Sea otter (Enhydra lutris)

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Californian sea otter anchored in kelp
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Sea otter fact file

Sea otter description

KingdomAnimalia
PhylumChordata
ClassMammalia
OrderCarnivora
FamilyMustelidae
GenusEnhydra (1)

The sea otter, the smallest marine mammal in the world, is well adapted for its predominately aquatic lifestyle, possessing a strong, rudder-like tail and large hind-feet that act as flippers (2). Unlike other marine mammals, sea otters do not have blubber and instead rely on their fur to keep warm in the water (4); their reddish-brown coat is the densest of any mammal, consisting of around 100,000 hairs per cm² (2). The natural oils produced by the fur provide a waterproof quality (5).

French
Loutre De Mer.
Spanish
Nutria Del Kamtchatka, Nutria Marina.
Size
Male length: 148 cm (2)
Female length: 140 cm (2)
Male weight: 45 kg (2)
Female weight: 32.5 kg (2)
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Sea otter biology

Highly aquatic, the sea otter rarely comes ashore, both resting and feeding in coastal waters. Alaskan sea otters are more at home out of the water than their Californian relatives, often hauling out on sandbars and ice (8). Otters have a high metabolic rate (9) and these resourceful, opportunistic predators need to consume 25 percent of their body weight a day (6). Diving to depths of up to 75 metres they retrieve invertebrates such as mussels, snails, crabs and urchins from the seabed (6). Pouches of skin at the armpit of each forelimb can be used to store food whilst it is carried to the surface (6). Otters float on their backs, using their chest as a table whilst they attempt to prize open the shells. Sometimes rocks are used to smash open the hard shells and sea otters are one of the only mammals (apart from primates) to have developed tool use (6). Sea otters are considered a 'keystone species' in some parts of their range, as they appear to be vital in the maintenance of kelp forest ecosystems by suppressing the number of sea urchins that would otherwise overgraze the forests (4).

These gregarious creatures can be found in large same-sex groups known as 'rafts'. Rafts in California rarely exceed 50 individuals but in Alaska, where the population density is higher, up to 2,000 otters can gather (2). Sea otters probably spend more time and energy grooming their fur than any other mammal; an important activity required to maintain the insulation of their fur, as it cleans and replenishes air to the under fur (4). Grooming involves rubbing, rolling and blowing air into the fur (4). Trapped air in the under fur is heated by the body to provide insulation and gives otters a silvery appearance underwater (5).

Sea otters are polygynous, with adult males generally defending territories that encompass the ranges of several females (2) (4). While mating, the male will grip the female's nose with his teeth and she is often left with a bloody souvenir of their encounter (4). Females usually give birth to a single pup and carry them on their chest, nursing them and grooming them meticulously to ensure the fur remains buoyant and insulated (9). Young pups are left on the surface whilst their mother dives for food, but as they mature they follow her, learning to forage by watching her technique (5). Pups will stay with their mother for around three to six moths (6).

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Sea otter range

Historically, the sea otter was found in coastal areas throughout the North Pacific (6). Sea otters can still be found in much of this former range although numbers are greatly reduced and populations fragmented (6). Three subspecies are currently recognised: the southern, or California sea otter (Enhydra lutris nereis), occurs from California to Mexico; the northern, or Alaskan sea otter (E. l. kenyoni), is found along the northwest coast of North America and into Alaska; and the Russian sea otter (E. l. lutris) occurs in the western north Pacific (7). Alaska plays host to possibly as much as 90 percent of the world population of this species (6).

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Sea otter habitat

Sea otters inhabit near-shore rocky or soft-bottom coastal waters, and in California are particularly associated with dense kelp forests (4).

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Sea otter status

Classified as Endangered (EN) on the IUCN Red List 2007 (1), and listed on Appendix II of CITES (3).

