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Background Pacific walruses (Odobenus rosmarus divergens) are a conspicuous and important component of the Bristol Bay ecosystem and human social systems, but very little is known about walrus ecology in this region, principally their feeding ecology. The present work provides contemporary data on the diets of walruses at four haulout locations throughout Bristol Bay between 2014 and 2018. Methods We analyzed scat and gastrointestinal tract samples from these animals using quantitative polymerase chain reaction to amplify prey DNA, which allowed for diet estimates based on frequencies of prey item occurrence and on the relative importance of dietary items as determined from DNA threshold cycle scores. Results Diets were highly diverse at all locations, but with some variation in composition that may be related to the time of year that samples were collected (summer vs. autumn), or to spatial variability in the distribution of prey. Overall, polychaetes and tunicates had the highest frequencies of occurrence and relative abundances in 2014–15, but a major change in diet appears to have occurred by 2017–18. While some sample sizes were small, diets in these later years contrasted sharply, with a greater prevalence of sea cucumbers and mollusks, and reduced importance of decapods and fishes compared to the earlier years. Prey identified in scat samples from one collection site also contrasted sharply with those reported from the same location in 1981. The apparent temporal shifts in walrus prey may represent a changing benthic ecosystem due to warming waters in recent decades.

Hideo Omura's concern over the killing of fin whales in the early 1950s, just a few years after the resumption of whaling following World War II, foreshadowed the demise of most of the remaining great whales in the following two decades as the slaughter expanded across the North Pacific (Springer et al. 2003). His concern was warranted because of the development of an unprecedented human capability for large-scale harvests of even the fastest whales using high-speed catcher boats and mechanized factory ships designed specifically for this purpose. In 1955, the nominal harvest of fin whales was about 2,100, and it doubled over the next 10 years to a peak of some 4,000 in 1965 before declining through 1975 when the harvest was ended. Other species were similarly exploited and depleted until pelagic hunting was successively halted for humpback and blue whales in 1965, for sei whales in 1975, and for sperm and Bryde's whales in 1979. Populations of most species were severely depleted by the mid-1980s (Stewart et al. 1987; Rice and Wolman 1982). Exceptions were humpback whales in the northeastern Pacific, which had begun to recover slowly after protection was enacted in 1965 (Calambokidis et al. 2001), and bowhead whales and gray whales that had been ravaged 100 years earlier (Raferty et al. 1995; Rugh et al. 1999). Reports of larger numbers of some species in the southeastern Bering Sea in the late 1980s in comparison with the midto late 1970s (Baretta and Hunt 1994) are difficult to interpret because changes in distribution rather than abundance could explain the difference in 'numbers between decades (Tynan 2004). Fin whales were possibly showing signs of slow recovery by the late 1990s in the northern Gulf of Alaska and Bering Sea, although sperm whales remained scarce (Moore et al. 2000; Tynan 2004). "

Pink salmon (Oncorhynchus gorbuscha) in the North Pacific Ocean have flourished since the 1970s, with growth in wild populations augmented by rising hatchery production. As their abundance has grown, so too has evidence that they are having important effects on other species and on ocean ecosystems. In alternating years of high abundance, they can initiate pelagic trophic cascades in the northern North Pacific Ocean and Bering Sea and depress the availability of common prey resources of other species of salmon, resident seabirds, and other pelagic species. We now propose that the geographic scale of ecosystem disservices of pink salmon is far greater due to a 15,000-kilometer transhemispheric teleconnection in a Pacific Ocean macrosystem maintained by short-tailed shearwaters (Ardenna tenuirostris), seabirds that migrate annually between their nesting grounds in the South Pacific Ocean and wintering grounds in the North Pacific Ocean. Over this century, the frequency and magnitude of mass mortalities of shearwaters as they arrive in Australia, and their abundance and productivity, have been related to the abundance of pink salmon. This has influenced human social, economic, and cultural traditions there, and has the potential to alter the role shearwaters play in insular terrestrial ecology. We can view the unique biennial pulses of pink salmon as a large, replicated, natural experiment that offers basin-scale opportunities to better learn how these ecosystems function. By exploring trophic interaction chains driven by pink salmon, we may achieve a deeper conservation conscientiousness for these northern open oceans.

We analyzed the fatty acid (FA) composition and carbon stable isotope ratios of indi- vidual FAs (δ 13 CFA) of 3 zooplankton species (Themisto libellula, Calanus marshallae/glacialis, and Thysanoessa raschii) sampled from the Bering Sea during winter maximum ice extent, spring ice melt, and summer ice-free conditions in 2009 and 2010. Our goal was to assess diets of these eco- logically important species and estimate the proportional contribution of pelagic and sympagic carbon sources to their diets. FA profiles showed little variation in diet within species between ice conditions or years but revealed differences in diet among species. FA biomarkers confirmed that T. libellula was predominately carnivorous and that C. marshallae/glacialis and T. raschii were primarily herbivorous. Estimates from 4 stable isotope mixing models using combinations of δ 13 CFA values of diatom FA markers (16:1n-7, 20:5n-3), and a flagellate FA marker (22:6n-3) showed that substantial, albeit highly variable, proportions of these FAs originated from organic matter originating from sea ice algae (T. libellula 36 to 72%, C. marshallae/glacialis 27 to 63%, and T. raschii 39 to 71%). Our results suggest that ice algae may be an important food source for zooplankton when water column phytoplankton are not available during critical periods in their life history. Predicted increases in water column phytoplankton production in the Bering Sea may help offset the expected reduction in ice algal production and any detrimental effects that this might have on consumers such as zooplankton.

TheSequentialMegafaunalCollapseHypothesis(SMCH)positsthatdecimationofgreat whales in the North Pacific Ocean in the 1950s and 1960s, by Russia and Japanfollowing the end of World War II, removed an important source of prey for marinemammal eating killer whales in southwest Alaska (the Aleutian Islands, southernBering Sea, and western and central Gulf of Alaska). The killer whales subsequentlybroadened their diets to include a larger proportion of other, much smaller species—harbor seals, Steller sea lions, fur seals, and sea otters—driving them into steepdecline. In the preceding letter, Wade, Ver Hoef, and DeMaster present informationand analyses that purportedly refute the SMCH. But, as explained below, we takeexception to their analytical procedures, use of data, and the strong claims they haveemployed in their continuing attempts to discredit the hypothesis.

