Health status of young Alaska Steller sea lion pups (Eumetopias jubatus) as indicated by blood chemistry and hematology

Blood chemistry and hematology were examined in 238 Steller sea lion pups (Eumetopias jubatus) to assess the health status of pups <1 month of age. Failure of juvenile recruitment (possibly due to nutritionally or physiologically compromised pups) into breeding populations has been proposed as a cause of recent declines of this endangered species in Alaska. To identify potential correlations with areas of high population decline, blood chemistry data were considered for three areas: eastern Aleutian Islands (low rates of population decline to stable populations), Gulf of Alaska (high rates of decline), and Southeast Alaska (stable to increasing population). Southeast Alaska pups showed elevated ketone body concentrations (β-hydroxybutyrate, (β-HBA)) and depressed glucose levels when compared with animals from the Aleutian Islands and lower blood urea nitrogen (BUN) and glucose levels than pups in the Gulf of Alaska. Over 40% of the pups from Southeast Alaska had elevated β-HBA concentrations suggesting they underwent longer periods of fasting than seen in pups from other areas. Hematocrit (Hct), hemoglobin concentration (Hb) and water content of the blood exhibited typical mammalian relationships. In summary, blood chemistry and hematology data showed no indication that Steller sea lion pups <1 month old from areas of population decline were nutritionally compromised.     

Inter-Population Movements of Steller Sea Lions in Alaska with Implications for Population Separation

Genetic studies and differing population trends support the separation of Steller sea lions (Eumetopias jubatus) into a western distinct population segment (WDPS) and an eastern DPS (EDPS) with the dividing line between populations at 144° W. Despite little exchange for thousands of years, the gap between the breeding ranges narrowed during the past 15–30 years with the formation of new rookeries near the DPS boundary. We analyzed >22,000 sightings of 4,172 sea lions branded as pups in each DPS from 2000–2010 to estimate probabilities of a sea lion born in one DPS being seen within the range of the other DPS (either ‘West’ or ‘East’). Males from both populations regularly traveled across the DPS boundary; probabilities were highest at ages 2–5 and for males born in Prince William Sound and southern Southeast Alaska. The probability of WDPS females being in the East at age 5 was 0.067 but 0 for EDPS females which rarely traveled to the West. Prince William Sound-born females had high probabilities of being in the East during breeding and non-breeding seasons. We present strong evidence that WDPS females have permanently emigrated to the East, reproducing at two ‘mixing zone’ rookeries. We documented breeding bulls that traveled >6,500 km round trip from their natal rookery in southern Alaska to the northern Bering Sea and central Aleutian Islands and back within one year. WDPS animals began moving East in the 1990s, following steep population declines in the central Gulf of Alaska. Results of our study, and others documenting high survival and rapid population growth in northern Southeast Alaska suggest that conditions in this mixing zone region have been optimal for sea lions. It is unclear whether eastward movement across the DPS boundary is due to less-optimal conditions in the West or a reflection of favorable conditions in the East.     

Parasites of forage fishes in the vicinity of Steller sea lion (Eumetopias jubatus) habitat in Alaska

Fish serve as intermediate hosts for a number of larval parasites that have the potential of maturing in marine mammals such as Steller sea lions (Eumetopias jubatus). We examined the prevalence of parasites from 229 fish collected between March and July 2002 near two islands used by Steller sea lions in Southeast Alaska and island habitats in the Aleutian Islands. Sea lion populations have remained steady in Southeast Alaska but have been declining over the last 30 yr in the Aleutian Islands. Even though the fish samples near the Southeast Alaska haul-outs were composed of numerous small species of fish and the Aleutian Islands catch was dominated by juveniles of commercially harvested species, the parasite fauna was similar at all locations. Eleven of the 20 parasite taxa identified were in their larval stage in the fish hosts, several of which have been described from mammalian final hosts. Four species of parasite were more prevalent in Southeast Alaska fish samples, and seven parasite species, including several larval forms capable of infecting marine mammals, were more prevalent in fish from the Aleutian Islands. Nevertheless, parasites available to Steller sea lions from common fish prey are not likely to be a major factor in the decline of this marine mammal species.     

