abstract
We constructed ecosystem models using the Ecopath with Ecosim software to evaluate whether predation by killer whales might explain the decline of Steller sea lions since the late 1970s in the western Aleutian Islands. We also sought to understand why sea lions increased in the presence of killer whales in Southeast Alaska. Modeling results reproduced the time series of abundances for exploited species and sea lions in both ecosystems. Simulation results suggest that killer whale predation contributed to the decline of sea lions in the western Aleutians, but that predation was not the primary cause of the population decline. Predation could however have become a significant source of mortality during the 1990s when sea lions numbers were much lower. In Southeast Alaska, predation was also found to be a significant source of mortality in the 1960s when sea lions were low, but ceased to control population growth through the 1980s and 1990s. Overall, the ecosystem models suggest that large populations of Steller sea lions can withstand predation, but that small populations are vulnerable to killer whales.

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.

abstract
Populations of killer whales in southeastern Alaska overlap with populations inhabiting Prince William Sound, Alaska and British Columbia, Canada. We synthesize the results of a 20-year study in Glacier Bay and Icy Strait, Alaska. Individuals were photo-identified and predation events documented. Foraging strategies of killer whales were compared to those documented in similar studies in adjacent areas. One hundred twenty of the resident form of killer whales, 150 of the West Coast transients, 13 of the Gulf of Alaska transients and 14 of the offshore form were photo-identified in the study area. Residents preyed primarily on silver salmon and Pacific halibut. The prey of transients were harbor seals (40 percent), harbor porpoise(23 percent), Steller sea lions (16 percent), seabirds (14 percent), Dall’s porpoise (5 percent) and minke whale (2 percent). Humpback whales were observed closely approaching transient groups that were attacking other marine mammals. Nonpredatory interactions also occurred between killer whales and Steller sea lions.

abstract
The hypothesis that commercial whaling caused a sequential megafaunal collapse in the North Pacific Ocean by forcing killer whales to eat progressively smaller species of marine mammals is not supported by what is known about the biology of large whales, the ecology of killer whales and the patterns of ecosystem change that took place in Alaska, British Columbia, and elsewhere in the world following whaling. A comparative analysis shows that populations of seals, sea lions and sea otters increased in British Columbia following commercial whaling, unlike the declines noted in the Gulf of Alaska and Aleutian Islands. The declines of seals and sea lions that began in western Alaska around 1977 were mirrored by increases in numbers of these species in British Columbia. A more likely explanation is the seal and sea lion declines and other ecosystem changes in Alaska stems from a major oceanic regime shift that occurred in 1977. Killer whales are unquestionably a significant predator of seals, sea lions and sea otters but not because of commercial whaling.

abstract
The foraging ecology and population dynamics of harbor seals (Phoca vitulina richardsi) were studied in Hood Canal, Washington from 1998 to 2005. Initial work was conducted in response to concerns over the potential impact seals may have on recovering populations of summer chum salmon. Direct observation of harbor seals consuming salmon within the inter-tidal regions of four rivers in Hood Canal were conducted from 1998-2001 and 2003. Seals were observed feeding on chinook, coho, pink, summer chum and fall chum salmon. Estimates of summer chum consumption by seals at each of the observation sites averaged 8.0% of returning adults across all sites and all years. The maximum percentage of returning chum consumed was 27.7% and the lowest was 0.84%. The number of seals observed foraging in the river for salmon averaged from two to seven seals. Summer chum populations in the study streams have increased over the time of the study to near historical highs. Because of thi! s increase, the levels of predation observed are not believed to significantly impact the recovery of summer chum in Hood Canal. A protocol for extraction of DNA and identification of seal sex from scats was developed to examine differential diets between male and female harbor seals. Scats from both sexes contained similar levels of Pacific hake, but male scats contained more salmon and female scats contained more Pacific herring. In 2003 and 2005, mammal-eating killer whales foraged exclusively within Hood Canal for 59 and 172 days respectively. Bio-energetic models and boat based observations were used to estimate harbor seal consumption by killer whales and, in both years, the predicted consumption was approximately 950 seals. However, aerial surveys conducted following the two foraging events have not detected a significant decline in the harbor seal population.