Four species of pagophilic phocid seals in the Western Arctic—bearded (Erignathus barbatus), ringed (Phoca hispida), ribbon (Phoca fasciata), and spotted (Phoca largha)—are particularly vulnerable to arctic warming trends. Documenting diet composition over time is one way in which the flexibility of a species, in the face of broad ecosystem changes, can be measured. Because the fatty acid (FA) composition of depot lipids has long been known to reflect diet, we analysed the FA composition of blubber samples collected from bearded (n = 30), ringed (n = 15), ribbon (n = 32) and spotted seals (n = 24). All animals were taken near Little Diomede Island in May and June 2003, providing a unique opportunity to study interspecies differences with minimal effects of spatial and temporal variation. Bearded seal FA composition was significantly different from all other seals (P < 0.001 in all cases), ringed seal FA composition was significantly different from that of both ribbon and spotted seals (P < 0.001), but ribbon and spotted seal FA compositions could not be distinguished from each other. Overall, the blubber FA compositions of ribbon, ringed and spotted seals implied diets dominated by pelagic prey. Inferences we made about current diets of bearded and ringed seals suggest that certain prey important to them in this area in the 1970s remain important today, despite notable changes in the ecosystem. We believe that blubber FA analysis will be a useful tool in assessing the response of ice-associated pinnipeds and their ecosystem to changes associated with arctic warming.

Northern fulmars Fulmarus glacialis in the North Pacific Ocean are opportunistic, generalist predators, yet their diets are poorly described; thus, relationships of fulmars to supporting food webs, their utility as indicators of variability in forage fish abundances, and their sensitivity to ecosystem change are not known. We employed fatty acid (FA) signature analysis of adipose tissue from adults (n = 235) and chicks (n = 33) to compare spatial, temporal, and age-related variation in diets of fulmars breeding at 3 colonies in Alaska. FA signatures of adult fulmars differed between colonies within years, and between seasons at individual colonies. Seasonal and spatial differences in signatures were greater than interannual differences at all colonies. Differences in FA signatures reflect differences in diets, probably because the breeding colonies are located in distinct ecoregions which create unique habitats for prey assemblages, and because interannual variation in the physical environment affects the availability of forage species. Differences between FA signatures of adults and chicks in 2003 and 2004 suggest that adults fed chicks different prey than they consumed themselves. Alternatively, if adults relied on the same prey as those fed to chicks, the differences in signatures could have resulted from partial digestion of prey items by adults before chicks were fed, or direct metabolism of FAs by chicks for tissue synthesis before FAs could be deposited into adipose tissue.

We explored correlation in diet trends for five piscivorous predators that reproduce on the Pribilof Islands as illustrative of the shifting structure of the Bering Sea ecosystem. We evaluated the size and species of prey consumed by adult female and juvenile northern fur seals ( Callorhinus ursinus ) and adults and chicks of black-legged kittiwakes ( Rissa tridactyla ), red-legged kittiwakes ( Rissa brevirostris ), thick-billed murres ( Uria lomvia ), and common murres ( Uria aalge ) from data collected between July and October 1960–2000. Sample sources included stomachs from seals and seabirds collected on pelagic foraging grounds in the eastern Bering Sea, seal scats from rookeries and seabird regurgitations and whole prey from nest sites on St. Paul and St. George Islands of the Pribilof Island archipelago. Typical prey included small fish and invertebrates (⩽20 cm for seals and ⩽12 cm for seabirds) that concentrate along frontal boundaries of the continental shelf/slope and in the epi-pelagic zone. Squids and fishes including walleye pollock ( Theragra chalcogramma ), capelin ( Mallotus villosus ), and sand lance ( Ammodytes hexapterus ) were variably important in the diet of all five predators. Some prey, such as capelin, were principal in predator diets during the 1960s (seals) and into the early 1980s (seabirds), but declined or disappeared from all predator diets thereafter while others, such as walleye pollock, occurred with increasing frequency from the 1970s forward. As the number of individuals consuming walleye pollock increased, the overall volume of pollock in seabird diets declined. This decline was coincident with a decrease in the age and body size of pollock consumed by both seabirds and fur seals. Squid and pollock were negatively correlated in the diets of their primary consumers, northern fur seals (Pearson's coefficient −0.71, p =0.016) and thick-billed murres (Pearson's coefficient=−0.74, p =0.015) from the 1970s forward. Inter-island variation in diet was evident to varying degrees for all predators, with a prevalence of fish on St. Paul Island and invertebrates on St. George Island. Bayesian time-series analysis of synthesized data described significant temporal cross-correlation in diet among northern fur seals, red- and black-legged kittiwakes, and thick-billed murres. For all correlated predators except common murres, beta-binomial modeling indicated that trends in the occurrence of four of the five primary prey (sand lance, capelin, squid, and pollock) evaluated, were significantly associated with eastern Bering Sea time-series trends in sea surface temperature, ice retreat or a combination of both. Data synthesis highlighted potential competition and a scenario for the effects of an altered prey field on the population stability of predators. The association between correlated diet changes among predators and indices of oceanographic shifts in the 1970s and the 1990s allow scrutiny of hypotheses concerning causal mechanisms in population declines.

