82 



Fishery Bulletin 105(1) 



size as an indication of the relative availability of poten- 

 tial prey (Table 5). During the 346 survey days in the 

 eastern Aleutians in June-September 2002-2004, the 

 largest number of pinniped sightings was of northern fur 

 seals (375 sightings), which were frequently encountered 

 as single individuals resting at the surface, or were 

 observed on the rookery at Bogoslof Island. Steller sea 

 lions (124 sightings) were counted during our repeated 

 monitoring of rookeries and haulouts in the region, 

 resulting in a relatively large average group size (53.4 

 sea lions) but were encountered only occasionally off 

 the rookeries. Other pinnipeds included harbor seals 

 (73 sightings), which were primarily observed hauled 

 out in small groups of less than 30 individuals, and two 

 sightings of individual California sea lions (Zalophus 

 californianus). Sea otters were observed on 81 occa- 

 sions. The most frequently encountered cetacean was 

 the humpback whale (Megaptera novaeangliae; 834 sight- 

 ings), followed by Ball's porpoise (Phocoenoides dalli; 521 

 sightings). Less frequently observed species were minke 

 whales (Baleanoptera acutorostrata; 42 sightings), fin 

 whales (Baleanoptera physalus; 15 sightings), sperm 

 whales (Physeter macrocephalus; 7 sightings), and Bairds 

 beaked whales (Berardius bairdii; 10 sightings). 



During 49 survey days in the False Pass-Unimak 

 Island region in May and early June 2003-2004, Steller 

 sea lions were the most frequently ncountered pinniped 

 (29 sightings), although some fur seals (13 sightings) 

 and harbor seals (9 sightings) were also observed. Sea 

 otters were also present (13 sightings). The most fre- 

 quently encountered cetaceans were gray whales (18 

 sightings) and harbor porpoises (Phocoena phocoena; 



18 sightings). Other cetaceans sighted in this region 

 were minke whales (3 sightings) and humpback whales 

 (1 sighting). 



Predation on marine mammals 



Predation on or harassment of marine mammals was 

 observed on 31 occasions and was attributed solely to the 

 transient killer whale ecotype. Attacks that did not yield 

 proof of a kill (i.e., tissue, blood, or prey in the mouth) 

 were considered harassments. Gray whales (Eschrichtius 

 robustus) were the most frequently taken species (Table 

 5), with 19 apparent harassments, of which 18 resulted 

 in kills, observed in False Pass in 2003 and 2004 (May 

 to early June). In all but one case, the gray whale was 

 already dead and was being consumed when we found 

 the whale. The only other predatory event during this 

 period was a single harassment of Steller sea lions that 

 were hauled out. During the summer season in the east- 

 ern Aleutians (Unimak Pass-Umnak Island), northern 

 fur seals were the most frequently harassed prey; four 

 of the five observed harassments resulted in kills. Other 

 species included Dall's porpoises (harassed on three 

 different occasions), two minke whales (harassed and 

 killed), one Steller sea lion (harassed and killed), and 

 one humpback whale (harassed). 



Discussion 



Our analysis indicates that the three killer whale eco- 

 types in the eastern North Pacific also are found in the 



