abstract
The endangered western stock of the Steller sea lion (Eumetopias jubatus) ˆ the largest of the eared seals ˆ has declined by 80% from population levels encountered four decades ago. Current overall trends from the Gulf of Alaska to the Aleutian Islands appear neutral with strong regional heterogeneities. A published inferential model has been used to hypothesize a continuous decline in natality and depressed juvenile survival during the height of the decline in the mid-late 1980‚s,followed by the recent recovery of juvenile survival to pre-decline rates. However, these hypotheses have not been tested by direct means, and causes underlying past and present population trajectories remain unresolved and controversial. We determined post-weaning juvenile survival and causes of mortality using data received post-mortem via satellite from telemetry transmitters implanted into 36 juvenile Steller sea lions from 2005 through 2011. Data show high post-weaning mortality by predation in the eastern Gulf of Alaska region. To evaluate the impact of such high levels of predation, we developed a conceptual framework to integrate density dependent with density independent effects on vital rates and population trajectories. Our data and model do not support the hypothesized recent recovery of juvenile survival rates and reduced natality. Instead, our data demonstrate continued low juvenile survival in the Prince William Sound and Kenai Fjords region of the Gulf of Alaska. Our results on contemporary predation rates combined with the density dependent conceptual framework suggest predation on juvenile sea lions as the largest impediment to recovery of the species in the eastern Gulf of Alaska region. The framework also highlights the necessity for demographic models based on age-structured census data to incorporate the differential impact of predation on multiple vital rates.

abstract
Public signaling plays an important role in territorial and sexual displays in animals; however, in certain situations, it is advantageous to keep signaling private to prevent eavesdropping by unintended receivers. In the northeastern Pacific, two populations of killer whales (Orcinus orca), fish-eating resident killer whales and mammal-eating transient killer whales, share the same habitat. Previous studies have shown that residents use whistles as private signals during close-range communication, where they probably serve to coordinate behavioral interactions. Here, we investigated the whistling behavior of mammal-eating killer whales, and, based on divergent social structures and social behaviors between residents and transients, we predicted to find differences in both whistle usage and whistle parameters. Our results show that, like resident killer whales, transients produce both variable and stereotyped whistles. However, clear differences in whistle parameters between ecotypes show that the whistle repertoire of mammal-eating killer whales is clearly distinct from and less complex than that of fish-eating killer whales. Furthermore, mammal-eating killer whales only produce whistles during milling after kill and surface-active behaviors, but are almost completely silent during all other activities. Nonetheless, whistles of transient killer whales may still serve a role similar to that of resident killer whales. Mammal-eating killer whales seem to be under strong selection to keep their communication private from potential prey (whose hearing ranges overlap with that of killer whales), and they appear to accomplish this mainly by restricting vocal activity rather than by changes in whistle parameters.

abstract
We used photographic mark-recapture methods to estimate the number of mammal-eating "transient" killer whales using the coastal waters from the central Gulf of Alaska to the central Aleutian Islands, around breeding rookeries of endangered Steller sea lions. We identified 154 individual killer whales from 6,489 photographs collected between July 2001 and August 2003. A Bayesian mixture model estimated seven distinct clusters (95% probability interval = 710) of individuals that were differentially covered by 14 boat-based surveys exhibiting varying degrees of association in space and time. Markov Chain Monte Carlo methods were used to sample identification probabilities across the distribution of clusters to estimate a total of 345 identified and undetected whales (95% probability interval = 255487). Estimates of covariance between surveys, in terms of their coverage of these clusters, indicated spatial population structure and seasonal movements from these near-shore waters, suggesting spatial and temporal variation in the predation pressure on coastal marine mammals.

abstract
Mammal-eating killer whales (Orcinus orca(L., 1758)) are a rare example of social predators that hunt together in groups of sexually dimorphic adults and juveniles with diverse physiological diving capacities. Dayˆnight ecological differences should also affect diving as their prey show diel variation in activity and mammal-eating killer whales do not rely on echolocation for prey detection. Our objective was to explore the extent to which physiological aerobic capacities versus ecological factors shape the diving behaviour of this breath-hold diver. We used suction-cup-attached depth recorders (Dtags) to record 7608 dives of 11 animals in southeast Alaska. Analysis of dive sequences revealed a strong bout structure in both dive depth and duration. Dayˆnight comparisons revealed reduced rates of deep dives, longer shallow dives, and shallower long-duration dives at night. In contrast, dive variables did not differ by ageˆsex class. Estimates of the aerobic dive limit (cADL) suggest that juveniles exceeded their cADL during as much as 15% of long dives, whereas adult males and females never exceeded their cADL. Mammal-eating killer whales in this area appear to employ a strategy of physiological compromise, with smaller group members diving nearer their physiological limits and large-bodied males scaling down their physiological performance.