Global warming and the loss of sea ice threaten to alter patterns of productivity in arctic marine ecosystems because of a likely decline in primary productivity by sea ice algae. Estimates of the contribution of ice algae to total primary production range widely, from just 3 to >50%, and the importance of ice algae to higher trophic levels remains unknown. To help answer this question, we investigated a novel approach to food web studies by combining the two established methods of stable isotope analysis and fatty acid (FA) analysis--we determined the C isotopic composition of individual diatom FA and traced these biomarkers in consumers. Samples were collected near Barrow, Alaska and included ice algae, pelagic phytoplankton, zooplankton, fish, seabirds, pinnipeds and cetaceans. Ice algae and pelagic phytoplankton had distinctive overall FA signatures and clear differences in delta(13)C for two specific diatom FA biomarkers: 16:4n-1 (-24.0+/-2.4 and -30.7+/-0.8 per thousand, respectively) and 20:5n-3 (-18.3+/-2.0 and -26.9+/-0.7 per thousand, respectively). Nearly all delta(13)C values of these two FA in consumers fell between the two stable isotopic end members. A mass balance equation indicated that FA material derived from ice algae, compared to pelagic diatoms, averaged 71% (44-107%) in consumers based on delta(13)C values of 16:4n-1, but only 24% (0-61%) based on 20:5n-3. Our estimates derived from 16:4n-1, which is produced only by diatoms, probably best represented the contribution of ice algae relative to pelagic diatoms. However, many types of algae produce 20:5n-3, so the lower value derived from it likely represented a more realistic estimate of the proportion of ice algae material relative to all other types of phytoplankton. These preliminary results demonstrate the potential value of compound-specific isotope analysis of marine lipids to trace C flow through marine food webs and provide a foundation for future work.

Colony-wide boat-based censuses suggest that numbers of Common Guillemots at Bluff, Alaska declined markedly in the late 1970s and remained low until 1983. Annual land-based counts of study plots since 1979 generally showed no trend. However, numbers declined substantially in one study area between 1978 and 1983. Reproductive success varied considerably among years and was highest in moderately warm summers. Simulation analyses suggest that the changes in numbers cannot be attributed to changes in reproductive success several years previously. Thus, factors affecting survival, presumably in winter, are apparently implicated in the declines.

Fatty acids (FA) have a diversity of structures that are transferred with little modification through food webs, making them valuable in assessing diets of animals that cannot be directly observed feeding. Before using FA to estimate diets, it is necessary to evaluate variation in FA signatures within and among individuals of a given species. To begin assessing diets and foraging of western Arctic bowhead whales ( Balaena mysticetus) , we examined the FA in blubber of 64 bowheads taken in the spring and fall subsistence hunts in 1997–2002 at Barrow and Kaktovik, Alaska. We found no significant differences in FA characteristics of inner blubber layers taken from either duplicate samples on the dorsal surface, or between dorsal and ventral sites. Significant differences were found in the FA composition between inner and outer layers of blubber at the same body site. We also found age, season and year to have significant effects on FA composition; however, gender was not found to be significant. While the importance of the Beaufort Sea as a feeding ground of bowhead whales remains uncertain, our results indicate that adults and sub-adults foraged to some extent on different prey and that both age classes consumed copepods there in summer at sufficient levels to significantly alter their blubber FA profiles. Both of these findings correspond with dietary conclusions reached from the analysis of stomach contents. Furthermore, we found compelling evidence that yearly variation in bowhead FA reflect changes in FA compositions of phytoplankton at the base of the food web, probably in response to climate variation. Variability in phytoplankton-derived FA in blubber was correlated significantly with yearly mean values of the Pacific Decadal Oscillation. FA in bowhead whale blubber, therefore, might be used to monitor effects of climate change on lower trophic levels and production processes in the western Arctic.

The Bering Sea has experienced dramatic warming in this century that has altered elements of the ecosystem, including the structure and productivity of the zooplankton community on the continental shelf, and the extent to which waters and associated plankton of oceanic origin have intruded onto the shelf. We documented temporal and spatial scales of these changes by monitoring diets of least auklets Aethia pusilla on the Pribilof Islands—least auklets are planktivores that specialize on the large calanoid copepods Neocalanus spp. from the basin and Calanus marshallae from the shelf. Diets were estimated in the summers of 1996 to 2006 by enumerating prey in regurgitated meals brought to chicks by adults, and by fatty acid analyses of live biopsy samples of adipose tissue from adult birds in 2003 and 2004, which provided additional insight. In the unusually warm 2000s, Neocalanus spp. apparently were excluded from regions of the outer shelf, where they typically occur in cooler years, and, concurrently, C. marshallae was depressed over a large region of the shelf because of chronic failures of spring cohorts to survive. Both changes were associated with anomalously high water temperatures over the middle shelf. The information provided by least auklets greatly improves our understanding of the consequences of environmental change and supplies clues about how communities and ecosystem processes respond to physical forcing. Continued warming of the magnitude seen in recent years could become a cause for concern for auklets and other planktivores in the eastern Bering Sea if it alters prey availability in ways detrimental to their populations.

The dynamics of predator-prey relationships, the structure of food webs, and the forag- ing behavior of individuals are critical to understanding animal ecology, interactions of predators with their prey, and effects of environmental variability on ecosystems. Like many other predators, seabirds are effective samplers of prey populations, and their diets can provide information about lower trophic levels over a range of spatial and temporal scales. Recognizing limitations of traditional methods of diet analysis, we validated the use of fatty acid (FA) signatures of subcutaneous adipose tissue biopsies for estimating diets of free-ranging seabirds. Calibration coefficients (CCs) for individ- ual FAs were determined from captive common murres (n = 13) fed a long-term, single-species diet. Quantitative FA signature analysis (QFASA), using these CCs, was then validated in murres (n = 26) and red-legged kittiwakes (n = 13) fed controlled mixed-species diets. FAs were analyzed from 426 free-ranging red-legged Rissa brevirostris and black-legged kittiwakes R. tridactyla, and common Uria aalge and thick-billed murres U. lomvia from the Bering Sea, 284 of which were also sampled for stomach contents analysis. Qualitatively, FA signatures revealed distinct separation of diets among all 4 species, and further separation by location and year. QFASA diet estimates were similar to those based on stomach contents, with diets of kittiwakes dominated by myctophids, while those of murres comprised a mixture of other forage species. QFASA estimates were indicative of regional habitat dif- ferences, and were consistent with other aspects of seabird ecology at our study sites. We conclude that seabird FAs provide important information about ecosystems, but this will likely depend on each species' foraging behavior and the complexities of the ecosystem it occupies.

