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.     

INCIDENTAL MORTALITY OF NORTHERN SEA LIONS IN SHELIKOF STRAIT, ALASKA

The incidental catch of northern sea lions ( Eumetopias jubatus ) in the walleye pollock ( Theragra chalcogramma ) joint-venture fishery in Shelikof Strait, Alaska, was studied during 1982–1984 to assess the nature and magnitude of the catch. Data were obtained by placing U.S. observers on foreign processing vessels. Dead sea lions recovered from trawl nets were counted, sexed and measured, teeth were removed for age determination by dental laminae; and stomach contents were analyzed. Although the fishery has continued to expand both in number of boats and estimated total catch (74,136 metric tons [t] in 1982 to 171,539 t in 1984), the estimated incidental catch of northern sea lions has declined (ranging from 958 to 1,436 in 1982, 216 to 324 in 1983 and 237 to 355 in 1984). Of the sea lions processed, 73 percent were caught between 2000 and 0500 h, probably during net retrieval. Most caught sea lions were females ranging in age from 1–25 yr with a mean age of 6.43 yr; 79 percent of the females were sexually mature and probably part of the reproducing population. Males had a mean age of 4.8 yr and only 12 percent were old enough to obtain and defend territories. Analysis of stomach contents showed that the sea lions consumed pollock the same size as that taken by the commercial fishery. The impact of the incidental catch on the Gulf of Alaska sea lion population is unknown.     

Effects of research disturbance on the behavior and abundance of Steller sea lions (Eumetopias jubatus) at two rookeries in Alaska

We examined the effects of research disturbance on the behavior and abundance of Steller sea lions (Eumetopias jubatus) at rookeries on Marmot and Ugamak Islands in Alaska. During 3 of 6 yr, researchers intentionally drove all adult and juvenile sea lions off at least part of the beach in order to permanently mark and measure sea lion pups. The research disturbance occurred after the majority of females had bred and when most pups were 1 mo old. We used generalized linear models to determine the relationship between research disturbance and sea lion behavior or abundance. Research disturbance was related to changes in the proportion of sea lions exhibiting two to three of nine behavior metrics: agonistic and resting females and active males at Marmot, and active and resting males and females at Ugamak. Model results indicated that changes lasted between 3 and 20 d depending on the sex, behavior, and rookery. Inclusion of research disturbance into Marmot abundance models did not improve the fit to the data, if variability between years was permitted. Optimally timed, low‐frequency research disturbance did not appear to have long‐term effects on sea lion behavior or abundance and was largely associated with changes that were similar to natural variation.     

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.     

RANGE-WIDE SURVEY AND ESTIMATION OF TOTAL NUMBER OF STELLER SEA LIONS IN 1989

Abstract: The first range-wide survey of Steller (northern) sea lions ( Eumetopias jubatus ) was completed in 1989 with a total of 68,094 adult and juvenile (nonpup) Steller sea lions counted. This total count includes 10,000 in Russia (15% of the range-wide count), 47,960 in Alaska (70%), 6,109 in British Columbia (9%), 2,261 in Oregon (3%), and 1,764 in California (3%). A range-wide pup count was not obtained. We estimated the 1989 world population based on a calculation for total pups and obtained a range-wide estimate of 116,000 total animals, or about 39–48% of the 240,000-300,000 estimated 30 yr ago.     

Population Trend and Elasticities of Vital Rates for Steller Sea Lions (Eumetopias jubatus) in the Eastern Gulf of Alaska: A New Life-History Table Analysis

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.     

DISPERSAL, ROOKERY FIDELITY, AND METAPOPULATION STRUCTURE OF STELLER SEA LIONS (EUMETOPIAS JUBATUS) IN AN INCREASING AND A DECREASING POPULATION IN ALASKA

Over the past 24 yr, 8,596 Steller sea lion ( Eumetopias jubatus ) pups were branded on their natal rookeries throughout Alaska with the objectives of determining survival rates, recruitment, movements, and site fidelity. Our objectives here were to examine the extent of dispersal of Steller sea lions away from their natal rookeries, movements between stocks, and degree of natal rookery fidelity. Pups (<1 yr old) usually remained within 500 km of their natal rookery. Branded juveniles dispersed widely and were resighted at distances up to 1,785 km from their natal rookeries. Adults generally remained within 500 km of their natal rookeries. No interchange of breeding animals between the ES (eastern stock) and WS (western stock) was observed. Although natal rookery fidelity was prevalent, 33% of the 12 observations of females branded in the WS during 1987–1988 and 19% of the 29 observations of females branded in the ES during 1994–1995 were observed with newly born pups at sites other than their natal rookeries. Steller sea lions generally conformed to the metapopulation concept as depicted by Hanski and Simberloff (1997), with local breeding populations (rookeries) and movements among these local populations having the potential of affecting local dynamics.     