abstract
Knowledge of the age distributions of killer whale Orcinus orca populations is critical to assess their status and long-term viability. Except for accessible, well-studied populations for which historical sighting data have been collected, currently there is no reliable benign method to determine the specific age of live animals for remote populations. To fill this gap in our knowledge of age structure, we describe new methods by which age can be deduced from measurements of specific lipids, endogenous fatty acids (FAs) and FA ratios present in their outer blubber layers. Whereas correlation of wax and sterol esters with age was reasonable for female ‘resident’ killer whales, it was less well-defined for males and ‘transients.’ Individual short-, branched-, and odd-chain FAs correlated better with age for transients and residents of both sexes, but these single parameter relationships were population specific and seemingly varied with long-term diet. Alternatively, a simple,empirical multi-linear model derived from the combination of 2 specific FA ratios enabled the ages of individual eastern North Pacific killer whales to be predicted with good precision (σ = ±3.8 yr), appeared to be independent of individual diet and was applicable to both genders and ecotypes. The model was applied to several less well-studied killer whale populations to predict their age distributions from their blubber FA compositions, and these distributions were compared with a population of known age structure. Most interestingly, these results provide evidence for the first time that adult male transient killer whales appear to have lower life expectancies than do their resident counterparts in Alaska.

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
Top predators in the marine environment integrate chemical signals acquired from their prey that reflect both the species consumed and the regions from which the prey were taken. These chemical tracers˜stable isotope ratios of carbon and nitrogen; persistent organic pollutant (POP) concentrations, patterns and ratios; and fatty acid profiles˜were measured in blubber biopsy samples from North Pacific killer whales (Orcinus orca) (n = 84) and were used to provide further insight into their diet, particularly for the offshore group, about which little dietary information is available. The offshore killer whales were shown to consume prey species that were distinctly different from those of sympatric resident and transient killer whales. In addition, it was confirmed that the offshores forage as far south as California. Thus, these results provide evidence that the offshores belong to a third killer whale ecotype. Resident killer whale populations showed a gradient in stable isotope profiles from west (central Aleutians) to east (Gulf of Alaska) that, in part, can be attributed to a shift from off-shelf to continental shelf-based prey. Finally, stable isotope ratio results, supported by field observations, showed that the diet in spring and summer of eastern Aleutian Island transient killer whales is apparently not composed exclusively of Steller sea lions.

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
Great whale populations are recovering in the North Pacific, perhaps even the right whale stock. As population numbers grow, so too will the roles they play in the ecosystem. Whether food webs and communities return to their former condition remains to be seen, as much has changed in the intervening years. The mean climate state over the northern North Pacific has undergone three major shifts since the end of the modern whaling era, and pinniped and sea otter populations throughout the Aleutian Islands and western Gulf of Alaska have collapsed. The fundamental rules governing rates and pathways of energy exchange in the ocean are likely still the same, but the constraints are certainly different now than they were in the hierarchy of the mature ecosystem 50 to150 years ago. Attention should be focused now on ways to improve our understanding of top-down oceanography (predator-prey interactions at all trophic levels, particularly high levels); how marine community structure and dynamics are influenced by those processes; and how ecosystems in their dramatically altered condition today behave in response to environmental change.

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
Springer et al. [Springer, A.M., Estes, J.A., van Vliet, G.B., Williams, T.M., Doak, D.F., Danner, E.M., Forney, K.A., Pfister, B., 2003. Sequential megafaunal collapse in the North Pacific Ocean: an ongoing legacy of industrial whaling? Proceedings of the National Academy of Sciences 100 (21), 12,223–12,228] hypothesized that great whales were an important prey resource for killer whales, and that the removal of fin and sperm whales by commercial whaling in the region of the Bering Sea/Aleutian Islands (BSAI) in the late 1960s and 1970s led to cascading trophic interactions that caused the sequential decline of populations of harbor seal, northern fur seal, Steller sea lion and northern sea otter. This hypothesis, referred to as the Sequential Megafaunal Collapse (SMC), has stirred considerable interest because of its implication for ecosystem-based management. The SMC has the following assumptions: (1) fin whales and sperm whales were important as prey species in the Bering Sea; (2) the biomass of all large whale species (i.e., North Pacific right, fin, humpback, gray, sperm, minke and bowhead whales) was in decline in the Bering Sea in the 1960s and early 1970s; and (3) pinniped declines in the 1970s and 1980s were sequential. We concluded that the available data are not consistent with the first two assumptions of the SMC. Statistical tests of the timing of the declines do not support the assumption that pinniped declines were sequential. We propose two alternative hypotheses for the declines that are more consistent with the available data. While it is plausible, from energetic arguments, for predation by killer whales to have been an important factor in the declines of one or more of the three populations of pinnipeds and the sea otter population in the BSAI region over the last 30 years, we hypothesize that the declines in pinniped populations in the BSAI can best be understood by invoking a multiple factor hypothesis that includes both bottom–up forcing (as indicated by evidence of nutritional stress in the western Steller sea lion population) and top–down forcing (e.g., predation by killer whales, mortality incidental to commercial fishing, directed harvests). Our second hypothesis is a modification of the top–down forcing mechanism (i.e., killer whale predation on one or more of the pinniped populations and the sea otter population is mediated via the recovery of the eastern North Pacific population of the gray whale). We remain skeptical about the proposed link between commercial whaling on fin and sperm whales, which ended in the mid-1960s, and the observed decline of populations of northern fur seal, harbor seal, and Steller sea lion some 15 years later.