abstract
The cost of vocal behaviour is usually expressed in energetic terms; however, many animals pay additional costs arising from predators or potential prey eavesdropping on their vocal communication. The northeastern Pacific is home to two distinct ecotypes of killer whales (Orcinus orca): resident killer whales feed on fish, a prey with poor hearing abilities, whereas transient killer whales hunt marine mammals, which 5 have sensitive underwater hearing at the frequencies of killer whale vocal communication. In this study, we investigated how the superior hearing ability of their prey has shaped the vocal behaviour of the transient ecotype. We recorded pulsed calls and the associated behavioural context of groups of transient and resident killer whales in British Columbia and southeastern Alaska. Transient killer whales emitted pulsed calls significantly less frequently than residents. Transient killer whales only exhibited significant amounts of vocal 10 behaviour after a marine mammal kill or when the whales where displaying surface-active behaviour. Vocal activity of transients increased after a successful attack on a marine mammal. Since marine mammals are able to detect killer whale pulsed calls and respond with anti-predator behaviour, the reduced vocal activity of transients is probably due to a greater cost for calling in this ecotype resulting from eavesdropping by potential prey. The increase in vocal behaviour after a successful attack may represent food calling (informing other animals in the area about the presence of food), but is more likely to reflect an increase in social interactions during feeding and/or the fact that the cost for vocal behaviour is comparatively low after a successful attack.

abstract
The discovery of flipper tags from 14 Steller sea lions (Eumetopias jubatus) in the stomach of a dead killer whale (Orcinus orca) in 1992 focused attention on the possible role of killer whale predation in the decline of Steller sea lions in western Alaska. In this study, mariners in British Columbia and Alaska were surveyed to determine the frequency and out-come of observed attacks on sea lions, the age classes of sea lions taken, and the areas where predatory attacks occurred. The 126 survey respondents described 492 killer whale/sea lion interactions, of which at least 32 were fatal attacks on the sea lion. The greatest rate of observed predation occurred in the Aleutian Islands. The stomach contents of dead and stranded whales also were examined. Stomachs that were not empty contained only fish or marine mammal remains, but not both. This supports earlier evidence of dietary segregation between fish-eating resident and marine mammal-eating transient killer whales in Alaska. Steller sea lion remains were found in two of 12 killer whale stomachs examined from Alaska between 1990 and 2001. Stomach contents fromtwo oVshore killer whales provided the first direct evidence that this third formof killer whale feeds on fish.

abstract
Steller sea lion populations in Alaska have declined precipitously over the last 25 years. Much research has been conducted on the role of anthropogenic factors in this decline. The retrieval of 14 sea lion flipper tags from a dead killer whale in 1992 underscored the need for a similar appraisal of predation. We used simulation models to examine (1) the extent to which killer whales contributed to the sea lion decline, and (2) the present effect of killer whale predation on depleted sea lion populations. We estimated the model parameters using three sources: a survey of researchers and mariners, the stomach contents of stranded killer whales, and killer whale identification photographs from several collections. The 126 survey respondents described 52 attacks including 32 reported kills. Eight out of 15 killer whale stomachs with identifiable contents contained marine mammals, and two contained Steller sea lion remains. The survey and stomach content data were consistent with earlier findings that only members of the transient killer whale population commonly prey on marine mammals. Based on identification photographs, we estimated that at least 250 transient killer whales feed in Alaskan waters. We ran Leslie matrix simulations under various assumptions concerning the functional responses of killer whales to changes in sea lion density. Our models suggest that killer whale predation did not cause the sea lion decline, but may now be a contributing factor. At present, approximately 18% of sea lions that die annually in Western Alaska may be taken by killer whales.