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.     

MAJOR DECLINE IN NUMBER OF HARBOR SEALS, PHOCA VITULINA RICHARDSI, ON TUGIDAK ISLAND, GULF OF ALASKA

Tugidak Island, located in the Gulf of Alaska, was once the site of one of largest local concentrations of harbor seals ( Phoca vitulina richardsi ) in the world. This population, which probably consisted of about 20,500 animals in the mid-1960s declined by about 85% between 1976 and 1988. The population appeared to decline more rapidly during the late 1970s than during the 1980s. Causes for the decline are not apparent. There appear to be both similarities and dissimilarities between this decline and recent declines in abundance of northern fur seals ( Callorhinus ursinus ) and Steller sea lions ( Eumetopias jubatus ) in the Bering Sea and Gulf of Alaska.     

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.     

The Effects of Birth Weight and Maternal Care on Survival of Juvenile Steller Sea Lions (Eumetopias jubatus)

Steller sea lions were listed as endangered following a collapse of the western distinct population beginning in the late 1970s. Low juvenile survival has been implicated as a factor in the decline. I conducted a multistate mark-recapture analysis to estimate juvenile survival in an area of the western population where sea lions are showing signs of recovery. Survival for males and females was 80% between 3 weeks and 1 year of age. Approximately 20% of juveniles continued to be nursed by their mothers between ages 1 and 2 and 10% between ages 2 and 3. Survival for juveniles that suckled beyond 1 year was 88.2% and 89.9% to ages 2 and 3, respectively. In contrast, survival for individuals weaned by age 1 was 40.6% for males and 64.2% for females between ages 1 and 2. Birth mass positively influenced survival for juveniles weaned at age 1 but had little effect on individuals continuing to suckle. Cumulative survival to age 4 was double that estimated during the population decline in this region. Evidence suggests that western Steller sea lions utilize a somewhat different maternal strategy than those in the eastern distinct population. Western adult females generally invest more in their pups during the first year but wean offspring by age 1 more often. This results in better survival to age 1, but greater mortality between ages 1 and 3 compared to the eastern population. Different maternal strategies may reflect density dependent pressures of populations at opposite levels of abundance.     

KILLER WHALES, WHALING, AND SEQUENTIAL MEGAFAUNAL COLLAPSE IN THE NORTH PACIFIC: A COMPARATIVE ANALYSIS OF THE DYNAMICS OF MARINE MAMMALS IN ALASKA AND BRITISH COLUMBIA FOLLOWING COMMERCIAL WHALING

The hypothesis that commercial whaling caused a sequential megafaunal collapse in the North Pacific Ocean by forcing killer whales to eat progressively smaller species of marine mammals is not supported by what is known about the biology of large whales, the ecology of killer whales, and the patterns of ecosystem change that took place in Alaska, British Columbia, and elsewhere in the world following whaling. A comparative analysis shows that populations of seals, sea lions, and sea otters increased in British Columbia following commercial whaling, unlike the declines noted in the Gulf of Alaska and Aleutian Islands. The declines of seals and sea lions that began in western Alaska around 1977 were mirrored by increases in numbers of these species in British Columbia. A more likely explanation is that the seal and sea lion declines and other ecosystem changes in Alaska stem from a major oceanic regime shift that occurred in 1977. Killer whales are unquestionably a significant predator of seals, sea lions, and sea otters—but not because of commercial whaling.     

High Natality Rates of Endangered Steller Sea Lions in Kenai Fjords,Alaska and Perceptions of Population Status in the Gulf of Alaska

Steller sea lions experienced a dramatic population collapse of more than 80% in the late 1970s through the 1990s across their western range in Alaska. One of several competing hypotheses about the cause holds that reduced female reproductive rates (natality) substantively contributed to the decline and continue to limit recovery in the Gulf of Alaska despite the fact that there have been very few attempts to directly measure natality in this species. We conducted a longitudinal study of natality among individual Steller sea lions (n = 151) at a rookery and nearby haulouts in Kenai Fjords, Gulf of Alaska during 2003–2009. Multi-state models were built and tested in Program MARK to estimate survival, resighting, and state transition probabilities dependent on whether or not a female gave birth in the previous year. The models that most closely fit the data suggested that females which gave birth had a higher probability of surviving and giving birth in the following year compared to females that did not give birth, indicating some females are more fit than others. Natality, estimated at 69%, was similar to natality for Steller sea lions in the Gulf of Alaska prior to their decline (67%) and much greater than the published estimate for the 2000s (43%) which was hypothesized from an inferential population dynamic model. Reasons for the disparity are discussed, and could be resolved by additional longitudinal estimates of natality at this and other rookeries over changing ocean climate regimes. Such estimates would provide an appropriate assessment of a key parameter of population dynamics in this endangered species which has heretofore been lacking. Without support for depressed natality as the explanation for a lack of recovery of Steller sea lions in the Gulf of Alaska, alternative hypotheses must be more seriously considered.     