Procellariiforms are unique among seabirds in storing dietary lipids in both adipose tissue and stomach oil. Thus, both lipid sources are potentially useful for trophic studies using fatty acid (FA) signatures. However, little is known about the relationship between FA signatures in stomach oil and adipose tissue of individuals or whether these signatures provide similar information about diet and physiology. We compared the FA composition of stomach oil and adipose tissue biopsies of individual northern fulmars (N = 101) breeding at three major colonies in Alaska. Fatty acid signatures differed significantly between the two lipid sources, reflecting differences in dietary time scales, metabolic processing, or both. However, these signatures exhibited a relatively consistent relationship between individuals, such that the two lipid sources provided a similar ability to distinguish foraging differences among individuals and colonies. Our results, including the exclusive presence of dietary wax esters in stomach oil but not adipose tissue, are consistent with the notion that stomach oil FA signatures represent lipids retained from prey consumed during recent foraging and reflect little metabolic processing, whereas adipose tissue FA signatures represent a longer-term integration of dietary intake. Our study illustrates the potential for elucidating short- versus longer-term diet information in Procellariiform birds using different lipid sources.

Dramatic multiyear fluctuations in water temperature and seasonal sea ice extent and duration across the Bering-Chukchi continental shelf have occurred in this century, raising a pressing ecological question: Do such environmental changes alter marine production processes linking primary producers to upper trophic-level predators? We examined this question by comparing the blubber fatty acid (FA) composition and stable carbon isotope ratios of individual FA (δ¹³CFA) of adult ringed seals (Pusa hispida), bearded seals (Erignathus barbatus), spotted seals (Phoca largha), and ribbon seals (Histriophoca fasciata), collectively known as "ice seals," sampled during an anomalously warm, low sea ice period in 2002-2005 in the Bering Sea and a subsequent cold, high sea ice period in 2007-2010. δ¹³C(FA) values, used to estimate the contribution to seals of carbon derived from sea ice algae (sympagic production) relative to that derived from water column phytoplankton (pelagic production), indicated that during the cold period, sympagic production accounted for 62-80% of the FA in the blubber of bearded seals, 51-62% in spotted seals, and 21-60% in ringed seals. Moreover, the δ¹³CFA values of bearded seals indicated a greater incorporation of sympagic FAs during the cold period than the warm period. This result provides the first empirical evidence of an ecosystem-scale effect of a putative change in sympagic production in the Western Arctic. The FA composition of ice seals showed clear evidence of resource partitioning among ringed, bearded, and spotted seals, and little niche separation between spotted and ribbon seals, which is consistent with previous studies. Despite interannual variability, the FA composition of ringed and bearded seals showed little evidence of differences in diet between the warm and cold periods. The findings that sympagic production contributes significantly to food webs supporting ice seals, and that the contribution apparently is less in warm years with low sea ice, raise an important concern: Will the projected warming and continuing loss of seasonal sea ice in the Arctic, and the associated decline of organic matter input from sympagic production, be compensated for by pelagic production to satisfy both pelagic and benthic carbon and energy needs?

Large body size, carnivory, and endothermic costs lead to exceptionally high caloric demands in many mammalian predators. The potential impact on prey resources may be marked but is difficult to demonstrate because of the mobility, sparseness, and cryptic nature of these animals. In this study, we developed a method based on comparative bioenergetics and demographic modeling to evaluate predator effects and then used this approach to assess the potential impact of killer whales on sea otter and Steller sea lion populations in the Aleutian Islands. Daily caloric requirements of killer whales determined from allometric regressions for field metabolic rate show that an adult killer whale requires 51-59 kcal·kg 21 ·d 21 (2.5-2.9 W/kg). Caloric values of prey items determined by bomb calorimetry ranged from 41 630 kcal for an adult female sea otter to sequentially higher values for male otters, sea lion pups, and adult Steller sea lions. Integrating these results with demographic changes in marine mammal populations show that fewer than 40 killer whales could have caused the recent Steller sea lion decline in the Aleutian archipelago; a pod of five individuals could account for the decline in sea otters and the continued suppression of sea lions. The collapse of the historical prey base of killer whales due to human whaling may have contributed to a sequential dietary switch from high to low caloric value prey, thereby initiating these declines. This study demonstrates that a combined physiological-demographic approach increases our ability to critically evaluate the potential impact of a predator on community structure and enables us to define underlying mechanisms

We surveyed Marbled Murrelets (Brachyramphus marmoratus) daily from small boats in Auke Bay and Fritz Cove, Alaska, from May through August 1992 and 1993. Differences in numbers of juveniles and in the timing of their presence in the study area between the two years indicated that breeding phenology was late and productivity was low in 1992 compared to 1993. This difference was consistent with variability in the physical environment. Of 99 fish identified in the bills of fish-holding adult murrelets, 81 (82%) were Pacific Sand Lance (Ammodytes hexapterus). Counts of fish-holding adult murrelets were significantly higher in the evening than at any other time of day. Time of day had no significant effects on counts of fledglings, indicating that juveniles were moving into and out of the study area during the day. Murrelets were predominantly found in groups of two or more, even during incubation, suggesting that murrelets incur an appreciable benefit, such as increased foraging efficiency, from for...

Steller sea lion (Eumetopias jubatus) numbers are beginning to recover across most of the western distinct population segment following catastrophic declines that began in the 1970s and ended around the turn of the century. This study makes use of contemporary vital rate estimates from a trend-site rookery in the eastern Gulf of Alaska (a sub-region of the western population) in a matrix population model to estimate the trend and strength of the recovery across this region between 2003 and 2013. The modeled population trend was projected into the future based on observed variation in vital rates and a prospective elasticity analysis was conducted to determine future trends and which vital rates pose the greatest threats to recovery. The modeled population grew at a mean rate of 3.5% per yr between 2003 and 2013 and was correlated with census count data from the local rookery and throughout the eastern Gulf of Alaska. If recent vital rate estimates continue with little change, the eastern Gulf of Alaska population could be fully recovered to pre-decline levels within 23 years. With density dependent growth, the population would need another 45 years to fully recover. Elasticity analysis showed that, as expected, population growth rate (λ) was most sensitive to changes in adult survival, less sensitive to changes in juvenile survival, and least sensitive to changes in fecundity. A population decline could be expected with only a 6% decrease in adult survival, whereas a 32% decrease in fecundity would be necessary to bring about a population decline. These results have important implications for population management and suggest current research priorities should be shifted to a greater emphasis on survival rates and causes of mortality.

