Sea Otters
Enhydra lutris


The biology of animals that have been traditionally hunted are usually well known. Throughout the eighteenth and nineteenth centuries Enhydra lutris, the sea otter, was heavily exploited by man, prized universally for its exceptionally warm, chic and tremendously valuable pelt. Subsequently, in order to maximize harvest yield, the human predatory machine studied sea otter biology intently. This cute, furry, charismatic species was so well hunted that it was extirpated along most of its historical range in North America. A second wave of study in sea otter biology was initiated at the beginning of the twentieth century in efforts to prevent the animal's extinction. Recent studies of E. lutris appear to emerge from all areas of science. Currently, numerous scientific papers are being published on many different scales in the disciplines of physiology, ecology, behaviour, pathology, and conservation of this animal. The resources utilized in the synthesis of this page represent a very minute fraction of the myriad of past and current publications dedicated to Enhydra lutris.


The sea otter (Enhydra lutris, Linnaeus 1758) is the only member of the genus Enhydra. It is the largest member of the family Mustelidae which includes approximately 70 species; the otter is also the smallest marine mammal and is the only one in the order Carnivora. Based on morphological differences in color, body and skull sizes, three sub-species of sea otters have been proposed historically: E. l. lutris Linn., "the Commander-Aleutian North American sea otter"; E. l. gracilis Bechstein, "the Kuril-Kamchatka sea otter"; and E. l. nereis Merriam, "the southern California sea otter". However, these sub-species determinations were based on limited data that were collected before the advent of modern molecular phylogenetic analysis. A recent study of skull characteristics has yielded claims that a new subspecies, E. l. kenyoni , is alive and kicking. More recent genetic analysis of otter mitochondrial DNA suggests that E. l. nereis has monophyletic mtDNA while E. l. lutris and E. l. kenyoni do not. The phylogenetic study of E. lutris is currently still in progress and the focus of heated debate between traditional and modern systemists. Despite recent advances in the field, some taxonomists still subscribe to the three original subspecies while others suggest that even those racial distinctions are unsubstantiated.

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Enhydra lutris is the heaviest of the otters, reaching a maximum weight of 45 kg. The body of the sea otter is relatively long and heavy, making progress on land clumsy and slow. At birth, the sea otter is covered with dense brownish fur and long silky yellowish-tipped guard hair; over a period of several weeks, the guard hair grows out, often giving the pup a distinct yellowish appearance. The sea otter has eight main physical characteristics that help to distinguish the animal from other mustelids: a coat of darkly colored (shades of brown), sparse guard hair and dense insulating fur that traps air and prevents water from contacting the skin; flattened hind feet or flippers for propulsion; retractile claws on the front feet; a loose flap or pouch of skin under each foreleg which is used to hold food items gathered from the sea bottom; flattened, rounded molar teeth with no cutting cusps; a horizontally flattened tail that aids in propulsion; a manner of swimming underwater by means of vertical undulations of the hind flippers and tail; and an external ear that resembles the ear of an otariid more than that of its closest relative, the river otter.
 Sea Otter in Motion
If you look closely you can see the bubbles of air escaping from its fur as it swims along.

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Historically, the sea otter was found on the coasts and islands around much of the north Pacific from northern Japan (Fig.2) in the west to the islands off southern California in the east. The populations were drastically reduced by human exploitation in the eighteenth and nineteenth centuries. Commercial hunting of E.lutris for its valuable pelt began in 1741, eventually eliminating it from most areas by the late 19th century. By the early 1900s, only about a dozen colonies containing greater than one thousand otters remained. Sea otters were exterminated from Near Island, westernmost of the Aleutian archipelago, where records from the fur trade indicated that they were once abundant. Legal protection and reintroduction efforts have reestablished stable sea otter colonies along much of its historical range. Recent studies of otter populations at Amchitka Island reported estimated population sizes ranging from 5500-8500. Non-migratory, resident populations of E.lutris have now been reported to extend from the Commander Islands east along the Aleutian Islands and from Prince William Sound south to southern California. Otters have recently been observed as far south as Mexico.

