Steller sea lion - Eumetopias jubatus

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Steller sea lions are the largest otariids and the fourth largest pinnipeds. Both sexes are robust and powerfully built at all ages. They are sexually dimorphic, with adult males weighing three times as much, and growing 20-25% longer than adult females. In addition to being larger, males have a mane of longer guard hairs extending from the back of the head to the shoulders and all around the neck. Breeding bulls are also very thick and wide in the neck and shoulder area. There is a sagittal crest on the skull that imparts a small to moderate forehead to adult and subadult males. In contrast, adult females and juveniles only have a minor forehead dip in front of the eyes and appear to be almost flat across the top of the head, from the crown to the tip of the nose. Males also have larger canines that are both longer and thicker than those of females.

Steller sea lions have a massive and wide head. The muzzle is thick, wide, long, and blunt on the end. The eyes are widely-spaced apart, and set well back from the end of the muzzle and, like the ear pinnae, appear small when compared with the size of the head. The vibrissae are whitish, conspicuous, and long, and several from the posterior end of the mystacial area can be very long in adults.

Both the fore and hind flippers are long and broad. The fore flippers have a sparse short fur that extends beyond the wrist onto the middle of the dorsal surface of the flipper in a ‘V’ pattern that does not reach the rounded tip. The rest of the dorsal surface, and the palms of both fore flippers are covered with a hairless black leathery skin. The first digit is the longest, widest and thickest, and curves posteriorly, giving the flipper a swept back look. Digits 2-4 are successively shorter. There is a small opening in the skin at the end of each digit for a claw that is usually reduced to a vestigial nodule, and rarely emerges above the skin. The claw openings are set back from the free edge of the flippers by cartilaginous rods that extend the length of each digit, and expand the size of the flippers. The hind flippers also have cartilaginous rods that extend the length of each toe. The bones of the three central toes terminate at the position of the small nails that emerge through the skin on the dorsal surface, set back from the end of the flipper. The first and fifth toes are longer than the three middle toes, and the first toe, or hallux, is longer and wider than the fifth toe. The hind flippers have short hair covering part of the proximal end of the flipper, and the rest of the dorsal surface, and the entire sole is covered in black leathery hairless skin.

Coloration in adults is pale yellow to light tan above, darkening around the insertion of the flippers to brown and shading to rust below. Unlike most pinnipeds, when wet, adult Steller sea lions are paler, appearing light grayish-tan. Juveniles are darker than adults and are dark tan to brown, and they appear darker in the water. Pups are born with a thick blackish-brown lanugo that is molted by about 6 months of age. Scars from bites and healed wounds are darker than the background color.

The maximum length of adult males is about 3.3 m and average weight is 1000 kg. The maximum length for adult females is about 2.5 m and average weight is 273 kg. Pups are born at an average of about 1 m and 18-22 kg.

The dental formula is I 3/2, C 1/1, PC 5/5. There is a wide diastema (gap) between the 4th and 5th post-canines.

Steller sea lions are readily separated from northern and Guadalupe fur seals that occur within their range by their massive size, large blocky head and muzzle, and pale color. California sea lions are most likely to cause confusion in the case of pale adult and subadult males and adult females. Careful attention to head and muzzle size and shape, overall coloration, and length and width of fore and hind flippers permits differentiation. Smaller Steller sea lions in the size range of large California sea lions will look like they are more muscular and powerfully built than similar-sized California sea lions, which look more filled-out with fat and streamlined. Also, smaller Steller sea lions have little or no sagittal crest development and a nearly flat-topped head whereas comparably-sized adult and subadult California sea lion males have a moderate to large sagittal crest and more pronounced forehead. Steller sea lion eyes also seem smaller and set farther apart, due to the proportionately larger head and wider muzzle.

Steller sea lions are found from central California (formerly southern California), north to the Aleutian Islands, and west along the Aleutian chain to the Kamchatka Peninsula, and from there south along the Kuril Islands to northern Japan, the Sea of Japan, and Korea. Steller sea lions also occur in the Sea of Okhotsk, west of Kamchatka and the Kuril Islands. From the Aleutians they range north across the Bering Sea to the Bering Strait. Throughout their range they are primarily found from the coast to the outer continental shelf and slope. However, they frequent and cross deep oceanic waters in some parts of their range.