IUCN Red List species status – Endangered

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Sea otter threats

Since the 1700s, prior to the commercial fur trade, native peoples throughout the otter’s range harvested sea otters for their pelts (2). An intensive commercial fur trade, from 1740 until about 1900, resulted in the sea otter being harvested almost to extinction, and by 1900, the sea otter was so rare that commercial harvesting was forced to cease (6). Sea otters were protected by an international treaty in 1911. Since then, numbers have recovered to an extent, but human activities, especially coastal development and marine pollution, now pose a threat to the sea otter (6). In 1989, hundreds, and possibly thousands, of sea otters were killed as a direct result of the Exxon Valdez oil spill in Prince William Sound in Alaska (4). The possibility of another major oil spill poses a continual threat to the sea otter, and could have devastating consequences. Additional threats include entanglement in fishing gear, particularly gill nets, and competition with commercial fisheries for food (6). In the central Aleutian Islands, Alaska, sea otters have declined by as much as 90 percent (4). Evidence indicates that this drastic decline is the result of increased predation by killer whales, which have switched to consuming more sea otters following the collapse of Steller’s sea lion and harbour seal populations in the region (4).

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Sea otter conservation

The sea otter is legally protected in the United States under the Marine Mammal Protection Act and in Canada sea otters are protected under the Species at Risk Act (10). Despite protection and various conservation measures, the Californian population has been slow to recover; a new Recovery Plan by the U.S. Fish and Wildlife Service was currently being developed with the aim of managing damaging human activities to enable the population to recover to a point where it can be removed from the List of Endangered and Threatened Wildlife (11). There have been several successful reintroduction attempts along the west coast of North America, restoring this highly appealing animal to much of its former range (2).

To help conserve this species by working in the field with Earthwatch, click here.
View information on this species at the UNEP World Conservation Monitoring Centre.
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Find out more

For more information on the sea otter see:

Find out more about otter conservation:

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Authentication

Authenticated (17/10/07) by Dr Jane Watson, Biology Department, Malaspina University-College.

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Glossary

Invertebrates
Animals without a backbone.
Polygynous
A pattern of mating in which a male has more than one female partner.
Subspecies
A different race of a species, which is geographically separated from other populations of that species.
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References

  1. IUCN Red List (September, 2007)
    http://www.iucnredlist.org
  2. Jefferson, T.A., Leatherwood, S. and Webber, M.A. (1993) FAO species identification guide. Marine mammals of the world. United Nations Food and Agriculture Organization, Rome.
  3. CITES (September, 2007)
    http://www.cites.org
  4. Estes, J.A. and Bodkin, J.L. (2002) Otters. In: Perrin, W.F., Wursig, B. and Thewissen, J.G.M. (Eds) Encyclopedia of Marine Mammals. Academic Press, London.
  5. SEA OTTERS: THE CLAM BUSTERS (Wildlife on One)(BBC tx. 16 January 1995).
  6. U.S. Fish and Wildlife Service - Alaska (September, 2007)
    http://alaska.fws.gov/fisheries/mmm/index.htm
  7. Perrin, W.F. (2002) Geographic Variation. In: Perrin, W.F., Wursig, B. and Thewissen, J.G.M. (Eds) Encyclopedia of Marine Mammals. Academic Press, London.
  8. A SEA OTTER STORY (Nature: Waddlers and Paddlers)(PBS tx. 17 March 1994).
  9. Kruuk, H. (2006) Otters. In: Macdonald, D.W. (Ed) The New Encyclopedia of Mammals. Oxford University Press, Oxford.
  10. IUCN Otter Specialist Group (September, 2007)
    http://otterspecialistgroup.org/Species/Enhydra_lutris.html
  11. US Fish and Wildlife Service. (2003) Final Revised Recovery Plan for the Southern Sea Otter. USFWS, Portland, Oregon. Available at:
    http://ecos.fws.gov/speciesProfile/SpeciesReport.do?spcode=A0A7
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Image credit

Californian sea otter anchored in kelp  
Californian sea otter anchored in kelp

© Doc White / naturepl.com

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