Large group
of sea lions

Population Status

In 1990, the Steller sea lion was listed as a threatened species under the US Endangered Species Act. In 1997, the Steller sea lion was listed as two distinct populations, listing those east of Prince William Sound as threatened, and relisting those to the west as endangered.

The earliest census of Steller sea lions was done in 1956. At that time there were about 250,000 Stellers in Alaska. The population rose to a peak of about 282,000 in the mid 1970s. Since 1980 it has decreased by over 70% (roughly 5% per year) to about 42,000 animals in 2000. Most of the decline took place in the Aleutian Islands and Kodiak region. However, since 1989, the population decline appears to have slowed significantly within two subareas of these large regions -- the eastern Aleutians and western Gulf of Alaska. Increases have been occurring in the smaller populations of southeast Alaska. It is not known why these changes have occurred.

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One way researchers learn about Steller sea lions is to study them at rookery or breeding sites.

A novel presence-only validation technique for improved Steller sea lion Eumetopias jubatus critical habitat descriptions. Gregr, E.J. and A.W. Trites. 2008. Marine Ecology Progress Series 365:247-261. abstract We used published information about foraging behaviour, terrestrial resting sites, bathymetry, and seasonal ocean climate to develop hypotheses relating life history traits and physical variables to the at-sea habitat of a wide-ranging marine predator, the Steller sea lion (Eumetopias jubatus). We used the hypotheses to develop a series of habitat models that predicted the probability of sea lions occurring within 3 x 3 km2 grids overlaid on the Gulf of Alaska and Bering Sea; and compared these deductive model predictions with opportunistic at-sea observations of sea lions (presence-only data) using 1) a likelihood approach in a small area where effort was assumed to be uniformly distributed, and 2) an adjusted skewness (Skadj) test that evaluated the distribution of the predicted values associated with true presence observations. We found the Skadj statistic was comparable to the likelihood test when using pseudo-absence data, but it was more powerful for assessing the relative performance of the different predictive spatial models. We also found that the habitat maps we produced for adult female sea lions using the deductive modelling approach captured a higher proportion of presence observations than the current habitat model (Critical Habitat) used by fisheries managers since 1993 to manage Steller sea lions. Such improved predictions of habitat are necessary to effectively design, implement, and evaluate fishery mitigation measures. The deductive approach we propose is suitable for modelling the habitat use of other age- and sex- classes, and for integrating these age/sex class specific models into a revised definition of Critical Habitat for Steller sea lions. It can also be readily used to identify the at-sea habitat of other central place foragers.
Quantification of terrestrial haul-out and rookery characteristics of Steller sea lions. Ban, S. and A.W. Trites. 2007. Marine Mammal Science 23:496-507. abstract Steller sea lions (Eumetopias jubatus)are known to have occupied the same terrestrial haul-out and rookery sites across the North Pacific rim for centuries, but it is not known why they choose and stay at these locations, or what defines their preferred habitat. Classifying and comparing the shoreline type of haulouts and rookeries against sites not used by Steller sea lions showed that they preferentially locate their haulouts and rookeries on exposed rocky shorelines and wave-cut platforms. However, no preference was found for selecting rookeries on sheltered shore-types. Shoreline types used less frequently by sea lions included fine-to-medium-grained sand beaches, mixed sand and gravel beaches, gravel beaches, and sheltered rocky shores. Quantifying the shoreline types used by sea lions confirms anecdotal reports of habitat preferences and may prove useful in identifying and protecting sea lion terrestrial habitat, or in forecasting how climate change might affect the distribution of sea lions.
Relationship between Steller sea lion diets and fish distributions in the eastern North Pacific. Bredesen, E.L., A.P. Coombs, and A.W. Trites. 2006. In A.W. Trites, S. Atkinson, D.P. DeMaster, L.W. Fritz, T.S. Gelatt, L.D. Rea and K. Wynne (eds), Sea Lions of the World. Alaska Sea Grant College Program, University of Alaska, Fairbanks. pp. 131-139. abstract Distributions of fish species were compared with diet information for Steller sea lions (Eumetopias jubatus) to assess the level of correspondence between potential prey availability and sea lion feeding habits. Fish distributions were compiled as part of the Sea Around Us Project at the UBC Fisheries Centre, and were based on published distributions and habitat preferences (e.g., latitude, depth). Sea lion scat samples were collected during the 1990s from seven geographic regions from Oregon to the western and central Aleutian Islands. The frequencies of occurrence of four prevalent species (walleye pollock, Theragra chalcogramma ; Pacific herring, Clupea pallasii ; Pacific cod, Gadus macrocephalus ; and North Pacific hake, Merluccius productus ) in the Steller sea lion diet were compared to their distributions in the North Pacific Ocean. The data suggest that Steller sea lion diets broadly reflect the distributions of these major prey species. However, some of the fish species that were regionally predicted to be present in high abundance were not proportionally reflected in the Steller sea lion diet, suggesting that other factors in addition to fish abundance influence their diets.