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Sea otters are generally found inhabiting nearshore coastal waters of less than 54 m in depth. E. lutris can be found in association with both rocky and soft-bottom habitats. The typical haunts of the sea otter are characterized by precipitous rocky shores, barrier reefs, tidewater stones, and dense kelp forests.

Estimates of otter home ranges in the literature vary considerably from author to author. This is probably because the ranges of otters themselves have been found to vary enormously among different individuals and is believed to be a function of resource distribution and availability. One estimate of the area used by individual otters during a 24h period is 6.9-1166.4 ha. Annual home ranges have been observed as small as 35 ha to larger than 540 ha. Regardless of the discrepancies bewtween these numbers, otters of all age and sex classes are most often found within 1-2 km of their locations on the previous day. However, individuals often remain within a small area for an extended period and then suddenly move a much greater distance within a short period of time.

When resting, sea otters often lie on their backs among kelp or in quiet water; the most common position is with the head up, and with folded paws and chin resting on the chest. Otters are usually found sleeping in kelp beds, and many have been observed to wrap kelp around their bodies to prevent them from drifting during resting periods. E. lutris spends a large proportion of its time grooming its fur. Sea otters are usually found in groups known as rafts or pods ranging in size from a few to several hundred animals. The animals are long-lived, interact frequently and typically remain in the same area for years. Olfaction is apparently well developed in the sea otter and is important in chemical communication during close-range social and reproductive interactions. The sexes tend to segregate into separate groups and to some extent, in separate areas also. In larger groups of otters, there is little evidence of avoidance or territorial behavior. Some independent males have been observed to exhibit territorial behaviour.

Recent spectrographic analysis has been performed on the vocalizations of young and adult sea otters, both in captivity and in the wild. Ten basic vocal categories have been recognized and it is believed that the vocal patterns have characteristics that are most suitable for short range communication among familiar individuals. The otter's 'whine' and 'squeal' that is commonly heard during courtship has been found to consist of graded signals that vary over a continuum. This degree of complexity and richness of communication patters is believed to have evolved as a result of complex social relationships.

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The sea otter has been described as a resourceful, voracious and gregarious, generalist predator. Recent studies have revealed that otters do not forage cooperatively, and rarely exhibit other types of coordinated group movement. Small feeding groups are common, and although large aggregations have been observed, they are rare. Individuals dive in search of prey and return to the surface to breathe and consume their catch. Otters have been observed to forage at night as well as during the day, and there is a great deal of individual variation in diet and foraging patterns. Diets are known to primarily consist of fish and marine invertebrates, including various species of mussels, tunicates, sea stars, bivalves, crabs, abalone and octopus. When quality prey abundance is limited, otters have been known to feed on relatively low caloric value prey items such as sand dollars. Occasionally, otters have been observed to feed on some kelp species, but algae is not believed to be a main part of their regular diet.

Dive duration has been observed to vary with prey type, with mean dive lengths of about 74s, and longer dives approaching 246s. Studies have revealed that otters prefer to feed in shallow (<10m), inshore waters and tend to forage in deeper waters (>20m) when food availability in the shallow waters is diminished.

E. lutris demonstrates significant selectivity and intelligence when foraging and consuming its prey. It has been shown that otters can detect and avoid toxic shellfish if suitable low toxicity prey are available. Otters are often observed using rocks to pound small, hard bodied prey items to gain access to the edible fleshy interior.

Among many mustelids, mating tends to be prolonged and aggressive. In sea otters, mating is always aquatic and often involves violent and prolonged copulations during which the male approaches the female from behind and grasps her face and nose with his teeth, sometimes pulling her head underwater while attempting to subdue her. Some females may form pair bonds with a single male while others may mate with up to three different males during a single estrous period. Most females reach sexual maturity between 2-5 years of age, with 88% maturing by age 4. Most females have been observed to have the first pup by 3-4 years of age. Reproductive rate is maximal at 5 years of age and remains stable through to age 15. Female otters typically mate numerous times during their estrous period, which lasts several days and occurs about once each year. If a female loses her pup before weaning age, she may enter estrus and mate repeatedly two or more times in a single year. Mean gestation period is approximately 218 days; mean pup dependency period is about 153 days. The majority of pups are born in spring (February) and early summer. Although implantation in most mustelid species is often controlled by environmental factors such as temperature and photoperiod, little synchrony has been observed in sea otter pupping activity. Groups of females periodically bring their young pups ashore to rest in a process known as hauling. Female reproductive rates and pup survivorship are generally higher in undisturbed areas with abundant food resources.