The importance of the subarctic gyres of the North Pacific Ocean to marine birds and mammals is poorly known because of a paucity of data spanning appropriate scales of time and space. The little information that is available indicates the western subarctic gyre (WSAG) is more productive than the eastern subarctic gyre (ESAG). In summer the WSAG supports a greater density and higher biomass of seabirds than the ESAG, including at least two species that are more abundant at nesting colonies in the eastern subarctic. Perhaps most revealing of the seabird distributions in this regard is that of southern hemisphere shearwaters (Puffinus spp. ) that overwinter in the North Pacific. Their biomass is an order of magnitude greater than that of any northern hemisphere species and is three-fold greater in the WSAG than in the ESAG. Several species of cetaceans also appear to be, or to have been prior to commercial depletions, more abundant in the WSA. Among the many prey species consumed by marine birds and mammals, squids and fishes in the family Myctophidae predominate overall. Other forage species, notably euphausiids, Pacific saury (Cololabis saira) and Atka mackerel (Pleurogrammus monopterygius) are important at times to certain species. The principal exceptions to this generalization are baleen whales and small seabirds that consume zooplankton. Interannual and decadal-scale variability in the physical environment and food web production affect seabirds and marine mammals at sea and at coastal breeding locations around the margins of the gyres.

Climate change in the last century was associated with spectacular growth of many wild Pacific salmon stocks in the North Pacific Ocean and Bering Sea, apparently through bottom-up forcing linking meteorology to ocean physics, water temperature, and plankton production. One species in particular, pink salmon, became so numerous by the 1990s that they began to dominate other species of salmon for prey resources and to exert top-down control in the open ocean ecosystem. Information from long-term monitoring of seabirds in the Aleutian Islands and Bering Sea reveals that the sphere of influence of pink salmon is much larger than previously known. Seabirds, pink salmon, other species of salmon, and by extension other higher-order predators, are tightly linked ecologically and must be included in international management and conservation policies for sustaining all species that compete for common, finite resource pools. These data further emphasize that the unique 2-y cycle in abundance of pink salmon drives interannual shifts between two alternate states of a complex marine ecosystem.

Steller sea lion (Eumetopias jubatus) numbers in the Western Distinct Population Segment are beginning to recover following the dramatic decline that began in the 1970s and ended around the turn of the century. Low female reproductive rates (natality) may have contributed to the decline and remain an issue of concern for this population. During the 2000s we found high natality among Steller sea lions in the Gulf of Alaska indicating a healthy population. This study extends these previous estimates over an additional three years and tests for interannual variations and long-term trends. We further examine the proportions of pups to adult females observed on the rookery and nearby haulouts during the birthing season to assess whether census data can be used to estimate natality. Open robust design multistate models were built and tested using Program MARK to estimate survival, resighting, and state transition probabilities in addition to other parameters dependent on whether or not a female gave birth in the previous year. Natality was estimated at 70% with some evidence of interannual variation but a long-term increasing or decreasing trend was not supported by the data. Bootstrap and regression comparisons of census data with natality estimates revealed no correlation between the two methods suggesting that census data are not an appropriate proxy for natality in this species. Longitudinal studies of individual animals are an appropriate method for estimating vital rates in species with variable detection over time such as the Steller sea lion. This work indicates that natality remains high in this region and is consistent with a population in recovery.

The concept of a highly productive habitat, or Green Belt, along the edge of the continental shelf in the Bering Sea is based upon compelling but fragmentary and often anecdotal observations of a variety of physical and biological features acquired from many sources over many years. Enhanced production at continental margins is not a novel concept, but in the case of the Bering Sea its importance has been overlooked during studies of the unusually broad continental shelf. The limited data reported from the vicinity of the shelf edge in the Bering Sea indicate that annual primary production can be as high as 175 to 275 g C m˜ year-, or approximately 60% greater than production in the adjacent outer shelf domain and 270% greater than in the oceanic domain. Estimates of annual secondary production at the eastern shelf edge also average approximately 60% higher than estimates for the outer domain and 260% higher than those for the oceanic domain. Physical processes at the shelf edge, such as intensive tidal mixing and transverse circulation and eddies in the Bering Slope Current, bring nutrients into the euphoric zone and contribute to enhanced primary and secondary production and elevated biomass of phytoplankton and zooplankton. Fishes and squids concentrate in this narrow corridor because of favourable feeding conditions and because of a thermal refuge from cold shelf-bottom temperatures that can be found at the shelf edge from fall to spring. The abundance of zooplankton, fishes and squids, in turn, attracts large numbers of marine birds and mammals. In aggregate, the observations suggest that sustained primary productivity, intense food web exchange and high transfer efficiency at the shelf edge are important to biomass yield at numerous trophic levels and to ecosystem production of the Bering Sea.

The advective supply of nutrients to the Bering-Chukchi continental shelf via a north-flowing “river” of oceanic water originating along the continental slope in the Bering Sea maintains a large portion of these shelf waters in eutrophic bloom summer-long. Known as the Anadyr Stream, this nutrient injection sustains conditions of high phytoplankton productivity and biomass in a region of the western Arctic that would otherwise be unproductive, as are adjacent shelf areas unaffected by the current. A production plume dominated by large chain-forming diatoms extends from the Gulf of Anadyr in the south to the southern Chukchi Sea in the north, has daily carbon uptake rates as high as 16 g C m −2 day −1 , and has an estimated annual production of about 470 g C m −2 year −1 . Maximum production occurs in three pools of especially prolific growth (Gulf of Anadyr, Chirikov Basin and southern Chukchi Sea) where rates could be as great as 720–840 g C m −2 year −1 . Outside of the plume, nutrients remain low following the spring bloom and a typical successional flora is dominated by flagellates and small diatoms throughout summer. Post-bloom productivity in this region is generally about 0.5 g C m −2 day −1 , and annual production is approximately 80 g C m −2 year −1 . The contrasting primary production regimes lead to major differences in food webs and in the energy transferred to higher trophic levels within the western Arctic.