Infectious disease and the decline of Steller sea lions (Eumetopias jubatus) in Alaska, USA: insights from serologic data

Serologic data were examined to determine whether infectious disease may have played a role in the decline of Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska and Aleutian Islands, USA. Available published data, unpublished data, and recent collections (1997-2000) were compared and reviewed. Data were stratified by geography to compare the declining western Alaskan population in the Aleutian Islands through eastern Prince William Sound to the increasing population in southeastern Alaska. Prevalences of antibodies from the 1970s to the early 1990s were noted for Leptospira interrogans, Chlamydophila psittaci, Brucella spp., phocid herpesvirus-1, and calciviruses. Serum samples collected from 1997-2000 were tested for antibodies to these agents as well as to marine mammal morbilliviruses, canine parvovirus, and canine adenovirus-1 and -2. Conclusions could not be drawn about changes in antibody prevalence to these agents during the decline of Steller sea lions, however, because data were incomplete or not comparable as a result of inconsistencies in testing techniques. Despite these shortcomings, results provided no convincing evidence of significant exposure of Steller sea lions to morbilliviruses, Brucella spp., canine parvovirus, or L. interrogans. Steller sea lions have been exposed to phocid herpesviruses, caliciviruses, canine adenovirus, and C. psittaci or to cross-reactive organisms in regions of both increasing and decreasing sea lion abundance. Based on similar antibody prevalence estimates from the increasing and decreasing populations, these agents are unlikely to have been the primary cause of the population decline. They may have contributed to the decline or impeded population recovery, however, because of undetected mortality and morbidity or reductions of fecundity and body condition in animals under other stresses. Systematic monitoring for disease agents and their effects is needed to determine whether infectious disease currently plays a role in the decline and lack of recovery of Steller sea lions.     

Differential Expression of Immune Response Genes in Steller Sea Lions (Eumetopias jubatus): An Indicator of Ecosystem Health?

Characterization of the polygenic and polymorphic features of the Steller sea lion major histocompatibility complex (MHC) provides an ideal window for evaluating immunologic vigor of the population and identifying emergence of new genotypes that reflect ecosystem pressures. MHC genotyping can be used to measure the potential immunologic vigor of a population. However, since ecosystem-induced changes to MHC genotype can be slow to emerge, measurement of differential expression of these genes can potentially provide real-time evidence of immunologic perturbations. MHC DRB genes were cloned and sequenced using peripheral blood mononuclear leukocytes derived from 10 Steller sea lions from Southeast Alaska, Prince William Sound, and the Aleutian Islands. Nine unique DRB gene sequences were represented in each of 10 animals. MHC DRB gene expression was measured in a subset of six sea lions. Although DRB in genomic DNA was identical in all individuals, relative levels of expressed DRB mRNA was highly variable. Selective suppression of MHC DRB genes could be indicative of geographically disparate environmental pressures, thereby serving as an immediate and sensitive indicator of population and ecosystem health.     

Health status of young Alaska Steller sea lion pups (Eumetopias jubatus) as indicated by blood chemistry and hematology

Blood chemistry and hematology were examined in 238 Steller sea lion pups (Eumetopias jubatus) to assess the health status of pups <1 month of age. Failure of juvenile recruitment (possibly due to nutritionally or physiologically compromised pups) into breeding populations has been proposed as a cause of recent declines of this endangered species in Alaska. To identify potential correlations with areas of high population decline, blood chemistry data were considered for three areas: eastern Aleutian Islands (low rates of population decline to stable populations), Gulf of Alaska (high rates of decline), and Southeast Alaska (stable to increasing population). Southeast Alaska pups showed elevated ketone body concentrations (beta-hydroxybutyrate,(beta-HBA)) and depressed glucose levels than pups in the Gulf of Alaska. Over 40% of the pups from Southeast Alaska had elevated beta-HBA concentrations suggesting they underwent longer periods of fasting than seen in pups from other areas. Hematocrit (Hct), hemoglobin concentration (Hb) and water content of the blood exhibited typical mammalian relationships. In summary, blood chemistry and hematology data showed no indication that Steller sea lion pups <1 month old from areas of population decline were nutritionally compromised.     

DEVELOPMENT OF DISPERSAL, MOVEMENT PATTERNS, AND HAUL-OUT USE BY PUP AND JUVENILE STELLER SEA LIONS (EUMETOPIAS JUBATUS) IN ALASKA

Population declines of Steller sea lions ( Eumetopias juhatus ) in western Alaska (west of 144°W) may be a result of reduced juvenile survival. We used satellite telemetry to study the at-sea distribution and movement patterns of pup (1.6–11.9 mo) and juvenile (12.0–35.1 mo) Steller sea lions. We studied trip distance, duration, and interhaul-out movements of sea lions in relation to age, sex, and month of year in the decreasing western population (WP; Prince William Sound, Kodiak, Aleutian Islands, Alaska) and the increasing eastern population (EP; Southeast Alaska). We deployed 103 satellite transmitters (29 WP; 74 EP) on sea lions between 1998 and 2001. Round trip distance and duration increased with age, trip distance was greater in the WP than the EP, trip duration was greater for females than males, and haul-out use was clustered. Changes in round trip distance and duration occurred from April to June for all age classes studied indicating that the annual timing of weaning may be less variable than the age of weaning. Overall, 90% of round trips were ≤ 15 km from haul-outs and 84% were <20 h, indicating nearshore areas adjacent to haulouts are critical to the developing juvenile.     

