The decline of Steller sea lions Eumetopias jubatus in Alaska: a review of the nutritional stress hypothesis

1. The decline of Steller sea lions Eumetopias jubatus in the Gulf of Alaska and Aleutian Islands between the late 1970s and 1990s may have been related to reduced availability of suitable prey. Many studies have shown that pinnipeds and other mammals suffering from nutritional stress typically exhibit reduced body size, reduced productivity, high mortality of pups and juveniles, altered blood chemistry and specific behavioural modifications. 2. Morphometric measurements of Steller sea lions through the 1970s and 1980s in Alaska indicate reduced body size. Reduced numbers of pups born and an apparent increase in juvenile mortality rates also appear to be nutritionally based. Blood chemistry analyses have further shown that Steller sea lions in the Gulf of Alaska and Aleutian Islands area exhibited signs of an acute phase reaction, or immune reaction, in response to unidentified physical and/or environmental stress. Behavioural studies during the 1990s have not noted any changes that are indicative of an overall shortage in the quantity of prey available to lactating female sea lions. 3. The data collected in Alaska are consistent with the hypothesis that Steller sea lions in the declining regions were nutritionally compromised because of the relative quality of prey available to them (chronic nutritional stress), rather than because of the overall quantity of fish per se (acute nutritional stress). This is further supported by captive studies that indicate the overall quality of prey that has been available to Steller sea lions in the declining population could compromise the health of Steller sea lions and hinder their recovery.     

Anthropogenic causes of the western Steller sea lion Eumetopias jubatus population decline and their threat to recovery

• 1 The western Steller sea lion Eumetopias jubatus population has experienced a chronic decline since the 1960s. The causes are likely multifactorial and a combination of anthropogenic and natural factors. A draft revised recovery plan for the Steller sea lion has been published by the US National Marine Fisheries Service, listing both anthropogenic and natural factors that may have contributed to the observed decline or which may be a threat to the recovery of the western Steller sea lion population. The purpose of this review is to consider the anthropogenic threats to this stock.
• 2 Anthropogenic sources of mortality include fisheries competition resulting in nutritional stress, mortality incidental to commercial fisheries (i.e. fisheries by‐catch), subsistence hunts, legal and illegal shooting, commercial hunts, anthropogenic‐related contamination, and research‐induced mortalities.
• 3 We present evidence that the following anthropogenic factors likely contributed to the decline of the western Steller sea lion population over the last 40 years: (i) mortality incidental to commercial fisheries (i.e. by‐catch); (ii) commercial hunting of western Steller sea lions; and (iii) legal and illegal shooting; whereas the subsistence hunts for western Steller sea lions and mortality incidental to research were not likely to be contributors to the observed decline.
• 4 Further, we present evidence that the following can be excluded as significant anthropogenic threats to the recovery of the western Steller sea lion population: (i) mortality incidental to commercial fishing; (ii) legal and illegal shooting; (iii) commercial hunts of Steller sea lions; (iv) subsistence hunting; and (v) mortality incidental to research.
• 5 Competition with fisheries resulting in nutritional stress, and the potential impacts of contaminants, are two anthropogenic factors that should continue to be a priority for the various organizations currently doing research on this population.

     

No Evidence of Metabolic Depression in Western Alaskan Juvenile Steller Sea Lions (Eumetopias jubatus)

Steller sea lion (Eumetopias jubatus) populations have undergone precipitous declines through their western Alaskan range over the last four decades with the leading hypothesis to explain this decline centering around changing prey quality, quantity, or availability for this species (i.e., nutritional stress hypothesis). Under chronic conditions of reduced food intake sea lions would conserve energy by limiting energy expenditures through lowering of metabolic rate known as metabolic depression. To examine the potential for nutritional stress, resting metabolic rate (RMR) and body composition were measured in free-ranging juvenile Steller sea lions (N = 91) at three distinct geographical locations (Southeast Alaska, Prince William Sound, Central Aleutian Islands) using open-flow respirometry and deuterium isotope dilution, respectively. Average sea lion RMR ranged from 6.7 to 36.2 MJ d⁻¹ and was influenced by body mass, total body lipid, and to a lesser extent, ambient air temperature and age. Sea lion pups captured in the Aleutian Islands (region of decline) had significantly greater body mass and total body lipid stores when compared to pups from Prince William Sound (region of decline) and Southeast Alaska (stable region). Along with evidence of robust body condition in Aleutian Island pups, no definitive differences were detected in RMR between sea lions sampled between eastern and western populations that could not be accounted for by higher percent total body lipid content, suggesting that that at the time of this study, Steller sea lions were not experiencing metabolic depression in the locations studied.     

