Overview

Steller sea lions are found in Puget Sound during much of the year. Their breeding areas are not found here (they are mostly north of the Sound, in British Columbia and Alaska), and sea lions are absent or very rare in inland Washington waters during their summer breeding season. Puget Sound is an area they use primarily for feeding. Although they were heavily hunted in the past, currently the main human-caused threats to the species are entanglement in fishing gear, and various forms of habitat degradation.

These animals feed on a wide variety of prey, and in local waters consume herring and eulachon, as well as some threatened stocks of salmonid fish. In western Alaskan waters, Steller sea lions have been declining in numbers, probably due to fisheries or habitat change issues. But, the stock off the west coast of the continental US and Canada does not appear to suffer from this problem. Both commercial and sport fishers often see Steller sea lions as competitors, and they sometimes take out their displeasure by shooting them or using other harmful methods. Two subspecies are recognized by the Society for Marine Mammalogy (western and eastern distinct population segments or stocks), but the US NMFS does not use these subspecies designations.

Status, trends & events

The graphs below show strong indications of increasing trends in the Steller sea lion population, largely driven by increasing pup counts.

Natural history

Distribution and occurrence

Steller sea lions are endemic to the North Pacific Ocean. They occur in coastal and offshore waters from central California around the rim of the North Pacific to southern Japan, including the Bering Sea (Jefferson et al. 2015). There are extralimital records from southern California and China. They occur in Puget Sound during the non-breeding season (i.e., autumn through spring), after they spread out from their breeding rookeries. 

Steller sea lions occur mainly over the continental shelf and slope, but they may be found a long distance from shore, for instance in the Bering Sea, where they may occur hundreds of nautical miles offshore. There are several haulouts used in Puget Sound and sea lions may also haul out on buoys and even on such human-made structures as Navy submarines (Jefferson et al. 2023).

What do they eat?

The main prey are walleye pollock, Pacific hake, rockfish, skates, smelt, shad, Pacific cod, Atka mackerel, herring, sand lance, salmon, rockfish, several species of flatfish, squid, octopus, bivalves, and gastropods. Bulls of this species may occasionally kill and eat northern fur seal and harbor seal young (see Loughlin et al. 1987; Loughlin and Gelatt 2017). In Puget Sound waters, detailed feeding habit studies have not been undertaken, but they are known to feed on herring and salmonids, such as salmon and steelhead.

What eats them?

The main predators of Steller sea lions are Bigg’s killer whales, but they are also attacked by large sharks.

Biology

The global population size of the species is around 143,000 individuals. Three stocks are recognized, one in Asian waters, and two in the US/Canada (Muto et al. 2022). The conservation status of the two US stocks is very different. The western stock found in Alaskan waters from the Gulf of Alaska westwards, has been declining for several decades, and is considered Endangered under the ESA. The Eastern Stock, which is found in US west coast waters from California to southeast Alaska, has been increasing in abundance at about 4-5% per year. There is some exchange among individuals across the stock boundary, though it is not considered to be extensive. There are no reliable recent estimates of its total size, but it is believed to consist of well over 77,000 individuals. Abundance in Washington peaks at about 2,000-3,000 animals in non-breeding months, but most of these animals remain along the outer coast (Wiles 2015). Within Puget Sound, aerial surveys from 2013-2016 suggest that about 110 Stellers, on average, are found in the peak season (fall), with as many as 600-700 occurring within the whole of inland Washington waters in that season (Jefferson et al. 2023). Influxes may occur when feeding opportunities are especially favorable, however.

Like virtually all species of pinnipeds, Steller sea lions were ruthlessly hunted in past centuries. They were killed for fur, oil, food, and as competitors of fishers. Native peoples in Alaskan waters have a long tradition of hunting this species, primarily as a food source, and Alaskan natives still take about 150-300 of them each year for subsistence (Schusterman 1981; Muto et al. 2022). Since protection in the late 20th century by the Marine Mammal Protection Act of 1972 (MMPA) and similar legislation in Canadian waters, Steller sea lions along the west coast of North American have been recovering.