Can we see a cohort effect on survival of Steller sea lions (Eumetopias jubatus) at Kozlova Cape rookery (eastern Kamchatka,Russia)?

Steller sea lions ( Eumetopias jubatus ) have experienced a decline in Russia as well as the United States. The causes for the decline of the Russian population may or may not overlap the causes of the decline in Alaska. The demographics of the Russian population are not well understood and are an area of interest for scientists wishing to compare and contrast the Alaskan and Russian stocks. This study uses a unique observational system to assess survival by age at a small rookery on the Kamchatka Peninsula, Kozlova Cape. We proceeded under the hypothesis that changes in environmental conditions between years would produce noticeable differences in survival of pups born in those years. We found no evidence to support our hypothesis, but did find some differences from the Alaskan population in estimated survival for the juvenile stage class. This study also provides some of the history of Steller sea lions in Russia for context.     

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.     

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.     

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.     

THE LIFE HISTORY OF FREE-RANGING ATLANTIC SPOTTED DOLPHINS (STENELLA FRONTALIS): AGE CLASSES, COLOR PHASES, AND FEMALE REPRODUCTION

Atlantic spotted dolphins (Stenella frontalis) were observed underwater and from the surface from 1985 to 1996 and photographed through successive years. Individuals were categorized into age classes by their degree of spotting and color phases. Dolphins spent an average of 3 yr in the two-tone color phase, 5 yr in the speckled phase, 7 yr in the mottled phase and up to 10 yr or more in the fused phase.
Sex ratios were close to parity, with old adults skewed towards females and juveniles and young adults skewed towards males. The average calving interval for 24 females was 2.96 years with a range of 1–5 yr. Females whose calves survived the first year had a significantly longer calving interval (3.56 years). The ages of first parturition for five females were estimated to be 10–12 yr. The age at sexual maturation was estimated to range from 8 to 15 yr.
Pregnancy rate fluctuated annually, with an average rate of 0.25 (range 0.07–0.57). Annual average birth rate was 0.08 (range 0.07–0.14), average calf production was 0.33 (range 0.06–0.52), average fecundity was 0.23 (range 0.13–0.30), and average recruitment was 0.06 (range 0.03–0.08). Most females who lost a calf conceived the same or following year.
Lactation lasted up to 5 yr, and 45% of visibly pregnant females were also lactating. Age of first parturition was associated with the mottled color phase. Average first-year mortality rate of calves was 0.24.     

Age Specific Survival Rates of Steller Sea Lions at Rookeries with Divergent Population Trends in the Russian Far East

After a dramatic population decline, Steller sea lions have begun to recover throughout most of their range. However, Steller sea lions in the Western Aleutians and Commander Islands are continuing to decline. Comparing survival rates between regions with different population trends may provide insights into the factors driving the dynamics, but published data on vital rates have been extremely scarce, especially in regions where the populations are still declining. Fortunately, an unprecedented dataset of marked Steller sea lions at rookeries in the Russian Far East is available, allowing us to determine age and sex specific survival in sea lions up to 22 years old. We focused on survival rates in three areas in the Russian range with differing population trends: the Commander Islands (Medny Island rookery), Eastern Kamchatka (Kozlov Cape rookery) and the Kuril Islands (four rookeries). Survival rates differed between these three regions, though not necessarily as predicted by population trends. Pup survival was higher where the populations were declining (Medny Island) or not recovering (Kozlov Cape) than in all Kuril Island rookeries. The lowest adult (> 3 years old) female survival was found on Medny Island and this may be responsible for the continued population decline there. However, the highest adult survival was found at Kozlov Cape, not in the Kuril Islands where the population is increasing, so we suggest that differences in birth rates might be an important driver of these divergent population trends. High pup survival on the Commander Islands and Kamchatka Coast may be a consequence of less frequent (e.g. biennial) reproduction there, which may permit females that skip birth years to invest more in their offspring, leading to higher pup survival, but this hypothesis awaits measurement of birth rates in these areas.     

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.