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There are a variety of pathogens and parasites that inflict otter populations. Most studies have focused on the more problematic southern populations. One of the main causes of death in California populations is Acanthocephalan peritonitis. This is caused by small worm-like parasites that enter the body with food, perforate the intestinal wall, resulting in infections of the abdominal cavity. Protozoal encephalitis is a disease that is also relatively common in otters. This disease involves swelling of the brain which has been found to be caused by several genera of protozoa. Coccidioidomycosis is an infection derived from a soil-borne fungus that is common in dry interior valleys. Cases of this type of infection are limited to populations in San Louis Obispo county. The nature of propagation of the disease is not well understood. Bacterial infection is another significant cause of otter mortality. The types of bacteria involved vary significant between the individuals examined. In the final stages of some infections, significant interstital fibrosis has been documented. Recent examinations of otters imported to Japan from Alaska have revealed intestinal parasites. Four species of acanthocephalan of the genus Corynosoma have been recovered from these sea otters.


Sea otter predation has a predictable and broadly generalizable influence on the structure of Alaskan kelp forests. The animals are well documented as 'keystone' predators in rocky marine communities. Otters have also been found to exert a strong influence on infaunal prey communities of soft -bottom sediments. In some areas of Alaska, sea urchin biomass has been found to decline by nearly 100% following the spread of sea otters into previously unoccupied habitats. Kelp forests are broadly dependent on sea otter predation for protection against destructive grazing.

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At present, the sea otter is considered endangered along much of its range, and is legally protected in the United States under the Marine Mammal Protection Act and the Endangered Species Act. Between 1965 and 1972 approximately 710 otters were translocated from Alaska in attempts to reestablish the species along portions of its range where it had been extirpated by human overexploitation. Several reintroductions have been attempted since then, with varying degrees of success. Gill nets used to be a major source of mortality to California populations; recent laws prohibiting gill nets at depths above 54m have resulted in 5-7% increases in populations that were previously stable or declining. Currently, industrial pollution is believed to be a major problem for southern populations of E. lutris.

Following the grounding of T/V Exxon Valdez in Prince William Sound, Alaska, in March, 1989, and subsequent oil spill, more than 781 sea otter carcasses were recovered at one location. It is estimated that more than 2650 otters were killed as a result of this oil spill. The large scale mortality of sea otters that resulted from this incident led to many studies (lots of post-mortem work) of otter reproductive biology and effects of water pollution on otter physiology. For Northern populations of sea otters, salmon gill nets are still a major cause of mortality. Presently in Alaska, the illegal shooting by fishermen and the legal hunting of otters by Indigenous Peoples are believed to be the main causes of mortality. Brown bear predation and eagle predation of pups have been observed, but their contributions to mortality are considered to be minimal.

In a recent conservation publication, criteria for delisting endangered species has been examined. It has been suggested that sea otter threats are currently minimal and populations have stabilized to the extent that the animal might no longer merit protection under the U.S. Endangered Species Act. Presently in Canada, there is no legislation protecting E. lutris . However, the Committee On the Staus of Endangered Wildlife in Canada (COSEWIC) lists the animal as threatened along the Canadian spans of its range.


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Bodkin, J.L., Mulcahy, D., and Lensink, C.J. 1993. Age-specific reproduction in female sea otters (Enhydra lutris) from south-central Alaska: analysis of reproductive tracts. Can. J. Zool. 71(9):1811-1815.

Chanin, P. 1985. The Natural History of Otters. Croom Helm, London, England.

Cohen, P.(ed.). 1962. The Sea Otter. Israel Program for Scientific Translations, Jerusalem.

Cronin, M.A., Bodkin, J., Ballachey, B., Estes, J., and Patton, J.C. 1996. Mitochondrial-DNA variation among subspecies and populations of sea otters (Enhydra lutris). Journal of Mammalogy 77(2):546-557.