Populations of several species of marine birds and mammals in the Bering Sea and Gulf of Alaska have been declining since the mid-1970s, with numbers of one, the Steller sea lion (Eumetopias jubatus), so depressed it was listed as threatened under the Endangered Species Act in spring 1990. All of the declining populations depend to an important extent on walleye pollock (Theragra chakogramma) for food, although they eat numerous other species as well. In contrast, certain animals that compete with pollock for common prey have been increasing in abundance. All of these changes could be related through food web connections mediated by pollock. Pollock is also important to people–it presently supports the largest single-species commercial fishery in the world, in large part because of its great biomass, which has averaged about 15 × 106t in the Bering Sea over the past 15 years. Pollock consume an inordinate proportion of the pelagic production in the Bering Sea, which further supports the conclusion that it is a key species in the ecosystem. However, there are conflicting hypotheses about the importance of the roles played by pollock as predator and prey, and about the effect that changes in pollock abundance might have on biomass yield at higher trophic levels.

Populations of sea otters, seals and sea lions have collapsed across much of southwest Alaska over the past several decades. The sea otter decline set off a trophic cascade in which the coastal marine ecosystem underwent a phase shift from kelp forests to deforested sea urchin barrens. This interaction in turn affected the distribution, abundance and productivity of numerous other species. Ecological consequences of the pinniped declines are largely unknown. Increased predation by transient (marine mammal-eating) killer whales probably caused the sea otter declines and may have caused the pinniped declines as well. Springer et al . proposed that killer whales, which purportedly fed extensively on great whales, expanded their diets to include a higher percentage of sea otters and pinnipeds following a sharp reduction in great whale numbers from post World War II industrial whaling. Critics of this hypothesis claim that great whales are not now and probably never were an important nutritional resource for killer whales. We used demographic/energetic analyses to evaluate whether or not a predator–prey system involving killer whales and the smaller marine mammals would be sustainable without some nutritional contribution from the great whales. Our results indicate that while such a system is possible, it could only exist under a narrow range of extreme conditions and is therefore highly unlikely.

The importance of food availability (bottom-up control) vs. predation (top-down control) in affecting population dynamics of individual species and the structure of marine communities and ecosystems is in most cases poorly known, but vigorously debated. This debate has recently focused on the dramatic declines of populations of four species of marine mammals in Alaska (Estes et al. 1998, Springer et al. 2003, 2008; Williams et al. 2004, Fritz and Hinckley 2005, Trites et al. 2007, Wade et al. 2007). Between the early 1970s and late 1990s, the abundance of harbor seals (Phoca vitulina), Steller sea lions (Eumetopias jubatus), and sea otters (Enhydra lutris) collapsed sequentially by 80%–90% throughout southwest Alaska—the Aleutian Islands, Bering Sea, and western Gulf of Alaska. The major portion of each collapse occurred over short intervals of about 10 yr. Harbor seals have since begun a slow recovery in at least a part of this range (Small et al. 2003); sea lions have variously increased somewhat, remained stable, or continued to decline (NMFS 2007); and sea otters were still declining in most areas through the early 2000s (Doroff et al. 2003, Estes et al. 2005, J. Estes1). Across that entire span of years, a fourth species, the northern fur seal (Callorhinus ursinus) on the Pribilof Islands in the Bering Sea, has experienced a less precipitous, but continuing, and now concerning, decline—for example, pup production, the index of total abundance, has fallen by 26% since 2002 and their abundance is just 24% of the peak in 1955 (Trites and Larkin 1989, Towell and Fowler 2006). Three principal hypotheses have been proposed to explain these collapses and the lack of significant recovery: direct mortality from human activities, nutritional limitation, and predation. Direct mortality of sea lions was caused primarily by commercial harvests, bycatch in commercial fisheries, and legal and illegal shooting (Atkinson et al. 2008): direct mortality has not been invoked as a cause of the declines of any of the other species in the region since the mid 1970s or earlier. The cause of purported nutritional limitation of pinnipeds is thought to be climate change or commercial fisheries, or both, which altered the availability of various species of

This chapter addresses temporal scales ranging from single anomalous years, groups of successive or near-successive years with similar behavior, decades, multiple decades, centuries, and millennia. Scales from local through meso-scales to the basin-wide scale are considered. Aleutian Low Pressure system (ALPS) and the North Pacific High (NPH) repetitively trade dominance in the northern Gulf with the change of the seasons. Geophysical forces and climate have been the two largest influences on the marine ecosystems of the Gulf of Alaska (GOA), but it is likely that human effects will be as great a factor in the future on this time scale. There is an underlying low-frequency decadal and multi-decadal forcing that is expressed in the atmosphere as changes in pressure, winds, temperature, and precipitation, and translated into the ocean as changes in spatial patterns of water temperature, mixing depth, upwelling, and down welling. Predominant meteorological and oceanographic phenomena are expressed in the biological components of the ecosystem, particularly in the lower trophic levels. The effects of changes in the atmospheric–oceanic system are assumed to be expressed through bottom-up forcing. The higher frequency pattern of change that influences much of the GOA is the El Nino/Southern Oscillation (ENSO). The chapter helps to understand the nature of ecosystem change that occurs on many interacting temporal and spatial scales in a more comprehensive way.

This chapter focuses on ecosystem structure. Ecosystem can be viewed as how the sun's energy is captured and passed from plants to animals and then from animals to animals and the strategies that animals have evolved to capture a share of that energy, use it, and flourish in the Gulf of Alaska. Marine animals thrive in the Gulf of Alaska (GOA) for two basic reasons: (1) the ecosystem is productive, and a lot of food is available to animals living there, and (2) a small number of successful species in the Gulf made the most of the available energy by organizing themselves, as revolving body forms, physiology, and behaviors that allow them to successfully withstand the rigors of the subarctic ocean, capture food, grow, and reproduce faster than they themselves are eaten, harvested, or fall victims to contaminants or disease. The chapter explores the changing structure of the ecosystem during a typical year. The survival strategies of the key species of fish, birds, and mammals that tie them to their habitats are also discussed.

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 oceanographic 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.