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.     

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.     

Predation on an upper trophic marine predator, the Steller sea lion: evaluating high juvenile mortality in a density dependent conceptual framework

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

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.     

REDUCED BODY SIZE OF FEMALE STELLER SEA LIONS FROM A DECLINING POPULATION IN THE GULF OF ALASKA

Nutritional stress is a leading hypothesis behind the decline in numbers of Steller sea lions in the Gulf of Alaska, the Aleutian Islands, and the Bering Sea. To evaluate this hypothesis we compared body growth of female Steller sea lions 1.0–13.9 yr of age collected in the Gulf of Alaska during two time periods, 1975–1978 just prior to or early in the decline and 1985–1986 when the decline was well established. We found that growth, as measured by standard length, axillary girth, and mass, was reduced during the 1980s, supporting the undernutrition hypothesis. We also found a suggestion of reduced growth in our 1970s and 1980s samples when compared to a collection of Steller sea lions obtained from the Gulf of Alaska in 1958. However, no direct link has been demonstrated between undernutrition and the actual decline in numbers.     

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.     

STELLER SEA LION STATUS AND TREND IN SOUTHEAST ALASKA: 1979–1997

Steller sea lion (Eumetopias jubatus) numbers in the United States declined by about 75% over the past 20+ yr. They are classified, under the U. S. Endangered Species Act, as “threatened” in the eastern portion of their range and as “endangered” in the western portion. We analyzed trends in numbers of pup and non-pup Steller sea lions counted in Southeast Alaska between 1979 and 1997. Sea lion numbers, based on counts of pups on rookeries, increased by an average of 5.9% per year between 1979 and 1997. However, numbers of pups increased at a much slower rate (+ 1.7% per year) between 1989 and 1997. For counts of non-pup Steller sea lions we used models that controlled for the effects of date, time, and tide at the time of the survey to analyze trends. This technique reduced bias and increased precision of the resulting trend estimates. Numbers of sea lions were stable (+0.5%) between 1989 and 1996, based on counts of non-pups. We estimated the Southeast Alaska breeding population of Steller sea lions at about 19,000 animals of all ages in 1997, a level that is probably near the highest in recorded history.     

Spatial patterns and scaling behaviors of Steller sea lion (Eumetopias jubatus) distributions and their environment

Fractal geometry and other multi-scale analyses have become popular tools for investigating spatial patterns of animal distributions in heterogeneous environments. In theory, changes in patterns of animal distributions with changes in scale reflect transitions between the controlling influences of one environmental factor or process over another. In an effort to find linkages between Steller sea lions (Eumetopias jubatus) and their environment, the objective of this study was to determine if the spatial distribution of Steller sea lions at sea displayed similar scaling properties to the variation of two environmental features, including bathymetry and sea surface temperature (SST). Additionally, distributions of Steller sea lion point patterns were examined with respect to measurements of bathymetric complexity. From February 2000 to May 2004, satellite transmitters were deployed on 10 groups of juvenile Steller sea lions (n=52) at eight different locations within the Aleutian Islands and Gulf of Alaska. Indices of fractal dimension were calculated for each group of sea lions using a unit square box-counting method, whereas indices of bathymetry and SST patchiness were derived by conducting a variance ratio analysis over the same scales. Distributions of Steller sea lions at sea displayed self-similar fractal patterns, suggesting that individuals were distributed in a continuous hierarchical set of clumps within clumps across scales, and foraging behavior was likely influenced by a scale invariant mechanism. Patterns of bathymetric variability also were self-similar, whereas patterns of SST variability were scale dependent and failed to retain self-similar spatial structure at larger scales. These results indicate that the distributions of Steller sea lions at sea were more influenced by bathymetry than SST at the scales examined, but scale-dependent patterns in the distribution of Steller sea lions at sea or linkages with SST may have been apparent if analyses were conducted at finer spatial scales.     

RISK OF EXTIRPATION OF STELLER SEA LIONS IN THE GULF OF ALASKA AND ALEUTIAN ISLANDS: A POPULATION VIABILITY ANALYSIS BASED ON ALTERNATIVE HYPOTHESES FOR WHY SEA LIONS DECLINED IN WESTERN ALASKA