These are the largest of all five sea lion species, with males much longer (20-25%) and heavier (up to 2.5-3 times) than females. Bulls can be up to 3.3 m in length and weigh on average 700-900 kg. Captive males can grow to over 1,000 kg! Females, on the other hand, only grow to about 2.5 m and 275 kg. Newborn pups are about 1 m long and weigh about 20-22 kg. These animals are much lighter in color than the California sea lions that share much of their range, appearing yellowish to tan on the upper parts, but with darker color on the undersides and around the flippers. They have a short, blunt snout, and adult males have a prominent mane of longer, coarser guard hairs. Adult males also have a sagittal crest, but it is not nearly as prominent as in the California sea lion (see Jefferson et al. 2015).

These massive pinnipeds can be very aggressive but are generally shy of humans when they are on shore and will often stampede into the water en masse when disturbed. While at sea, they tend to be solitary, but may aggregate in groups of up to about a dozen in the water. They are very gregarious on haul outs and rookeries. Individual animals may range widely when they go to sea, generally in search of good feeding opportunities.

Stellers breed mostly on offshore islands with rocky shores, and they do not give birth in inland Washington waters. In fact, their only significant pupping sites in the state are at Carroll Island and Sea Lion Rock, on the west side of the Olympic Peninsula (Wiles 2015). The breeding season for Stellers is from late spring to summer, with older males arriving on the rookeries first, to set up territories and establish dominance, using both their distinctive roar and overt aggression to do so. Pups are born mainly in May through July, and the mother attends to the pup on shore for 8-11 days, before beginning a series of foraging trips to sea, which usually last less than a day. Breeding is annual, and pups are usually weaned before the next breeding season, but some females nurse their pups for 2-3 years. Therefore, occasionally females may be nursing more than one pup at a time. A couple of weeks after giving birth, females are ready to mate and do so with the bull whose territory they occupy. Male Stellers may live to the age of 16 years, and females to 23.

While the species is considered an opportunistic feeder, they do prey on some commercially important fish species, such as herring, salmon, and steelhead. Their habits of feeding very visibly at the surface on fish that fishers target has meant that they often earn the ire of both commercial and sport fishers. Concerns about their impacts on commercial fisheries have resulted in several management actions, including removal from sensitive areas. Lethal removal of “problem” individuals that prey on threatened fish stocks at “pinch points” have been suggested. Diving is generally not very deep, up to about 400 m maximum, but most dives are less than about 200 m in depth and 2 minutes or so in length (see Trites 2021).

The roar of an adult male Steller sea lion is low frequency, but very loud (as might be expected from a sea “lion”), and they use these vocalizations to defend their breeding territories. Females also vocalize, and use pup contact calls as one method of keeping track of their offspring. It is not generally thought that Steller sea lions have a functional system of echolocation.

What threatens them?

Disease outbreaks can cause large-scale mortality for sea lions, as their gregariousness on shore and at haul outs can lead to disastrous spreading of contagious pathogens. However, in the Steller sea lion this does not appear to be as serious a concern as it is for the California sea lion, which often suffers from prey shortages due to El Niño and other ocean-warming events.

Some individuals become seriously injured or killed on rookeries, or during stampedes. Steller sea lions occasionally strand on shore, both live and dead, and live-stranded individuals may be taken to rehabilitation facilities for medical care. Almost yearly, 1-3 individuals become entangled in, or swallow portions of, fishing gear, such as salmon flasher hooks and line, and require at-sea sedation and gear removal (Muto et al. 2022). Bulls can be difficult to handle, due to their very large size and aggressive nature.

The overall species is listed as Near Threatened on the IUCN Red List, and the Western Stock is considered Endangered under the US Endangered Species Act (ESA). The Eastern Stock (the one that occurs in Puget Sound) is increasing in numbers, and is not listed under the ESA, nor considered depleted under the MMPA. Although there are critical uncertainties about stock boundaries (currently set at 144°W for the two US stocks) and overall population size, the stock is considered to be within its Optimum Sustainable Population (Muto et al. 2022).

The main reason for the decline in numbers of the Western Stock of the species is controversial but is thought to be related to either prey reduction caused by fisheries, or shifts to lower quality food, which is more habitat related. Recent studies favor the second hypothesis, in which poor nutrition of juveniles results in delayed weaning and therefore reduced birth rates (Trites 2021). This is sometimes called the “junk food hypothesis”. Anthropogenic threats for the Eastern Stock include fisheries bycatch (in both commercial and recreational/subsistence fisheries), entanglement and ingestion of marine debris, shooting (both illegal and legally mandated for predator control), vessel strikes, disturbance causing stampedes, and explosives use. Oil/chemical spills and contaminants are also a concern. Climate change/ocean warming appears to be causing a northward shift in the southern range off California, as California sea lions are better adapted to warmer waters (Muto et al. 2022).