A CRITICAL REVIEW OF THE REGIME SHIFT-"JUNK FOOD"-NUTRITIONAL STRESS HYPOTHESIS FOR THE DECLINE OF THE WESTERN STOCK OF STELLER SEA LION

Steller sea lions ( Eumetopias jubatus ) in the central and western Gulf of Alaska, Aleutian Islands, and Bering Sea have declined by 80% in the last 30 yr. One hypothesis for the decline in this western Steller sea lion population is that a climate regime shift in 1976–1977 changed the species composition of the fish community and reduced the nutritional quality (energy density) of the sea lion prey field. This in turn led to nutritional stress and reduced individual fitness, survival, and reproduction of sea lions. Implications of this regime shift-"junk food" hypothesis are that (1) the recruitment and abundance of supposed high quality species ( e.g. , Pacific herring, Clupea pallasi ) decreased while those of supposed low quality ( e.g. , species in the family Gadidae) increased following the regime shift, (2) Steller sea lion diets shifted in response to this change in fish community structure, and (3) a diet composed principally of gadids ( e.g. , walleye pollock, Theragra chalcogramma ) is detrimental to sea lion fitness and survival. We examine data relating to each of these implications and find little support for the hypothesis that increases in the availability and consumption of gadids following the regime shift are primarily responsible for the decline of the western population of Steller sea lion.     

Impact of changing diet regimes on Steller sea lion body condition

A leading theory for the cause of the decline of Steller sea lions is nutritional stress, which led to chronic high juvenile mortality and possibly episodic adult mortality. Nutritional stress may have resulted from either poor quality or low abundance of prey. The objective of this study was to determine whether we could predict shifts in body condition (i.e., body mass or body fat content) over different seasons associated with a change in diet (i.e., toward lower quality prey). Captive Steller sea lions (n= 3) were fed three different diet regimes, where Diet 1 approximated the diet in the Kodiak area in the 1970s prior to the documented decline in that area, Diet 2 approximated the species composition in the Kodiak area after the decline had begun, and Diet 3 approximated the diet in southeast Alaska where the Steller sea lion population has been increasing for over 25 yr. All the animals used in this study were still growing and gained mass regardless of diet. Body fat (%) varied between 13% and 28%, but was not consistently high or low for any diet regime or season. Mean intake (in kg) of Diet 2 was significantly greater for all sea lions during all seasons. All animals did, however, tend to gain less body mass on Diets 2 and 3, as well as during the breeding and postbreeding seasons. They also tended to gain more mass during the winter and on Diet 1, though these differences were not statistically significant. Thus, changing seasonal physiology of Steller sea lions appears to have more impact on body condition than quality of prey, provided sufficient quantity of prey is available. Steller sea lions are opportunistic predators and are evidently able to thrive on a variety of prey. Our results indicate that Steller sea lions are capable of compensating for prey of low quality.     

Linking seasonal distribution patterns with prey availability in a central-place forager, the Steller sea lion

Aim We used a novel approach to infer foraging areas of a central‐place forager, the Steller sea lion (Eumetopias jubatus), by assessing changes in the temporal and spatial distribution patterns of sea lions at terrestrial sites. Specifically, our objectives were (1) to classify seasonal distribution patterns of Steller sea lions and (2) to determine to what extent the seasonal distribution of Steller sea lions is explained by seasonal concentrations of prey. Location Southeast Alaska, USA. Methods Steller sea lions of all age classes were counted monthly (2001–04) by aerial surveys at 28 terrestrial sites. Hierarchical cluster analysis and principal components analysis were used to classify seasonal distribution patterns of Steller sea lions at these terrestrial sites. We estimated the proportion of sea lions in the study area that were associated with each seasonal distribution pattern. Results Multivariate ordination techniques revealed four distinct seasonal distributional patterns. During December, 55% of the sea lions in the study area were found at Type 1 sites, located near over‐wintering herring aggregations. During May, 56% of sea lions were found at Type 2 sites, near aggregations of spring‐spawning forage fish. In July, 78% of sea lions were found at Type 3 sites, near summer migratory corridors of salmon. During September, 44% of sea lions were found at Type 4 sites, near autumn migratory corridors of salmon. Main conclusions Seasonal attendance patterns of sea lions were commonly associated with the seasonal availability of prey species near terrestrial sites and reflected seasonal foraging patterns of Steller sea lions in Southeast Alaska. A reasonable annual foraging strategy for Steller sea lions is to forage on herring (Clupea pallasii) aggregations in winter, spawning aggregations of forage fish in spring, salmon (Oncorhynchus spp.) in summer and autumn, and pollock (Theragra chalcogramma) and Pacific hake (Merluccius productus) throughout the year. The seasonal use of haulouts by sea lions and ultimately haulout‐specific foraging patterns of Steller sea lions depend in part upon seasonally available prey species in each region.     