Modelling and characterization of Steller sea lion haulouts and rookeries using oceanographic and shoreline type data. Ban, S. 2005. Graduate Thesis, University of British Columbia, Vancouver. 103 pages abstract Steller sea lions range across the Pacific rim from Southern California in the east to northern Japan in the west, where they have continuously occupied terrestrial resting sites (haulouts) and breeding sites (rookeries) for hundreds of years, if not longer. Why they choose (and stay) at these locations, and what their preferred habitat is, remains unknown. Thus, two aspects of the Steller sea lion?s habitat usage were examined?the oceanographic and the terrestrial. For the oceanographic aspect, spatial models were constructed to determine which oceanographic factors are associated with haulouts and rookeries, and how conditions near sites might differ from conditions elsewhere. The two modelling techniques employed (logistic regression and supervised classification) were evaluated using the kappa statistic (Kno), and receiver-operating characteristic(ROC) plots. Supervised classification was found to produce better-fitting models than logistic regression. In general, Steller sea lions showed preferences for sites associated with waters that were relatively shallow, well-mixed, had higher average tidal speeds and less-steep bottom slopes. Conditions within 1 nautical mile of land were better predictors of haulout and rookery locations than were conditions within 10, 20, and 50 nautical miles. No consistent differences were found in the physical characteristics of waters surrounding sites in the eastern and western populations of Steller sea lions, or between haulouts and rookeries. Regarding the terrestrial aspect of their habitat, anecdotal accounts describe Steller sea lions as predominantly occupying exposed, rocky shorelines, but this habitat preference has never been quantified. Locations of haulouts and rookeries were compared against a coastline type database to identify the shoreline preferences of Steller sea lions and to look for other spatial trends in site characteristics. Haulouts and rookeries were preferentially located on exposed rocky shorelines and wave-cut platforms. No relationship was found between either latitude or longitude of a site and its average non-pup count. The results indicate that there are differences in both the oceanographic and terrestrial characteristics of sites used by Steller sea lions versus areas of coastline where they are not found. The models could be used to predict changes in habitat use given changing physical conditions, and could be applied to any central-place forager.
Prey consumption of Steller sea lions (Eumetopias jubatus) off Alaska: how much prey do they require? Winship, A.J. and A.W. Trites. 2003. Fishery Bulletin 101:147-163. abstract The effects of seasonal and regional differences in diet composition on the food requirements of Steller sea lions (Eumetopias jubatus)were estimated by using a bioenergetic model. The model considered differences in the energy density of the prey, and differences in digestive effciency and the heat increment of feeding of different diets. The model predicted that Steller sea lions in southeast Alaska required 45–60% more food per day in early spring (March) than after the breeding season in late summer (August) because of seasonal changes in the energy density of the diets (along with seasonal changes in energy require ments).The southeast Alaska population,at 23,000 (±1660 SD)animals (all ages), consumed an estimated 140,000 (±27,800) of prey in 1998. In contrast, we estimated that the 51,000 (±3680) animals making up the western Alaska population in the Gulf of Alaska and Aleutian Islands consumed just over twice this amount (303,000 [±57,500 ] t). In terms of biomass removed in 1998 from Alaskan waters,we estimated that Steller sea lions accounted for about 5% of the natural mortality of gadids (pollock and cod) and up to 75% of the natural mortality of hexagram mids (adult Atka mackerel).These two groups of species were consumed in higher amounts than any other.The predicted average daily food require ment per individual ranged from 16 (±2.8)to 20 (±3.6)kg (all ages com bined). Per capita food requirements differed by as much as 24% between regions of Alaska depending on the rel ative amounts of low–energy-density prey (e.g.gadids)versus high–energy density prey (e.g. forage fish and salmon)consumed. Estimated require ments were highest in regions where Steller sea lions consumed higher proportions of low—energy-density prey and experienced the highest rates of population decline.