Estes, J.A. 1990. Growth and equilibrium in sea otter populations. Journal of Animal Ecology 59: 385-401.

Estes, J.A. and Duggins, D.O. 1995. Sea otters and kelp forests in Alaska: generality and variation in a community ecological paradigm. Ecological Monographs 65(1):75- 100.

Fisher, E.M. 1939. Habits of the southern sea otter. J. Mammalogy 20:21-36.

Green, G.A., and Brueggeman, J.J. 1991. Sea otter diets in a declining population in Alaska. Marine Mammal Science 7(4): 395-401.

Haseltine, A.W.(pres.). 1997. NWH necropsy data, 1992-1995. Friends of the sea otter website: [].

Jameson, R.J., Kenyon, K.W., Johnson, A.M., and Wight, H.M. 1992. History and Status of translocated sea otter populations in North America. Wildife Society Bulletin 10(2). .

Johnson, A.M. 1982. The sea otter, Enhydra lutris. Mammals in the seas 4: 525-531. Fisheries series-Food and Agriculture Organization of the United Nations, No.5.

Kenyon, K.W. 1969. The Sea Otter in the Eastern Pacific Ocean. North American Fauna No.68, Bureau of Sport and Fisheries Wildlife, Washington, D.C.

Kenyon, K.W. 1981. Sea otter, Enhydra lutris. Handbook of marine mammals volume 1: the walrus, sea lions, fur seals and sea otter. Academic Press, London

Kikuchi, S., and Nakajima, M. 1993. Corynosoma enhydri from sea otters Enydra lutris (Acanthocephala).Japanese Journal of Parasitology 42(4):331-339.

Kvitek, R.G., DeGange, A.R., and Beitler, M.K. 1991. Paralytic shellfish poisoning toxins mediate feeding behaviour of sea otters. Limnol. Oceanogr. 36: 393-404.

Kvitek, R.G., Oliver, J.S., DeGange, A.R., and Anderson, B.S. 1992. Changes in Alaskan soft-bottom prey communities along a gradient in sea otter predation. Ecology 73(2):413-428.

Lensink, C.J. 1960. Status and distribution of sea otters in Alaska. J. Mammalogy 41: 172-182.

McShane, L.J., Estes, J.A., Reidman, M.L., and Staedler, M.M. 1995. Repertoire, structure, and individual variation of vocalizations in the sea otter. J. Mammalogy 76(2):414-427.

Monson, D.H., and DeGange, A.R. 1995. Reproduction, preweaning survival, and survival of adult sea otters at Kodiak Island, Alaska. Can. J. Zool. 73(3):1161- 1169.

Ralls, K., Hatfield, B.B., and Siniff, D.B. 1995. Foraging patterns of California sea otters as indicated by telemetry. Can. J. Zool. 73:523-531.

Ralls, K., Eagle, T.C., and Siniff, D.B. 1996. Movement and spatial use of California sea otters. Can. J. Zool. 74(10): 1841-1849.

Schneider, K.B. 1978. Sex and age segregation of sea otters. Alaska Department of Fish and Game, Juneau, Alaska.

Smith, D.R., Niemeyer, S., and Flegal, A.R. 1992. Lead sources to California sea otters: Industrial inputs circumvent natural lead biodepletion mechanisms. Environmental Research 57(2): 163-174.

Staedler, M. and Riedman, M. 1993. Fatal mating injuries in female sea otters (Enhydra lutris nereis). Mammalia 57(1):135-139.

Watson, Jane - Jane has spent a tremendous amount of time studying the ecology of sea otters. Any paper by this author will be really insightful.

Wayre, P. 1979. The Private Life of the Otter. B.T. Batsford Ltd., London, England.

Williams, T.D., Allen, D.D., Groff, J.M., and Glass, R.L. 1992. An analysis of California sea otter (Enhydra lutris) pealge and integument. Marine Mammal Science 8(1):1-18.

Wilson, D.E., Bogan, M.A., Brownell, R.L., Jr., Burdin, A.M., and Maminov, M.K. 1991. Geographic variation in sea otters, Enhydra lutris. Journal of Mammalogy 72(1):22-36

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