Examining the potential for nutritional stress in young Steller sea lions: physiological effects of prey composition

The effects of high- and low-lipid prey on the body mass, body condition, and metabolic rates of young captive Steller sea lions (Eumetopias jubatus) were examined to better understand how changes in prey composition might impact the physiology and health of wild sea lions and contribute to their population decline. Results of three feeding experiments suggest that prey lipid content did not significantly affect body mass or relative body condition (lipid mass as a percent of total mass) when sea lions could consume sufficient prey to meet their energy needs. However, when energy intake was insufficient to meet daily requirements, sea lions lost more lipid mass (9.16±1.80 kg±SE) consuming low-lipid prey compared with eating high-lipid prey (6.52±1.65 kg). Similarly, the sea lions lost 2.7±0.9 kg of lipid mass while consuming oil-supplemented pollock at maintenance energy levels but gained 5.2±2.7 kg lipid mass while consuming identical energetic levels of herring. Contrary to expectations, there was a 9.7±1.8% increase in metabolism during mass loss on submaintenance diets. Relative body condition decreased only 3.7±3.8% during periods of imposed nutritional stress, despite a 10.4±4.8% decrease in body mass. These findings raise questions regarding the efficacy of measures of relative body condition to detect such changes in nutritional status among wild animals. The results of these three experiments suggest that prey composition can have additional effects on sea lion energy stores beyond the direct effects of insufficient energy intake.     

Assessment of Competition between Fisheries and Steller Sea Lions in Alaska Based on Estimated Prey Biomass,Fisheries Removals and Predator Foraging Behaviour

A leading hypothesis to explain the dramatic decline of Steller sea lions (Eumetopias jubatus) in western Alaska during the latter part of the 20th century is a change in prey availability due to commercial fisheries. We tested this hypothesis by exploring the relationships between sea lion population trends, fishery catches, and the prey biomass accessible to sea lions around 33 rookeries between 2000 and 2008. We focused on three commercially important species that have dominated the sea lion diet during the population decline: walleye pollock, Pacific cod and Atka mackerel. We estimated available prey biomass by removing fishery catches from predicted prey biomass distributions in the Aleutian Islands, Bering Sea and Gulf of Alaska; and modelled the likelihood of sea lions foraging at different distances from rookeries (accessibility) using satellite telemetry locations of tracked animals. We combined this accessibility model with the prey distributions to estimate the prey biomass accessible to sea lions by rookery. For each rookery, we compared sea lion population change to accessible prey biomass. Of 304 comparisons, we found 3 statistically significant relationships, all suggesting that sea lion populations increased with increasing prey accessibility. Given that the majority of comparisons showed no significant effect, it seems unlikely that the availability of pollock, cod or Atka mackerel was limiting sea lion populations in the 2000s.     

Steller sea lion spatial‐use patterns derived from a Bayesian model of opportunistic observations

Despite acquisition of a substantial catalog of telemetry data from Steller sea lions (Eumetopias jubatus) over the past two decades, scientists still lack comprehensive regionally explicit knowledge about Steller sea lion habitat use. The Platforms of Opportunity data contain records of Steller sea lion sightings throughout the species’ entire range and have potential to fill gaps in knowledge about their spatial use; however, the data have not previously been used because effort (e.g., time spent surveying or area sampled) was not recorded when sightings were obtained. For this study a novel approach was used to overcome the lack of effort data through development of an effort index and a Bayesian negative binomial model. The model quantified Steller sea lion encounter rates and associated uncertainty within 15 × 15 km² grid cells across the species’ entire range. Year‐round, as well as breeding and nonbreeding season encounter rates were estimated. The results of this analysis identify several previously undocumented areas of high use by Steller sea lions, indicate that only 37% of Steller sea lion high‐use areas fall within designated critical habitat, and demonstrate that use of depth and distance from shore as indicators of Steller sea lion habitat is contraindicated.     