We estimated the risk that the Steller sea lion will be extirpated in western Alaska using a population viability analysis (PVA) that combined simulations with statistically fitted models of historical population dynamics. Our analysis considered the roles that density‐dependent and density‐independent factors may have played in the past, and how they might influence future population dynamics. It also established functional relationships between population size, population growth rate and the risk of extinction under alternative hypotheses about population regulation and environmental variability. These functional relationships can be used to develop recovery criteria and guide research and management decisions. Life table parameters (e.g., birth and survival rates) operating during the population decline (1978–2002) were estimated by fitting simple age‐structured models to time‐series of pup and non‐pup counts from 33 rookeries (subpopulations). The PVA was carried out by projecting all 33 subpopulations into the future using these estimated site‐specific life tables (with associated uncertainties) and different assumptions about carrying capacities and the presence or absence of density‐dependent population regulation. Results suggest that the overall predicted risk of extirpation of Steller sea lions as a species in western Alaska was low in the next 100 yr under all scenarios explored. However, most subpopulations of Steller sea lions had high probabilities of going extinct within the next 100 yr if trends observed during the 1990s were to continue. Two clusters of contiguous subpopulations occurring in the Unimak Pass area in the western Gulf of Alaska/eastern Aleutian Islands and the Seguam–Adak region in the central Aleutian Islands had relatively lower risks of extinction. Risks of extinction for a number of subpopulations in the Gulf of Alaska were reduced if the increases observed since the late 1990s continue into the future. The risks of subpopulations going extinct were small when density‐dependent compensation in birth and survival rates was assumed, even when random stochasticity in these vital rates was introduced.     

FORAGING STRATEGIES OF SYMPATRIC KILLER WHALE (ORCINUS ORCA) POPULATIONS IN PRINCE WILLIAM SOUND, ALASKA

Killer whales ( Orcinus orca ) feed on a wide variety of fish, cephalopods, and marine mammals throughout their cosmopolitan range; however, the dietary breadth that characterizes the species is not reflected in all populations. Here, we present the findings of a 14-yr study of the diet and feeding habits of killer whales in Prince William Sound, Alaska. Two non-associating forms of killer whale, termed resident and transient (Bigg et al. 1987), were identified. All prey seen taken by transients were marine mammals, including harbor seals ( Phoca vitulina ), Dall's porpoises ( Phocoenoides dalli ), Steller sea lions ( Eumetopias jubatus ), and harbor porpoises ( Phocoena phocoena ). Resident killer whales appeared to prey principally on salmon ( Oncorhynchus spp.), preferring coho salmon ( O. kisutch ) over other, more abundant salmon species. Pacific herring ( Clupea pallasi ) and Pacific halibut ( Hippocampus stenolepis ) were also taken. Resident killer whales frequently were seen to interact in non-predatory ways with Steller sea lions and Dall's porpoises, while transients were not. Differences in the social organization and behavior of the resident and transient killer whales in Prince William Sound are discussed in the light of the dietary differences documented here.     

THE RELIABILITY OF SKINFOLD-CALIPERS FOR MEASURING BLUBBER THICKNESS OF STELLER SEA LION PUPS (EUMETOPIAS JUBATUS)

Twelve dead Steller sea lion pups ( Eumetopias jubatus ) aged 3-14 d were recovered from rookeries in Southeast Alaska. They had a wide range of body sizes and conditions (small to large and fat to no fat). The ability of calipers to estimate the thickness of their blubber layer was assessed with a set of skinfold calipers. Average error of measurement for skin and blubber thickness was an acceptable 5.4%, but the skin and blubber of the pups were highly compressible. Skinfold thickness increased with body mass but did not necessarily reflect the development of blubber, given that pups with no blubber also showed an increase in skinfold thickness with increases in body mass. Skinfold thickness of sea lion pups appears to predict body size better than it predicts blubber thickness, making it difficult if not impossible to develop a simple index of body condition or a calculation of percent body fat for Steller sea lion pups from skinfold caliper measurements.     

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.     

The effects of age, season and geographic region on thyroid hormones in Steller sea lions (Eumetopias jubatus)

The purpose of this study was to investigate thyroid hormone concentrations, thyroxine (T4) and triiodothyronine (T3), in order to determine basal levels in Steller sea lions of different ages and over seasons. Serum concentrations of total T4 were highest in Steller sea lions followed by total T3 concentrations. Concentrations of free T4 and free T3 were three to four orders of magnitude lower. Concentrations for all four thyroid hormone measurements tended to a lower level as animals matured beyond the neonatal stage. When thyroid hormones from captive sea lions were evaluated across seasons, all thyroid hormones were highest in the July to September period. When compared across the geographic range, animals in southeast Alaska tended to have lower thyroid hormone levels, while the Steller sea lions west of Prince William Sound and animals from the Russian Far East had significantly higher concentrations. Significant inter-annual differences in concentrations were also observed across the geographic range. With an understanding of the basic changes in thyroid hormone concentrations, changes in plane of nutrition or life history states (i.e. fasting, pregnancy or lactation) can now be evaluated for their effect on the overall health of this endangered species.