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
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
Conventional and electronic tags were used to investigate social segregation, distribution, movements and migrations of salmon sharks Lamna ditropis in Prince William Sound, Alaska. Sixteen salmon sharks were tagged with satellite transmitters and 246 with conventional tags following capture, and were then released in Prince William Sound during summer 1999 to 2001. Most salmon sharks sexed during the study were female (95%), suggesting a high degree of sexual segregation in the region. Salmon sharks congregated at adult Pacific salmon Oncorhynchus spp. migration routes and in bays near Pacific salmon spawning grounds in Prince William Sound during July and August. Adult Pacific salmon were the principal prey in 51 salmon shark stomachs collected during summer months in Prince William Sound, but the fish appeared to be opportunistic predators and consumed sablefish Anoplopoma fimbria, gadids, Pacific herring Clupea pallasi, rockfish Sebastes spp. and squid (Teuthoi dea) even when adult Pacific salmon were locally abundant. As Pacific salmon migrations declined in late summer, the salmon sharks dispersed; some continued to forage in Prince William Sound and the Gulf of Alaska into autumn and winter months, while others rapidly moved south-east thousands of kilometres toward the west coasts of Canada and the U.S. Three movement modes are proposed to explain the movement patterns observed in the Gulf of Alaska and eastern North Pacific Ocean: ‘focal foraging’ movements, ‘foraging dispersals’ and ‘direct migrations’. Patterns of salmon shark movement are possibly explained by spatio-temporal changes in prey quality and density, an energetic trade-off between prey availability and water temperature, intra-specific competition for food and reproductive success. Transmissions from the electronic tags also provided data on depth and water temperatures experienced by the salmon sharks. The fish ranged from the surface to a depth of 668 m, encountered water temperatures from 4.0 to 16.8 C and generally spent the most time above 40 m depth and between 6 and 14 C (60 and 73%, respectively).

abstract
Ecosystem models were constructed for the Antarctic and the Bering Sea that incorporate current understanding of biological interactions of species within the ecosystem (i.e., who eats whom and how much). Within the limitations that are inherent to simulations, both models suggest that removal of large whales had little measurable effect on lower trophic levels or on the dynamics of other species in their polar ecosystems. Trophic interactions failed to explain the magnitude of changes in the biomass of the major species groups in the Antarctic and Bering Sea. Nor did fin-fisheries appear to have had a significant effect on the abundance of non-targeted species. This may mean that environmental effects (which were not modeled) play an important role in influencing the dynamics of marine ecosystems. Oceanographic factors such as changes in water temperature or ocean currents likely result in variations in ecosystem production and species recruitment patterns which are not captured by our Ecopath models. The Ecopath modeling approach is a powerful means of synthesizing knowledge about ecosystems and the factors that influence ecosystem dynamics. They provide a straightforward means for estimating trophic levels and niche overlaps with other species to assess the potential for resource competition. While the models failed to support the hypotheses that large whales play a significant structural role in the Antarctic and Bering Sea ecosystems, they do support what most already know ?- i.e., that populations of large whales are easily reduced to low numbers, but take a long, long time to recover. They also help in recognizing the need to consider factors other than food web interactions when assessing the status of cetaceans, as well as highlighting the potential tradeoffs that can result when other species are removed from ecosystems.

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
The western stock of Steller sea lions has declined from over 140,000 individuals in the 1960s to possibly fewer than 40,000 individuals in 2000. The primary hypotheses put forth by the National Marine Fisheries Service (NMFS) explaining this decline centers around food limitation.One alternative hypothesis that has recently received attention is that the decline or lack of recovery is due to the effects of predation by killer whales or sharks.Reports of large numbers of killer whales surrounding longline and trawl fishing vessels in western Alaska suggest that there are many killer whales in the region. In order to assess the impact of killer whale predation on this popula- tion decline,we need the following information:

1.Number of Steller sea lions.

2.Intrinsic growth rate of Steller sea lion population.

3.Number of killer whales that prey on Steller sea lions.

4.Percentage of the killer whale diet that consists of Steller sea lions and age class of sea lion that is consumed.

keywords     Predation

abstract
Marine mammal predator-prey interactions occur over different spatial and temporal scales, making it difficult to empirically decipher the influences they have on one another and on their ecosystems. However, their coexistence suggests that marine mammal predators and their prey have had profound influences on each other’s behaviors, physiologies, morphologies, and life history strategies. The diversity of niches filled by marine mammals makes if difficult to generalize about the evolutionary consequences of their interactions with prey, beyond stating the obvious: marine mammals have adapted to catch food, while their prey have adapted to avoid being caught. On the shorter ecological time scale, marine mammals can affect the abundance of other species by consuming or out-competing them. They can also indirectly affect the abundance of nontargeted species by consuming one of their predators, and can have strong impacts on the overall dynamics and structure of their ecosystems. One of the best tools for understanding marine mammal predator-prey interactions is the ecosystem model. However, more work is required through experimental manipulations and observational studies to evaluate the choices made by marine mammals and the costs of obtaining different species of prey.

keywords     predation

abstract
Calculating trophic levels is necessary first step to quantifying and understanding trophic interactions between marine mammals and other species in marine ecosystems. This can be achieved using dietary information collected from stomachs and scats, or by measuring isotopic ratios contained in marine mammal tissues. These data indicate that marine mammals occupy a wide range of trophic levels beginning with dugong and manatees (trophic level 2.0), and followed by baleen whales (3.35), sea otters (3.45), seals (3.95), sea lions and fur seals (4.03), toothed whales (4.23), and polar bears (4.08). With the aid of ecosystem models and other quantitative analyses, the degree of competition can be quantified, and the consequences of changing predator-prey numbers can be predicted. These analyses show that many species of fish are major competitors of marine mammals. A number of field studies have also shown negative effects of reduced prey abundance on body size and survival of marine mammals. However, there are fewer examples of marine mammal populations affecting their prey due perhaps to the difficulty of monitoring such interactions, or to the complexity of most marine mammal food webs.

keywords     PhdTLmarine mammalsdietbackground

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.