Population growth and colonization of Steller sea lions in the Glacier Bay region of southeastern Alaska: 1970s–2009

We estimated trends in numbers of Steller sea lions in the Glacier Bay region of the eastern population from the 1970s to 2009. We documented the colonization of several new haul‐outs and the transition of one haul‐out (Graves Rocks) to a rookery, assessed seasonal patterns in distribution, and compared counts from different observation platforms. Sea lions increased in the region by 8.2%/yr (95%CI = 6.4%–10.0%), with the most growth at South Marble Island in Glacier Bay (16.6%/yr, 1991–2009) and rapid growth in Cross Sound. Seasonal patterns in the distribution of sea lions were likely influenced by new breeding opportunities and the seasonal availability of prey. Factors that likely contributed to the exceptional growth include availability of new habitat following deglaciation, immigration, redistribution, decreases in mortality, and ecosystem‐level changes. The rapid increase in sea lion numbers in this region is of particular interest in light of dramatic declines in the western population and evidence that Steller sea lions from both the eastern and western populations colonized the Graves Rocks rookery. The colonization and rookery development in this dynamic area may signal the reversal of the reproductive isolation of the two populations.     

Comparing total body lipid content of young‐of‐the‐year Steller sea lions among regions of contrasting population trends

Steller sea lion (Eumetopias jubatus) young‐of‐the‐year (YOY) are nutritionally dependent upon their dam through the majority of their first year. Several indices of body condition were measured in YOY 1.5–9 mo of age captured in Southeast Alaska (n = 122), the Gulf of Alaska (n = 182), and the Aleutian Islands (n = 38) to test the hypothesis that nutritional stress impacted the ability of adult female Steller sea lions to adequately nourish their late gestation YOY in the central Aleutian Islands in the early 2000s. Body mass (kg) and percent total body lipid content (%TBL) increased with age in all three regions of Alaska that were sampled (P < 0.05). Young‐of‐the‐year 7–9 mo of age were leaner in Southeast Alaska (27.6% ± 1.0%) and Gulf of Alaska (29.5% ± 0.8%) than in the Aleutian Islands (35.7% ± 1.2%, P < 0.001). Condition indices calculated from morphometric measures did not strongly predict the %TBL measured by isotope dilution. The trend for Aleutian Island YOY to have larger body mass and larger body fat reserves are counter to what would be expected if dams were unable to adequately provision their late lactation YOY due to inadequate food availability in the central Aleutian Islands.     

Body mass and composition responses to short-term low energy intake are seasonally dependent in Steller sea lions (Eumetopias jubatus)

Steller sea lions (Eumetopias jubatus) were fed restricted iso-caloric amounts of Pacific herring (Clupea pallasi) or walleye pollock (Theragra chalcogramma) for 8–9 days, four times over the course of a year to investigate effects of season and prey composition on sea lion physiology. At these levels, the sea lions lost body mass at a significantly higher rate during winter (1.6 ± 0.14 kg day⁻¹), and at a lower rate during summer (1.2 ± 0.32 kg day⁻¹). Decreases in body fat mass and standard metabolic rates during the trials were similar throughout the seasons and for both diet types. The majority of the body mass that was lost when eating pollock derived from decreases in lipid mass, while a greater proportion of the mass lost when eating herring derived from decreases in lean tissue, except in the summer when the pattern was reversed. Metabolic depression was not observed during all trials despite the constant loss of body mass. Our study supports the hypothesis that restricted energy intake may be more critical to Steller sea lions in the winter months, and that the type of prey consumed (e.g., herring or pollock) may have seasonally specific effects on body mass and composition.     