Implications of three viability models for the conservation status of the western population of Steller sea lions (Eumetopias jubatus). Gerber, L.R. and G.R. VanBlaricom. 2001. Biological Conservation 102:261-269. abstract Two distinct viability models are developed for Steller sea lions (Eumetopias jubatus )to evaluate the sensitivity of extinction risk to various levels of stochasticity,spatial scale and density dependence.These models include a metapopulation model,Analysis of the Likelihood of Extinction (ALEX;Possingham et al.,1992; Possingham,H., Davies,I.A.,Noble,I.1992.ALEX 2.2 Operation Manual.Department of Applied Mathematics,University of Adelaide,Adelaide,SA 5005;Australia.),and a model that incorpo- rates both sampling and process error in estimating population parameters from timeseries data (Gerber and DeMaster,1999; Gerber,L.R.,DeMaster,D.P.1999.An approach to endangered species act classification of long-lived vertebrates:a case study of north Pacific humpback whales.Conservation Biology 13 (5);1203 –1214.).Results are compared with a third model that encompasses three different geographic scales (York et al.,1996;York,A.E.,Merrick,R.L.,Loughlin,T.R.1996.An analysis of the Steller Sea lion metapopulation in Alaska.In:McCullough,D.R.(Ed.),Metapopulations and Wildlife Conservation.Island Press, Covelo,CA pp.259 –292).The combination of modeling approaches provides a basis for considering how model parameterization and the selection of classification criteria affect both model results and potential status determinations.Results from the models generally agree with regard to central tendency,25th and 75th percentile times to extinction.For Steller sea lions,the distributions of time to extinction for each model were narrower than the range of extinction distributions between models.If this finding applies generally to listed species,it would suggest that more than one viability model should be considered when listing decisions are made.On a more applied basis,the results of our analysis provide a quantitative assessment of extinction risk of Steller sea lions in the context of its status pursuant to the US Endangered Species Act.
Forage fish abundance and distribution at Forrester Island, Alaska. Norcross, B.L., B.A. Holladay and F. Mueter. 2000. Institute of Marine Science, University of Alaska, Fairbanks, Alaska. pp. 75 + appendices abstract This study examined the abundance and species composition of forage fishes near sea lion and seabird rookeries in Southeast Alaska, East Aleutians, Pribilofs, Central Aleutians, and West Aleutians (Figure 1). Bottom trawls, midwater trawls, surveys of large predatory fish stomach contents, and measurements of physical parameters were conducted at some or all of these five regions during summer 1997. Sixty-one bottom trawl tows from among all regions collected 4539 fishes of 62 taxa. There were significant differences among regional values of depth, % mud, bottom temperature, bottom salinity, towing speed, cumulative fish abundance, and size of fish. Differences were not detected among regional mean values of %gravel, %sand, and species diversity. Among the tows on sand substrate, there were significant regional differences in species diversity. Regional species abundance and species distribution relative to physical parameters are reported and contrasted; parameters important to distribution are identified. Fourteen midwater trawl tows from Southeast and the Pribilof Islands collected 23,345 fishes of 10 taxa. Salinity, number of taxa caught, and species diversity were all significantly greater in Southeast Alaska than in the Pribilofs. Regional differences were not detected between depths, temperatures, towing speed, or cumulative fish abundance. Regional values of species abundance are reported. The stomach contents of 126 Pacific halibut and Pacific cod captured in the Pribilofs and East, Central, and West Aleutians were examined. Frequency of occurrence and numerical composition of prey taxa are reported for each predator species and region. Fish species diversity, composition, abundance, and size differed between Southeast Alaska and western locations. There were higher species diversity, different species, and fewer individuals in Southeast Alaska than in the Aleutians and Pribilofs. There were also considerable differences between sampling sites in Southeast Alaska and the Aleutian and Pribilof Islands. Trawlable areas within Southeast Alaska were significantly deeper than in the other regions, and were all of sand substrate. Bottom temperatures at bottom trawl tow sites in Southeast Alaska and in the East Aleutian Islands were warmer than in the Pribilofs, Central Aleutians and West Aleutian Islands. Samples were collected from a different vessel in Southeast Alaska than in other regions, in part resulting in a higher tow speed in Southeast Alaska. While these physical differences between regions compound the regional differences detected in fish distribution and abundance, the differences in fish are real, just their magnitude is uncertain.