abstract
Steller sea lions (Eumetopias jubatus) increased in the eastern portion of their range while declining in the Gulf of Alaska and Aleutian Islands from the late 1970s to late 1990s. We constructed ecosystem models of the central and western Aleutians and of Southeast Alaska to simultaneously evaluate four hypotheses explaining sea lion dynamics: killer whale (Orcinus orca) predation, ocean productivity, fisheries, and competition with other species. Comparisons of model predictions to historical time series data indicate that all four factors likely contributed to the trends observed in sea lion numbers in both ecosystems. Changes in ocean productivity conveyed by the Pacific Decadal Oscillation influenced the abundance trajectory of several species. Fishing could have affected the ecosystem structure by influencing the abundance of Atka mackerel (Pleurogrammus monopterygius) in the Aleutians, and herring (Clupea pallasii) in Southeast Alaska. Halibut (Hypoglossus stenolepis) in the Aleutians and arrowtooth flounder (Reinhardtius stomias) in Southeast Alaska appear to impede sea lion population growth through competitive interactions. Predation by killer whales was important when sea lions were less abundant in the 1990s in the Aleutians and in the 1960s in Southeast Alaska, but appear to have little effect when sea lion numbers were high.

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
Populations of seals, sea lions, and sea otters have sequentially collapsed over large areas of the northern North Pacific Ocean and southern Bering Sea during the last several decades. A bottom-up nutritional limitation mechanism induced by physical oceano-graphic change or competition with fisheries was long thought to be largely responsible for these declines. The current weight of evidence is more consistent with top-down forcing. Increased predation by killer whales probably drove the sea otter collapse and may have been responsible for the earlier pinniped declines as well. We propose that decimation of the great whales by post-World War II industrial whaling caused the great whales’ foremost natural predators, killer whales, to begin feeding more intensively on the smaller marine mammals, thus ‘‘fishing-down’’ this element of the marine food web. The timing of these events, information on the abundance, diet, and foraging behavior of both predators and prey, and feasibility analyses based on demographic and energetic modeling are all consistent with this hypothesis. food web dynamics brought about by human overharvesting initiated the change.

abstract
Johnstone Strait provides important summer habitat for the northern resident killer whales Orcinus orca of British Columbia. The site is also an active whale-watching area. A voluntary code of conduct requests that boats do not approach whales closer than 100 m to address perceived, rather than demonstrated, effects of boat traffic on killer whales. The purpose of the study was to test the relevance of this distance guideline. Relationships between boat traffic and whale behaviour were studied in 1995 and 1996 by shore-based the odolite tracking of 25 identifiable focal animals from the population of 209 whales. Individual killer whales were repeatedly tracked in the absence of boats and during approaches by a 5.2 m motorboat that paralleled each whale at 100 m. In addition, whales were tracked opportunistically, when no effort was made to manipulate boat traffc. Dive times, swim speeds, and surface-active behaviours such as breaching and spy-hopping were recorded. On average, male killer whales swam significantly faster than females. Whales responded to experimental approaches by adopting a less predictable path than observed during the preceding, no-boat period, although males and females used subtly different avoidance tactics. Females responded by swimming faster and increasing the angle between successive dives, whereas males maintained their speed and chose a smooth, but less direct, path. Canonical correlations between whale behaviour and vessel proximity are consistent with these conclusions, which suggest that weakening whale-watching guidelines, or not enforcing them, would result in higher levels of disturbance. High variability in whale behaviour underscores the importance of large sample size and extensive experimentation when assessing the impacts of human activity on killer whales.

abstract
Generalized survival models were applied to growth curves published for 17 species of cetaceans (5 mysticetes, 12 odontocetes) and 13 species of pinnipeds (1 odobenid, 4 otariids, 8 phocids). The mean mass of all individuals in the population was calculated and plotted against the maximum body length reported for each species. The data showed strong linearity (on logarithmic scales), with three distinct clusters of points corresponding to the mysticetes (baleen whales), odontocetes (toothed whales), and pinnipeds (seals, sea lions, and walruses). Exceptions to this pattern were the sperm whales, which appeared to be more closely related to the mysticetes than to the odontocetes. Regression equations were applied to the maximum lengths reported for 76 species of marine mammals without published growth curves. Estimates of mean body mass were thus derived for 106 living species of marine mammals.

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.