Data sources & gaps

Monitoring and information gaps for this species exist, as there is overlap in southeast Alaska of the ranges of Steller sea lions from the two US stocks. Total abundance for the US Eastern Stock cannot be determined from available data, as the counts only include animals visible during the survey, and do not account for animals that were in the water (and therefore not visible) at the time.

Methods & statistics

The abundance time series graph above was constructed from 1971 to 2017 non-pup count data collected at rookeries and haul-outs and modeled using agTrend (from Muto et al. 2022). The distribution map was compiled from all available data sources (from Jefferson et al. 2015). The data on numbers in Puget Sound and inland Washington waters are from a recent paper by Jefferson et al. (2023) in Aquatic Mammals.

References

The scientific name of the Steller sea lion is Eumetopias jubatus. The species was named after Georg Wilhelm Steller, the German naturalist on Vitus Bering’s Russian expedition that first exposed Europeans to Alaska and its wildlife.

Jefferson, T. A., M. A. Webber and R. L. Pitman. (2015). Marine Mammals of the World: A Comprehensive Guide to Their Identification. Academic Press/Elsevier.

Jefferson, T. A., M. A. Smultea, and E. J. Ward. (2023). Distribution and abundance of California (Zalophus californianus) and Steller (Eumetopias jubatus) sea lions in the inshore waters of Washington, 2013-2016. Aquatic Mammals 49:366-381.

Loughlin, T. R., M. A. Perez and R. L. Merrick. (1987). Eumetopias jubatus. Mammalian Species, 283, 7 pp.

Loughlin, T. R. and T. S. Gelatt. (2017). Steller sea lion, Eumetopias jubatus. In B. Würsig, J. G. M. Thewissen and K. M. Kovacs (Eds.), Encyclopedia of Marine Mammals (Third Edition). (pp. 931-935). Academic Press.

Muto, M. M., B. J. D. V. T. Helker, N. C. Young, J. C. Freed, R. P. Angliss, N. A. Friday, P. L. Boveng, J. M. Breiwick, B. M. Brost, M. F. Cameron, P. J. Clapham, J. L. Crance, S. P. Dahle, M. E. Dahlheim, B. S. Fadely, M. C. Ferguson, L. W. Fritz, K. T. Goetz, R. C. Hobbs,Y. V. Ivashchenko, A. S. Kennedy, J. M. London, S. A. Mizroch, R. R. Ream, E. L. Richmond,K. E. W. Shelden, K. L. Sweeney, R. G. Towell, P. R. Wade, J. M. Waite, and A. N. Zerbini. (2022). Alaska marine mammal stock assessments, (2021). NOAA Technical Memorandum, NMFS-AFSC-441, 295 pp.

Schusterman, R. J. (1981). Steller sea lion Eumetopias jubatus (Schreber, 1776). In S. H. Ridgway and R. J. Harrison (Eds.), Handbook of Marine Mammals, Volume 1: The walrus, sea lions, fur seals and sea otter. (pp. 119-141). Academic Press.

Trites, A. W. (2021). Behavioral insights into the decline and natural history of Steller sea lions. In C. Campagna and R. Harcourt (Eds.), Ethology and Behavioral Ecology of Otariids and the Odobenid. Ethology and Behavioral Ecology of Marine Mammals. (pp. 489-519). Springer.

Wiles, G. J. (2015). Washington state periodic status review for the Steller sea lion. Washington Department of Fish and Wildlife, Olympia, Washington. 38 pp.

Acknowledgments

The authors thank the Puget Sound Partnership for funding the analysis of species data in inland Washington waters. Funding for this article was provided by the University of Washington Puget Sound Institute with support from the U.S. Environmental Protection Agency and the Puget Sound Ecosystem Monitoring Program. 

Licensing & attribution

Data and products from the PSEMP Marine Mammal Work Group are governed by a Creative Commons BY-NC-SA license. Attribution should be to: “PSEMP Marine Mammal Work Group” with the link https://psemp.net/mmwg.