Remotely releasable instruments for monitoring the foraging behaviour of pinnipeds. Andrews, R.D. 1998. Marine Ecology Progress Series 175:289-294. abstract The use of stomach temperature data loggers to record prey ingestion has proven to be very valuable when combined with time-depth recorders and satellite tracking devices in studies of seabird foraging ecology. This paper presents a similar system that will allow biologists to determine the precise timing and location of foraging by pinnipeds. The system includes a stomach temperature transmitter and an animal-mounted instrument package. The instrument package contains a satellite transmitter, for remote tracking of movements, and a data logger, for recording dive depth, swim speed, water temperature, and stomach temperature (made possible by an incorporated telemetry receiver). The instrument package can be remotely released upon command to allow data recovery without animal recapture. The system was tested on 6 Steller sea lions Eumetopias jubatus in Southeast Alaska and found to be a powerful tool for quantifying foraging behaviour, although some suggestions for improvement are presented. keywords     swimming, telemetry, foraging, food, Steller sea lion
Competition between fisheries and marine mammals for prey and primary production in the Pacific Ocean. Trites, A.W., V. Christensen and D. Pauly. 1997. Journal of Northwest Atlantic Fishery Science. 22:173-187. abstract The degree of competition between fisheries and marine mammals in the Pacific Ocean was estimated for 7 statistical areas defined by the Food and Agriculture Organization of the United Nations (FAO). Catch statistics compiled from FAO sources show that the amount of fish caught in the Pacific Ocean rose from 2 million tons in the late-1940s to over 50 million tons in the early-1990s. Recent stagnation and declines occurring in some areas of the Pacific suggest that Pacific fisheries cannot continue to expand as they had previously. Based on estimates of population size, total biomass and daily consumption rates, it was estimated that the 84 species of marine mammals inhabiting the Pacific Ocean con-sume about three times as much food as humans harvest. A large fraction (>60%) of the food caught by marine mammals consisted of deep sea squids and very small deep sea fishes not harvestable by humans, thus limiting the extent of direct competition between fisheries and marine mammals. Moreover, the most important consumers of commercially exploited fish are other predatory fish, not marine mammals. Although direct competition between fisheries and marine mammals for prey appears rather limited, there may be considerable indirect competition for primary production. The primary production required to sustain marine mammals in each of the 7 FAO areas varies within a narrow range, suggesting that the diversity and abundance of marine mam-mals may have slowly evolved to fully exploit their niche and maximize their use of avail-able primary production. This contrasts with the rapid expansion of fisheries and their relatively recent dependence on primary production, which may have led to what we call ‘ food web competition’. keywords     competition, fisheries, food, feeding, marine mammals, Pacific Ocean, #3
Changes in the abundance of Steller sea lions (Eumetopias jubatus) in Alaska from 1956 to 1992: how many were there? Trites, Andrew W. and Peter A. Larkin. 1996. Aquatic Mammals 22:153-166. abstract The size of Steller sea lion populations in the Gulf of Alaska and Aleutian Islands was estimated by applying life table statistics to counts of pups and adults (non-pups) at rookery sites. Total population size was 5.10 times the number of pups counted or 3.43 times the number of adults counted. Only 55% of the adult population return to rookeries during the summer. Data compiled from published and unpublished sources for all 39 major rookeries in Alaska suggest that the total number of Steller sea lions (including pups) rose from 250 000 to 282 000 between the mid 1950s and the mid 1970s. Since 1980 it has decreased by over 70% (roughly 5% per year) to about 76 000 animals in 1992. Most of the decline appears to have slowed or stopped within two subareas of these large regions- the eastern Aleutians and western Gulf of Alaska. Increases have been occurring in the smaller populations of southeast Alaska. It is not known why these changes have occurred. keywords     abundance, Steller sea lions, Alaska, decline, #2