Observations of killer whales in the act of predation are rare because feeding bouts are infrequent in relation to sighting opportunities, and they apparently feed at night as well as during the day. A pop‐up autonomous recording unit (ARU) was used to expand the observation window of transient (mammal‐eating) killer whales by monitoring them continuously from 22 June to 12 July 2006 near a northern fur seal rookery at St. Paul Island, AK, a predation hot‐spot in the Bering Sea. The ARU recorded at a sampling rate of 0.5–16 000 Hz. Killer whale vocalizations were detected on 19 of 20 days. Most vocalizations occurred from the end of civil twilight (00:26 on 1 July) through mid‐morning. Fewest vocalizations occurred between 1800 and 2400. Vocal whales were most likely feeding, based on the paradigm that transients are mostly silent except when attacking prey. Call types are being identified and vocalizations are being correlated with visual observations and fur seal activity at the rookery. Nocturnal hunting is likely one important reason why it has been difficult to understand the foraging behavior of these elusive predators and must be considered when drawing conclusions about the roles transient killer whales play in marine ecosystems.

Acoustic recorders can be superior to visual techniques for detecting marine mammals. Monitoring the presence of predators that exploit depleted prey populations, such as killer whales and northern fur seals at the Pribilof Islands in the Bering Sea, At, facilitates a better understanding of the role predators play in prey population dynamics and ecosystem function. Killer whales frequent the Pribilofs and prey upon fur seals when they are abundant during spring‐fall, but their broader geographic range, stock structure, and fidelity to this predation hot spot are unknown. Identifying killer whale call types can address these questions because killer whale groups share calls. This study documented nine killer whale call types at the Pribilofs during the summers of 2006–2008. Calls were characterized by spectral and temporal properties, component structure and contour shape. The two most common calls were detected at one island more than another 64 km away, which suggests site preference. Calls during feedi...

The Pribilof Islands, located in the eastern Bering Sea, are the site of the largest concentration of northern fur seals in the world. Despite the occurrence of transient killer whales on a near daily basis during the fur seal breeding season, these whales are unknown in terms of their social structure and overall range. Orcas were recorded with autonomous acoustic recorders at the Pribilof Islands in the summers of 2006 and 2008. This is the first study of call types and whistles of these killer whales. Whistles have been described for other odontocete species but not yet for transient killer whales. This study also reports on temporal and spatial dynamics of killer whale occurrence at the two major Pribilof Islands, St. Paul and St. George Islands. Call matches between the years and locations were analyzed to ascertain visitation patterns, predation sounds, and diurnal activity near the fur seal rookeries. Whale vocalizations from the Pribilofs were compared to other regions in Alaska. Thus far, there is no match between the calls of the Pribilof whales and those in regions of Southeast Alaska and Prince William Sound, but some calls bear a structural resemblance to calls produced by whales recorded near False Pass, AK.

We examined effects of season, time of day, tide stage, tidal oscillation, and sea surface temperature on Marbled Murrelet (Brachyramphus marmoratus) abundance and distribution at sea. We also evaluated whether constraining surveys to specific time periods or tide stages would reduce temporal variability in counts. Murrelets were surveyed daily from small boats and from shore in Auke Bay and Fritz Cove, Alaska, from May through August in 1992 and 1993. Murrelet numbers were high before egg-laying, declined by more than half during egg-laying and incubation and were highly variable during chick-rearing and fledging. Murrelet numbers were highest in early and late morning and declined throughout the day, sometimes increasing slightly in the evening. Peak murrelet numbers occurred on high or falling morning tides, especially in shallow areas where Pacific sand lance (Ammodytes hexapterus) were abundant. Differences between years in murrelet abundance and breeding phenology probably resulted from interannual differences in the pattern of seasonal warming and subsequent effects on production at lower trophic levels. We recommend that surveys for trends in abundance in Southeast Alaska be conducted in early morning, in June, at high or falling tides. Power analyses indicated that surveys conducted in this manner would minimize the number of years required to detect a significant change in abundance. Received 6 April 2000, accepted 3June 2000.

Two distinct environmental settings in the Bering Strait region of the northern Bering Sea lead to characteristic pathways of energy flow through primarily pelagic food webs to avian consumers. In Norton Sound, a large, shallow embayment on the northeastern coast, the physical environment is dominated by the discharge of the Yukon River and by a large seasonal temperature signal. Seabirds breeding at Bluff, the largest colony in Norton Sound, number in the order of 5 × 104 and require 1.2 × 106 g C d−1. Two piscivorous species constitute the bulk of all seabirds there and are supported by a pelagic food web typical of the coastal zone of the Bering and Chukchi seas. This food web also is present around St. Lawrence Island, on the northwestern shelf, and is important to at least one species of seabird there. In addition, and generally more important, St. Lawrence Island is in a biologically rich environment resulting from the northward flow of water that originates along the continental shelf break of the Bering Sea. This flow apparently accounts for the unexpected presence of oceanic zooplankton and a diversity of forage fishes on the shallow northern shelf that support an abundant and taxonomically rich avifauna. In comparison to Norton Sound, breeding seabirds on St. Lawrence Island number in the order of 2 × 106, with planktivores consuming about 8 × 106 g C d−1 and piscivores consuming about 16 × 106 g C d−1.

Studies undertaken to determine the causes of continuing reproductive failures of populations of peregrine falcons (Falco peregrinus anatum) in Texas and California demonstrate exceptionally high levels of DDE (1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene) in prey species in western Texas and unchanged high levels of DDE since 1969 in the peregrine population of the central coast of California. Although DDE is a derivative of DDT (p,p'-DDT; 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane), levels of DDT were very low in in these samples. These observations follow a recent pattern of DDE contamination in the South and Southwest that is characterized by locally high environmental levels of DDE and low levels of DDT. The virtual absence of DDT indicates that the possible illegal uses of DDT, or the transport of DDT compounds from Latin America by migrating birds, cannot account for all of the DDE in these samples. Another source of environmental DDE is therefore indicated.