Diet of Pacific sleeper shark, a potential Steller sea lion predator, in the north-east Pacific Ocean

Pacific sleeper sharks Somniosus pacificus were captured near Steller sea lion Eumetopias jubatus rookeries during the period when Steller sea lion pups are most vulnerable to Pacific sleeper shark predation (first water entrance and weaning). Analysis of stomach contents revealed that teleosts were the dominant prey in August and cephalopods were the dominant prey in May ( n = 198). Marine mammals were found in 15% of stomachs regardless of season, but no Steller sea lion tissues were detected. Molecular genetic analysis identified grey whale Eschrichtius robustus and harbour seal Phoca vitulina remains in some Pacific sleeper shark stomachs. Most mammals were cetacean and at least 70% of the cetaceans were probably scavenged. Although Pacific sleeper shark and Steller sea lion ranges overlapped, so predation could potentially occur, the diet study suggested that predation on Steller sea lions is unlikely, at least when pups first enter the water or during weaning. Harbour seals were infrequent prey and may have been consumed alive. Pacific sleeper sharks consume fast-swimming prey like Pacific salmon Oncorhynchus sp., most likely live animals rather than scavenged animals. Pacific sleeper sharks appeared to be opportunistic consumers of the available prey and carrion, feeding both on the bottom and in the water column, and their diet shifted to teleosts and cetacean carrion as the fish grew larger.     

Depth and movement behaviour of the Pacific sleeper shark in the north-east Pacific Ocean

Satellite-linked radio telemetry was used to study the geographic movements and vertical movement behaviour of the Pacific sleeper shark Somniosus pacificus . The fish were tagged near Steller sea lion Eumetopias jubatus rookeries in the Gulf of Alaska during periods when Steller sea lions pups were most vulnerable to predation; when Steller sea lion pups first enter the water (July to August) and when Steller sea lion pups are weaned (April to May). Final locations recovered from most Pacific sleeper sharks (76%) were within 100 km of release locations, 16% were within 100–250 km and 8% were within 250–500 km. The most striking behavioural feature was their extensive, nearly continuous vertical movements. Median daily depth range was 184 m; the most time (61%) was spent between 150 and 450 m, but ascents above 100 m were common (58% of days). Median vertical movement rate was 6 km day⁻¹ and steady. The longest period of continuous vertical movement (> 60 m h⁻¹) was 330 h. Systematic vertical oscillations were most common (60%), followed by diel vertical migrations (25%) and irregular vertical movements (15%). The Pacific sleeper sharks travelled below the photic zone during the day and approached the surface at night. Pacific sleeper sharks appear to employ a stealth and ambush hunting strategy that incorporates slow vertical oscillations to search for prey, and cryptic colouration and cover of darkness to avoid detection by potential prey. The depth and geographic range of Pacific sleeper shark and Steller sea lions overlap near four important Steller sea lion rookeries in the northern Gulf of Alaska, so the potential exists for predation to occur. None of the tissues in the stomachs of the 198 Pacific sleeper sharks collected during a companion diet study, however, were identified as Steller sea lion.     

Characterization of eight microsatellite loci in Steller sea lions (Eumetopias jubatus)

Steller sea lions (Eumetopias jubatus) are listed as an endangered species in western Alaska and have exhibited a significant population decline throughout their range. Eight microsatellite loci were isolated from genomic DNA libraries. In addition, all these markers were found to be variable in nine individuals of the California sea lion (Zalophus californicus). This panel of markers was developed to analyse population structure in Steller sea lions throughout their range.     

ASSESSING KILLER WHALE PREDATION ON STELLER SEA LIONS FROM FIELD OBSERVATIONS IN KENAI FJORDS, ALASKA

The behavioral and predatory patterns of Gulf of Alaska (GOA) transient killer whales ( Orcinus orca ) were studied between 2000 and 2005 using remote video and vessel-based observations near the Chiswell Island Steller sea lion ( Eumetopias jubatus ) rookery and in the broader Kenai Fjords (KF) region of the northern GOA. GOA transient killer whales were observed on 118 d over the 6-yr period; the median group size was two (range: 1–9). Nine predation events were observed from vessels and an additional sixteen were inferred from remote video studies; all involved Steller sea lions. Estimates from field observations suggest that fifty-nine sea lions were consumed over the summer seasons of 2002–2005; whereas estimates based on published caloric requirements of transient killer whales would suggest a loss of 103 sea lions over the same time period. GOA transients spent a large proportion (43%) of their time resting which may be a strategy for conserving energy. Predation on sea lion pups at the Chiswell Island rookery was greatest during years when a single killer whale was foraging alone and when a 1.5-yr-old calf was evidently being trained to handle prey. Predation on pups was low during years when killer whales were foraging in groups and were observed and presumed to be taking mostly juvenile sea lions. Our study suggests that GOA transients are having a minor effect on the recovery of Steller sea lions in the GOA.     

