Steller Sea Lions are the largest otariids and the fourth largest pinniped. Both sexes are robust and powerfully built. They are sexually dimorphic, with adult males weighing three times as much as, and growing 20–25% longer than, adult females. Pups are born with a thick blackish-brown lanugo that is molted by about six months of age. The maximum length of adult males is about 3.3 m and average weight is 1,000 kg. The maximum length for adult females is about 2.5 m and average weight is 273 kg. Pups are born at an average size of about 1 m and 18–22 kg (Loughlin 2009).

The age of sexual maturity is 3–6 years for females, and 3–7 years for males (Calkins and Pitcher 1982). Males are not able to defend territories before they are nine years old. The annual pregnancy rate of mature females in the western population declined during the 1970s and 1980s and was estimated to be 55% in the 1980s based on collections at sea (Pitcher et al. 1998). Age-structured modelling based on population counts from the central Gulf of Alaska indicates that the birth rate in 2004 was 36% lower than in the 1970s (Holmes et al. 2007). Gestation lasts one year, including a delay of implantation of about three months. Females may live to be up to 30 years old and males to about 20 years (Loughlin 2009). Here we use a generation time of 10 years based on life tables derived from collections made in the 1970s and 1980s (Calkins and Pitcher 1982), and other analyses (York 1994, Holmes and York 2003, Van de Kerk et al. 2013).

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 defend. Pups are born from May through July, and females stay continuously ashore with their newborns for the first 7–10 days after giving birth. Following this period of attendance, females make foraging excursions for periods of 18–25 hours, followed by time ashore to nurse their pup. Females come into oestrus and mate about two weeks after giving birth. Weaning can occur before the next breeding season, but it is not unusual to see females nursing yearlings or older juveniles (Loughlin 2009). Recent work on vital rates indicates that age specific survival may differ across the range and second year survival is likely dependent on the age at weaning (Hastings et al. 2011, Fritz et al. 2014, Altukhov et al. 2015, Maniscalco et al. 2015).

Steller Sea Lions are primarily found from the coast, where they haul out on rocky shores, to the outer continental shelf and slope where they feed. However, they frequent and cross deep oceanic waters in some parts of their range. They sometimes leave haulouts in very large groups but 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 discarded bycatch. Steller Sea Lions sometimes haul out on sea ice where it is available. They may move long distances but are not considered migratory; juveniles and subadults make the longest trips. Adults usually forage and live near their natal colonies and return to those sites to breed. The area used by adult females for foraging in winter is much greater than the area used in the summer, and females tend to dive deeper in winter than summer. Overall, adults usually forage and use haul-out sites near their natal rookeries, where they typically return to breed. Adult females satellite-tagged in the US portion of the western stock made long pelagic foraging trips during winter, whereas their foraging range was more restricted during summer (Merrick and Loughlin 1997, Fadely and Lander 2012, Fadely et al. 2013). Adult females are capable of diving to depths of at least 427 m but average approximately 45 m; dives are deeper during winter than summer (NMFS, unpublished data). Dive duration is usually two minutes or less, but can exceed 13 minutes (Rehberg and Burns 2008). Dives of juveniles are generally shallow and short. Diving ability of pups increases with age (Loughlin et al. 2003, Fadely et al. 2005, Lander et al. 2010), with dives ranging on average from 5-35 m and up to 2 min (Loughlin et al. 2003, NMFS, unpublished data). Juvenile Sea Lions satellite-tagged in Southeast Alaska and Washington State made deeper average dives, and had higher dive rates and longer dive durations than juveniles in western Alaska (Loughlin et al. 2003, Pitcher et al. 2005). The diving of adult males has not been studied.

Steller Sea Lions feed on many types of fishes and invertebrates. Much of the information on diet comes from Alaska, where they feed on Walleye Pollock, Pacific Cod, Atka Mackerel, Herring, Sand Lance, several species of Flatfish, Salmon, and Rockfish, and invertebrates such as Squid and Octopus (Sinclair and Zeppelin 2002). Food habits analysis using samples collected between 1990 and 2009 indicate that Western Steller Sea Lions in the U.S. consumed similar prey by region throughout the time period, but the distribution range of prey use increased over time (Sinclair et al. 2013). Adult females with young pups feed primarily at night, switching to foraging at any time of day after the breeding season. Steller Sea Lions are known to kill and consume young northern Fur Seals at the Pribilof Islands, as well as Harbor and Ringed Seals.

The primary predators of Steller Sea Lions are Killer Whales (Loughlin 2009). Sleeper Sharks have been suggested as a potential predator of juvenile Western Steller Sea Lions in the Gulf of Alaska (Horning and Mellish 2014), although previous work in the same area found that none of the 198 Sharks examined near rookeries during summer contained Steller Sea Lion remains (Sigler et al. 2006).

Steller Sea Lions normally occur from central California north along the west coast of North America, westward through the Gulf of Alaska and the Aleutian Islands to the Kamchatka Peninsula, and from there south along the Kuril Islands to northern Japan and the Sea of Japan (Loughlin 2009). They also occur in the Sea of Okhotsk and the Bering Sea north to Bering Strait. Vagrants have been recorded in China, and at Herschel Island in the Canadian Beaufort Sea (Rice 1998). The western subspecies primarily occupies the area west of 144° W longitude. The eastern subspecies occurs from east of 144° W longitude south along the North American coast to central California.

Western Steller Sea Lions are listed as vulnerable in the Threatened Wildlife of Japan Red Data Book. Hattori and Yamamura (2014) reported that over 200 Steller Sea Lions were culled annually between 1960-1993 to reduce predation on commercial fish stocks. Recent work indicates that the annual culling was then reduced to a limit of 116/year until 2010 at which time a new 5 year quota of 1,030 culled Sea Lions was imposed. This resulted in an increased annual average take (Matsuda et al. 2015). In 2015 approximately 400 Steller Sea Lions were reported to be culled in Japan (Yamamura et al. 2015). In Russia, the major Steller Sea Lion rookeries were given protection under the Northern Fur Seal and Sea Otter Conservation Act in the late 1950s. They were listed as endangered (category 2) in the Russian Red Data Book in 1994 and harvest was prohibited. These measures had a positive effect in the western portion of the range as the population increased around Sakhalin Island, the Kuril Islands, and in the northern Sea of Okhotsk. However, abundance along the eastern coast of Kamchatka and in the Commander Islands has not recovered for unknown reasons (V. Burkanov pers. comm.).

In the USA the Steller Sea Lion is listed as depleted under the US Marine Mammal Protection Act. The species was listed as threatened under the ESA in 1990, and in 1997 the western population was uplisted to endangered. A recovery plan for Steller Sea Lions was approved in 1992, and a revised recovery plan was published in 2008. Critical habitat was designated under the ESA in 1993. No-entry zones were established around rookeries at the time of listing, and fisheries, particularly those operating in critical habitat, have been managed to reduce the likelihood of competitive interactions. Substantial funding has been made available for Steller Sea Lion research to develop information on ecology, behavior, genetics, population dynamics, and movements. Results have been used to assist in the development of management activities, to attempt to understand the reasons for the decline, and to promote recovery of the species (NMFS 2008). The fact that the overall growth rate of the western population is now positive suggests that at least some of these conservation efforts have had a beneficial effect. A species status review conducted in 2013 found that the Loughlin’s Steller Sea Lion population increased more than 3% per year since the 1970s and therefore it was removed from the ESA threatened species list (NMFS 2013). The western population in the USA is still listed as endangered under the ESA.