waters. It is not known 

 whether these transient whales move offshore and dis- 

 perse, follow the gray whales into the Bering Sea, or 

 move into other unstudied regions. 



The distribution of most, if not all, transient killer 

 whales that we identified undoubtedly extends well 

 beyond our survey area. Technical advances in satellite 

 and radio tagging procedures that could be applied to 

 killer whales would aid considerably in understanding 

 the movements and range of transients in this region. 

 Without a better understanding of the range of these 

 whales, it is impossible to fully assess their impact on 

 prey populations. 



Northern fur seals appear to be an important prey 

 for killer whales from late June to September west 

 of Unimak Pass. This finding is based on observed 

 kills compared to kills of other species. A substantial 

 number of the fur seals sighted in summer were likely 

 associated with the recently established and expanding 

 fur seal rookery on Bogoslof Island. This population 

 increased rapidly from 898 pups in 1992 to 5096 pups 

 in 1999 (Angliss and Lodge, 2004). Additionally, peak 

 numbers of migrating fur seals pass through Unimak 

 Pass into the Bering Sea in June on their way to the 



Pribilof Islands and then migrate back to the Pacific in 

 peak numbers during October-November (Bigg, 1990). 



Northern fur seals have long been indicated as an 

 important prey for killer whales in the Pribilof Region 

 (Hanna, 1923; Zenkovich, 1938; Tomilin, 1957). How- 

 ever, their importance as prey in the eastern Aleutians 

 has not been previously documented and may have de- 

 veloped with the growth of the Bogoslof rookery. This 

 geographic region presents an opportunity to examine 

 the effects of killer whale predation on an apparently 

 stable or increasing population of fur seals — a species 

 that is declining in other areas. 



Minke whales made up a substantial proportion of 

 summer predation despite the relative low frequency 

 with which they were sighted, and the apparent dif- 

 ficulty that killer whales have in capturing this fast 

 swimming species in open water (Ford et al., 2005). 

 Minke whales appear to be a minor part of the diet of 

 killer whales from Washington State to northern south- 

 eastern Alaska (Ford et al., 2005). 



We observed the harassment of a humpback whale by 

 killer whales once; during the attack, other humpback 

 whales rapidly converged on the attackers and appeared 

 to drive the killer whales away. No injuries were appar- 

 ent. Harassments of humpbacks have been reported in 

 other regions of Alaska (Saulitis et al., 2000), but did 

 not result in a kill or apparent injury. Photographs of 

 scars indicate that most killer whale attacks on baleen 

 whales target young animals, probably calves on their 

 first migration from low-latitude breeding and calving 

 areas to high-latitude feeding grounds (Mehta^). 



Although none of the attacks that we observed on 

 Dall's porpoises resulted in confirmed kills, Dall's por- 

 poises could be a significant prey as has been indicated 

 in other regions (Ford et al., 1998; Saulitis et al., 2000); 

 however, more observations are needed. Harbor seals 

 were conspicuously absent from our prey observations 

 despite also being an important prey in other regions 

 of Alaska (Saulitis et al., 2000; Matkin et al., in press). 

 Harbor seals are found in relatively low numbers in the 

 eastern Aleutians. 



Although Steller sea lions were observed as prey on 

 one occasion and were harassed on another, they did 

 not appear to be a primary target of the transient killer 

 whales we observed during our spring and summer 

 surveys. Whether or not they are an important prey 

 during other seasons (fall and winter) is not known and 

 will require additional study or the application of other 

 methods to be fully assessed. 



Although our study was limited by a small sample 

 size in the summer, it provided significant information 

 on the distribution of transient killer whale prey and 

 the importance of fur seals in the killer whale diet dur- 

 ing summer west of Unimak Pass and the importance 

 of grey whales in killer whale diet during spring from 

 Unimak Pass east. An increase in sample size of ob- 



^ Mehta, A. 2005. Unpubl. data. Woods Hole Oceanographic 

 Institute, Woods Hole, MA 02543-1050. 