Walleye pollock Theragra chalcogramma are important prey of piscivorous seablrds at offshore breeding colonies in the southeastern Bering Sea. The reproductive success of kittiwakes Rissa spp. is related to the abundance of juvenile pollock. It is proposed that poor pollock recruitment in recent years, as determ~ned by fisheries surveys, led to persistent kittiwake breeding failures in 1981-84. Declines in numbers of k~ttiwakes and other piscivorous seabirds at 2 colonies also may be related to the decline in pollock abundance. In contrast, planktivorous seabirds, which possibly compete with juvenile pollock for zooplankton, particularly copepods, might benefit from a low abundance of pollock. A conceptual model of food limitation and trophic interactions at several levels in the offshore pelagic food web of the eastern Bering Sea can be developed by relating relations between seabirds and their prey to ( l ) the high grazing efficiency of the herbivorous zooplankton community, which is dominated by large calanoid copepods, (2) estimates of a high annual loss of copepods, presumably to vertebrates, and (3) regional carbon budgets. The possibility that populations of several species respond to changes in pollock abundance should be considered in management decisions for the commercial pollock fishery

The ecosystem of the northern Bering-Sea shelf was studied using data from the NOAA Very High Resolution Radiometer and AVHRR and the Landsat MSS and Thematic Mapper (TM) in conjunction with shipboard measurements. Emphasis was placed on the influence of the Yukon River on this inner shelf environment and on the evaluation of the utility of the new Landsat TM data for oceanography. It was found that the patterns of water mass distribution obtained from satellite images agreed reasonably well with the areal patterns of temperature, salinity, and phytoplankton distributions. The AVHRR, MSS, and TM data show that the Yukon-River discharge is warmer and more turbid than the surrounding coastal water that originates to the south; thus, the Yukon water contributes to the higher temperatures and lower transmissivity of the coastal water. The high resolution of the TM thermal IR band made it possible to observe complex patterns and structures in the surface water that could not be resolved on previous data sets.

Food habits of thick-billed murres (Uria lomvia), common murres (U. aalge), and black-legged kittiwakes (Rissa tridactyla) were examined at two breeding colonies in the eastern Chukchi Sea between 1976 and 1980. Cods (Gadidae), sculpins (Cottidae), sand lance (Ammodytes hexapterus), and capelin (Mallotus villosus) were the dominant (biomass) prey of thick-billed murres. Cods, sand lance, and capelin were the dominant prey of common murres and kittiwakes. Annual changes in the consumption of fishes by murres and kittiwakes suggest that available fish biomass in the eastern Chukchi Sea was low in 1976 and 1977, but increased each year through 1979 and 1980. The average size of several taxa of fishes eaten by the birds increased also between most years. Invertebrates were less important than fishes in all years, were eaten most frequently by thick-billed murres, and were eaten more frequently in 1976–77 than in 1978–79. A pattern of climatic cooling in the early 1970's followed by warming in the second half of the decade caused annual differences in the extent and duration of sea ice, and apparently in the spacial and temporal development of Alaskan Coastal Water, a major oceanographic feature of the Bering–Chukchi shelf. Fluctuations in the physical environment could have led to changes in fish populations through direct physiological and behavioral effects, or indirectly by altering the abundance of important zooplankton prey populations. Variability in the reproductive success of murres and kittiwakes in the northern Bering Sea and eastern Chukchi Sea corresponded with the apparent changes in fish stocks: reproductive success was very low in 1976 but improved in successive years. Short-term responses of seabirds to environmental fluctuation and its proposed effect on supporting food webs suggest that the stability of regional seabird populations could also be affected by long-term changes in weather and climate.

DDE (1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene), the DDT derivative that causes thinning of the eggshells of sensitive bird species, is metabolically formed in mallard ducks fed Kelthane®, a widely used acaricide that consists of technical dicofol, a mixture of compounds synthetically derived from technical DDT. Technical dicofol mixtures contain very low levels of p,p'-DDT, which has been assumed to be the only precursor to environmental DDE. Applications of Kelthane and of other products containing technical dicofol would therefore increase the environmental input of DDE, accounting at least in part for recent observations of high levels of DDE in environmental samples from Texas, New Mexico, Arizona and California that contained only trace amounts of DDT. Among the components of Kelthane, chloro-DDT (1,1,1,2-tetrachloro-2,2-bis(p-chlorophenyl)ethane) is converted to DDE under certain laboratory conditions. Levels of chloro-DDT in the Kelthane mixture employed could account for all of the DDE metabolically formed in the ducks. The data leave unresolved the question of whether DDE may be derived from p,p'-dicofol (1,1-bis(p-chlorophenyl)-2,2,2-trichloroethanol), the principal ingredient of technical dicofol, under either experimental or environmental conditions.

Anadyr Water, a “river” of oceanic water originating over the continental slope of the Bering Sea, transports oceanic zooplankton onto the shallow northern shelf and into the Chukchi Sea through western Bering Strait. The oceanic copepods Neocalanus cristatus, N. plumchrus, Eucalanus bungii and Metridia pacifica dominated the biomass of herbivorous zooplankton in Anadyr Water on the Bering-Chukchi shelf, averaging about 5 g m −2 (dry weight) in midsummer and 2 g m −2 in later summer of 1985 and 1986. The biomass of copepods was at times augmented considerably by that of a larvacean, Oikopleura spp. Oceanic water was replaced to the east, and often overlain by, lighter water formed on the continental shelf. The oceanic species were not present in waters of shelf origin, where Calanus marshallae was the major herbivore, averaging about 1 g m −2 in summer 1985, but only 0.2–0.5 g m −2 in 1986. In the low salinity, nearshore water, C. marshallae was uncommon and Pseudocalanus spp. and Acartia longiremis predominated. A preliminary estimate indicated that in the order of 1.8 × 10 12 g C of boreal zooplankton were carried into the Chukchi Sea during summer 1985. On average, the grazers were unable to control prodigious diatom growth that resulted from concentrated nutrients also carried by Anadyr Water into the area. In contrast, the smaller biomass of grazers in the shelf community, away from the region of high diatom biomass, might have had a much greater influence over the standing stock of phytoplankton following the spring bloom, and, at times, could have consumed the daily primary production. The highly productive pelagic food web of oceanic origin in the north does not have close coupling between primary producers and zooplankton, or between zooplankton and their predators, and contrasts with the oceanic community in the southeastern Bering Sea, where the same species of herbivores apparently have a much greater effect on phytoplankton biomass, and provide an efficient transfer of the available fixed energy to upper trophic levels in pelagic food webs.