Steller's sea lion (Eumetopias jubatus)

Also known as: Northern sea lion
KingdomAnimalia
PhylumChordata
ClassMammalia
OrderCarnivora
FamilyOtariidae
GenusEumetopias (1)
SizeMale weight: 700 kg (2)
Female weight: 300 kg (2)
Female length: 2.3 m (2)
Male length: 3.0 m (2)

Classified as Near Threatened (NT) on the IUCN Red List (1).

Steller's sea lion is the largest of the eared seals (members of the family Otariidae), a group that includes fur seals and sea lions (3) (4). The impressive adult males are two and a half times the size of the females; they have large necks, and shoulders covered with a mane of long, coarse hair (2) (3). Both sexes are a light brown, whilst pups are originally black, moulting to the adult coat after three to four months (5). The common name of the species comes from the German naturalist who first described these seals in 1741, George Wilhelm Steller (4). Eared seals are able to control their hind flippers independently, allowing them to be particularly agile on land and, unlike the true seals (family Phocidae), they swim using their foreflippers (4).

Steller's sea lions are found from the northern Californian coast, north to Alaska and across the Bering straits to the coasts of Russia and Japan (3). Recently, the species has undergone a worrying decline (especially in Alaska), and the United States population as a whole has declined by 75 percent since the mid to late 1970s (4).

Found in the cool waters of the northern Pacific Ocean, hauling out on beaches and rocky coastline (6).

Steller's sea lions breed in massive, noisy rookeries common to most members of this family (4). The males arrive at the haul-out sites in spring and establish their territory on the limited space of the beach; they usually do not feed throughout the breeding season, as they cannot afford to relinquish their hard-won position (7) (8). Females arrive in mid-May to late June and give birth to a single pup; only four days later the female is ready to mate again and the most successful males will aggressively guard, and mate with, up to 30 females (9). Despite mating straight after giving birth, the fertilised egg will not be implanted into the female's uterus until October (3). Both males and females reach sexual maturity at around three to six years of age, but males are unlikely to breed successfully until their eighth to tenth year due to the fierce competition at rookeries (3). Around nine days after giving birth the female will resume foraging trips to the sea. Most pups are weaned when about one year old, but a mother may continue to suckle her young for up to two or three years (6). The breeding season draws to an end in early July but these sea lions maintain their social lifestyle, being commonly seen on shore throughout the year in groups of tens to hundreds of animals (4) (7).

Steller's sea lions are opportunistic hunters, feeding on a wide range of fish and cephalopods such as squid (6). In Alaska some of the more important prey species are walleye pollock (Theragra chalcogramma), Atka mackerel (Pleurogrammus monopterygius) and Pacific herring (Clupea harengus) (6).

The world population of Steller's sea lions has been undergoing a mysterious decline; since 1980, numbers have dropped from over 300,000 individuals to fewer than 100,000 (10). Despite this well-documented and worrying decline, the causes are still being debated; various hypotheses cite pollution, bycatch, parasites and disease, rookery disturbance and predation by killer whales (10). Research into dietary factors have revealed that Steller's sea lions in the northeast Pacific have suffered a decrease in the diversity and energy content of their diet since the mid 1970s, corresponding to changes in fish species available due to natural climatic changes (11). A diet dominated by low energy fish (such as pollock) can cause sea lions to lose condition, and can result in reduced pregnancy rates and increased susceptibility to disease or predation. This may be one of the major causes of the population decline (12).

Steller's sea lion was listed as Threatened on the United States Endangered Species Act in 1990 throughout its range; the western stock in Alaska was listed as Endangered in 1997 (6). The United States National Marine Fisheries Service (NOAA) has established a number of protection measures relating to fishing bans around major rookeries and feeding areas, in an attempt to slow the decline in population numbers (6). A consortium of North Pacific Universities is carrying out ongoing research into the causes of the perplexing population decline (10). The battle to understand the factors involved in the decline in Steller's sea lion numbers may also provide better understanding of the complex marine ecosystem and the effects of fish stock changes (by both natural and man-made causes) on other marine mammals and sea birds (10).

For more information on Steller's sea lion see:

Authenticated (12/02/04) by Dr. Andrew Trites, Director of the Marine Mammal Research Unit, University of British Columbia.


http://www.marinemammal.org


http://mmru.fisheries.ubc.ca

  1. IUCN Red List (February, 2013)
    http://www.iucnredlist.org
  2. Winship, A.J., Trites, A.W. and Calkins, D.G. (2001) Growth in Body Size of the Steller Sea Lion (Eumetopias jubatus). Journal of Mammalogy, 82: 500 - 519. Available at:
    http://www.marinemammal.org/pdfs/Growth_in_body_size_2001.pdf
  3. Alaskan Department of Fish and Game (December, 2002)
    http://www.state.ak.us/local/akpages/FISH.GAME/notebook/marine/sealion.htm
  4. Macdonald, D. (2001) The New Encyclopedia of Mammals. Oxford University Press, Oxford, UK.
  5. Daniel, R.G. (2003) The Timing of Moulting in Wild and Captive Steller Sea Lions (Eumetopias jubatus). University of British Columbia, Canada. Available at:
    http://www.marinemammal.org/pdfs/Daniel2003.pdf
  6. National Marine Mammal Laboratory (December, 2002)
    http://nmml.afsc.noaa.gov/AlaskaEcosystems/sslhome/stellerhome.html
  7. Trites, A.W. (2004) Pers. comm.
  8. Seal Lion Caves (December, 2002)
    http://www.sealioncaves.com/
  9. American Museum of Natural History (December, 2002)
    http://www.amnh.org/nationalcenter/Endangered/sealion/sealion.html
  10. North Pacific Universities Marine Mammal Research Consortium (April, 2008)
    http://www.marinemammal.org
  11. Benson, A.J. and Trites, A.W. (2002) Ecological effects of regime shifts in the Bering Sea and eastern North Pacific Ocean. Fish and Fisheries, 3: 95 - 113. Available at:
    http://www.marinemammal.org/pdfs/bensontrites2002.pdf
  12. Trites, A.W. and Donnelly, C.P. (2003) The decline of Steller sea lions in Alaska: a review of the nutritional stress hypothesis. Mammal Review, 33: 3 - 28. Available at:
    http://www.marinemammal.org/pdfs/TritesDonnelly2003.pdf