An analysis of groundfish fishing activities near Steller sea lion rookeries in Alaska. Sampson, D. 1995. Oregon State University, Hatfield Marine Science Centre, Newport, Oregon, OR 97365. pp. 40 abstract During the past few decades large commercial fisheries for groundfish developed in the Gulf of Alaska and Bering Sea. There has been speculation that these fishing operations may have reduced the available fish stocks and thereby contributed to the dramatic declines in the Alaskan populations of Steller sea lion (Eumetopias jubatus) that occurred during the same period. Previous studies that attempted to relate estimates of sea lion abundance with annual catches of walleye pollock (Theragra chalcogramma) produced inconclusive results. In this investigation principal component analysis was applied to data from 1979-90 on sea lion counts for 25 sea lion rookeries in the Gulf of Alaska and Aleutian Islands, and independently to fishery observer data from 1980-89 for the commercial groundfish fishing operations that occurred within a distance of about 37 kilometers of these rookeries. The component scores from the two data sets were then correlated to explore for similarities between the pattern of sea lion decline and the pattern of fishing operations. There was an unusually large correlation between the second principal component for the adult sea lion declines and the second component for the winter pollock catches. Rookeries that suffered relatively large declines in sea lion counts early in the study period generally experienced large winter pollock catches, but rookeries that suffered declines late in the study period experienced either no winter pollock catches or ones that occurred late in the study period. There were no strong correlations between the components for the adult sea lion declines and any other fishery components (quarterly fishing effort and total catches of groundfish, catches of Pacific cod, Gadus macrocephalus, and of Atka mackerel, Pleurogrammus monopterygius). Also, there were no strong correlations between the components for the sea lion pups and any fishery components.
Final report on an investigation of image processing techniques for the problem of automatic counting of sea lions from aerial video. Gosine, R.G. and L. Gamage. 1994. University of British Columbia, Industrial Automation Laboratory, Department of Mechanical Engineering, 2324 Main Mall, Vancouver, B.C. V6T 1Z4, Vancouver, B.C. V6T 1Z4. abstract A coalition of North Pacific fishing industry groups has been working to address pending restrictions on the Gulf of Alaska and Eastern Bering Sea commercial fishery as a result of possible decline of the Steller sea lion population. A component of the resea.rch into this problem is the investigataioonf techniques to automate the counting of sea lions from aerial video of the Alaska coast. Currently, sea-lion counts are completed mmually from 35mm slides, and there is some concern regarding t,he accuracy and repeatabilityof such an approach. It is proposed that computer-assisted counting from video tape or digitised slides (CDROM) could provide a better alternative to manual counting in terms of improved spatial coverage, improved reliability/consistency and reduced labour costs.
Statistical considerations in assessing recent adult/juvenile census trends of Steller sea lions. Schaffner, A.A., S.B. Mathews and J.E. Zeh. 1994. Report to the North Pacific Universities Marine Mammal Research Consortium, University of Washington, Fisheries Research Institute, WH-10, Seattle WA. abstract From June 1985 through June 1992 regular aerial surveys over Southeast Alaska, the Gulf of Alaska, and Aleutian Islands were conducted to monitor the distribution and abundance of Steller sea lions (Eumetopias jubatus). Furthermore, during 1992 the use of repetitive aerial surveys was introduced to study survey variability. The purposes of our present study were two-fold: (1) To investigate alternative statistical procedures for estimating population trend rates from aerial survey counts, and (2) to apply these procedures to the 1989-92 counts. We focused on 1989-92 because of the more abundant data for that period and to determine the most recent direction of population abundance relative to the well-documented steep decline prior to 1989. We concluded that parametric bootstrapping was the most appropriate interval estimation procedure. For this procedure the 90% confidence interval for the rate of change between the 1989 and 1992 Kenai-Kiska Recovery Plan trend site counts was (-10.19%, 2.62%). For glJ rookery and haul-out sites within the Kenai-Kiska region for which counts were made for both 1989 and 1992, the 90% confidence interval was (-6.20%, 6.81%). For sites outside the Kenai-Kiska region, the 90% confidence interval for the rate of change between 1989 and 1992 was (-37.92%, -6.93%). The parametric bootstrap procedure had the weakness of including data from only pairs of years; thus, 1990-91 information was excluded from the above interval estimates. Consequently, we investigated the use of both generalized linear modeling (GLM) and generalized estimating equations (GEE) as tools for analyzing all four years' data together. GLM seemed less appropriate than GEE, and the latter procedure yielded conclusions similar to those of parametric bootstrapping: 1989-92 stability for sea lion abundance in the Kenai-Kiska region, and some likelihood of decline from 1989-1992 considering all sites together.