Steller sea lions are highly polygynous and breed in the late spring and summer. Adult males arrive before females and those that are nine years or older establish themselves on territories, which they aggressively and vociferously defend. Steller sea lions have deep voices and produce powerful low-frequency rolling roars and can be heard for long distances over the noise of wind and waves. Roaring males often bob their head up and down while vocalizing. This is in contrast to the side to side head wave California sea lion males often make when they produce their characteristic repetitive bark during interactions with con-specifics.

Pups are born from May through July, and females stay continuously ashore with their newborns for the first week to ten days after giving birth. Following this period of attendance, females make foraging excursions, primarily at night for periods of 18-25 hours, followed by time ashore to nurse their pup. Females come into estrous and mate about two weeks after giving birth. Weaning often takes place before the next breeding season, but it is not unusual to see females nursing yearlings, older juveniles, or multiple offspring.

Steller sea lions occasionally leave haul-outs in very large groups; however, sightings at sea are most often of groups of 1-12 animals. They aggregate in areas of prey abundance, including near fishing vessels, where they will feed on netted fish and by-catch. They are not considered migratory, and juveniles and subadults make most long distance trips. Adults usually forage and live near their natal colonies and return to these sites to breed. The area used by adult females for foraging in winter increases dramatically over the area used in the summer. Diving is generally to depths of 200 m or less and dive duration is usually two minutes or less, with both parameters varying by season and age of the animal. Adult females tend to dive deeper in winter than summer. Diving ability of pups and juveniles increases with age, and they routinely dive to depths of around 140 m for periods of two minutes as yearlings. The diving of adult males has not been studied. Predators include killer whales and sharks.

Steller sea lions feed on many varieties of fish and invertebrates. Much of the information on diet comes from animals living in Alaska, where Steller sea lions feed on walleye pollock, Pacific cod, Atka mackerel, herring, sand lance, several varieties of flatfish, salmon and rockfish, and invertebrates such as squid, octopus, bivalves and gastropods. Adult females with young pups feed extensively at night, switching to foraging at any time after the breeding season.

Steller sea lions have been important to the subsistence cultures of people living near them for long periods. Native Alaskans currently take about 300 a year for food and other products. The worldwide population of Steller sea lions declined by 64% during the period from 1960 to 1989, and is currently estimated to be approximately 100,000 animals. The decline has been most dramatic in the large populations from the Gulf of Alaska (-54%), west throughout the Aleutian Island chain (-81%), to the moderately-sized Russian population (-74%). During the same period, the moderately-sized southeastern Alaska population increased (+70%). The reasons for these changes and the overall serious decline are unclear, but are the subject of intensive ongoing investigations. Factors hypothesized include, taken both alone and in combination with each other: the direct and indirect effects of large-scale commercial fisheries on key Steller sea lion prey species, long-term ecosystem shifts, and changes in behavior by a primary predator, the killer whale.

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Loughlin, T.R., M.A. Perez and R.L. Merrick. 1987. Eumetopias jubatus. Mammalian Species 283:1-7.

Loughlin, T.R. 2002. Steller’s sea lion Eumetopias jubatus. pp. 1181-1185 in W.F. Perrin, B. Würsig and J.G.M. Thiewissen, eds. Encyclopedia of marine mammals. Academic Press.

Merrick, R.L., T.R. Loughlin, G.A. Antonelis and R. Hill. 1994. Use of satellite-linked telemetry to study Steller sea lions and northern fur seal foraging. Polar Research 13:105-114.

Raum-Suryan, K.L., K.W. Pitcher, D.G. Calkins, J.L. Sease and T.R. Loughlin. 2002. Dispersal, rookery fidelity, and metapopulation structure of Steller sea lions (Eumetopias jubatus) in an increasing and a decreasing population in Alaska. Marine Mammal Science 18:746-764.

Pitcher, K.W. and D. G. Calkins. 1981. Reproductive biology of Steller sea lions in the Gulf of Alaska. Journal of Mammalogy 62:599-605.

Sease, J.L. and A.E. York. 2003. Seasonal distribution of Steller’s sea lions at rookeries and haul-out sites in Alaska. Marine Mammal Science 19(4):745-763.

Trites, A.W. and C.P. Donnelly. 2003. The decline of Steller seal lions Eumetopias jubatus in Alaska: a review of the nutritional stress hypothesis. Mammal Review 33(1)3-28.