Organochlorine exposure and health effects in stranded Steller sea lions (Eumetopias jubatus) and Pacific harbor seals (Phoca vitulina richardii) from Oregon and southern Washington coasts

Organochlorine (OC) pesticides and polychlorinated biphenyls (PCBs) have been detected in a variety of marine mammal species at levels associated with adverse health effects. Little is known about OC levels and impacts on health in pinnipeds with different life histories. We determined the health and levels of 18 OC pesticides and 16 PCB congeners in blubber samples from 20 Steller sea lions and 39 Pacific harbor seals stranded from Oregon and Southern Washington. The most commonly detected OC at the highest concentration was p,p′- dichlorodiphenyldichloroethylene (DDE). PCBs were detected in all samples as well. Hypothesis testing indicated that diseased Steller sea lions (males and females combined) had higher contaminant concentrations than healthy Steller sea lions, and diseased Pacific harbor seals had higher concentrations of total OCs than healthy animals. Differences were also noted between diseased and healthy animals when looking at individual sexes of each species. Diseased Steller sea lions had higher mean contaminant levels than diseased harbor seals and healthy Steller sea lions had higher mean contaminant concentrations than healthy Pacific harbor seals. These results show that species differences exist in both contaminant loads and sensitivity to contaminants, which may be due to differences in life histories and physiology.     

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The role of Steller sea lions in a large population decline of harbor seals

We provide the first direct evidence that Steller sea lions will prey on harbor seals. Direct observations of predation on marine mammals at sea are rare, but when observed rates of predation are extrapolated, predation mortality may be found to be significant. From 1992 to 2002, harbor seals in Glacier Bay declined steeply, from 6,200 to 2,500 (∼65%). After documenting that Steller sea lions were preying on seals in Glacier Bay, we investigated increased predation by sea lions as a potential explanation for the large decline. In five independent data sets spanning 21–25 yr and including 14,308 d of observations, 13 predation events were recorded. We conducted a fine-scale analysis for an intensively studied haul-out (Spider Island) and a broader analysis of all of Glacier Bay. At Spider Island, estimated predation by sea lions increased and could account for the entirety of annual pup production in 5 of 8 yr since 1995. The predation rate, however, was not proportional to the number of predators. Predation by Steller sea lions is a new source of mortality that contributed to the seal declines; however, life history modeling indicates that it is unlikely that sea lion predation is the sole factor responsible for the large declines.     

Temporal records of δ13C and δ15N in North Pacific pinnipeds: inferences regarding environmental change and diet

Sea lion and seal populations in Alaskan waters underwent various degrees of decline during the latter half of the twentieth century and the cause(s) for the declines remain uncertain. The stable carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) isotope ratios in bone collagen from wild Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus) and harbor seals (Phoca vitulina) from the Bering Sea and Gulf of Alaska were measured for the period 1951-1997 to test the hypothesis that a change in trophic level may have occurred during this interval and contributed to the population declines. A significant change in '¹⁵N in pinniped tissues over time would imply a marked change in trophic level. No significant change in bone collagen '¹⁵N was found for any of the three species during the past 47 years in either the Bering Sea or the Gulf of Alaska. However, the ¹⁵N in the Steller sea lion collagen was significantly higher than both northern fur seals and harbor seals. A significant decline in '¹³C (almost 2 ‰ over the 47 years) was evident in Steller sea lions, while a declining trend, though not significant, was evident in harbor seals and northern fur seals. Changes in foraging location, in combination with a trophic shift, may offer one possible explanation. Nevertheless, a decrease in '¹³C over time with no accompanying change in '¹⁵N suggests an environmental change affecting the base of the foodweb rather than a trophic level change due to prey switching. A decline in the seasonal primary production in the region, possibly resulting from decreased phytoplankton growth rates, would exhibit itself as a decline in '¹³C. Declining production could be an indication of a reduced carrying capacity in the North Pacific Ocean. Sufficient quantities of optimal prey species may have fallen below threshold sustaining densities for these pinnipeds, particularly for yearlings and subadults who have not yet developed adequate foraging skills.