Enhydra lutris

Sea Otter

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Sea Otter



The Sea Otter is the largest member of family Mustelidae, and the smallest marine mammal. Sea Otters are more aquatic even than seals and sea lions, because they mate and give birth in the water. They are tool users, using rocks to pound open hard-shelled prey, such as abalone. Once hunted almost to extinction for their fur, but now protected, they have made a comeback. This has stirred controversy where their predation on abalone, crabs, clams, and sea urchins affects shellfisheries. Ecologists are beginning to understand the larger, long-term role they play in shaping the marine environment, by eating creatures such as sea urchins. Sea urchins feed on and if unchecked, can destroy kelp. Vast underwater kelp forests are at the base of the coastal food web and also provide shelter for countless organisms.

Adaptation: In the sea otter (Enhydra lutris), adaptations to crushing marine invertebrates mark the skull through its powerful chewing muscles, which leave muscles scars in the form of raised ridges of bone and a rough surface texture. Note also the large, flat cheek teeth.


Historically, Sea Otters occurred across the North Pacific Rim, ranging from Hokkaido, Japan, through the Kuril Islands, the Kamchatka Peninsula, the Commander Islands, the Aleutian Islands, peninsular and south coastal Alaska and south to Baja California, Mexico (Kenyon 1969). In the early 1700s, the worldwide population was estimated to be between 150,000 (Kenyon 1969) and 300,000 individuals (Johnson 1982). Although it appears that harvests periodically led to local reductions of Sea Otters (Simenstad et al. 1978), the species remained abundant throughout its range until the mid-1700s. Following the arrival in Alaska of Russian explorers in 1741, extensive commercial harvest of Sea Otters over the next 150 years resulted in the near extirpation of the species. When Sea Otters were afforded protection by the International Fur Seal Treaty in 1911, probably fewer than 2,000 animals remained in 13 remnant colonies (Kenyon 1969). Remnant populations were located in the Kuril Islands, Kamchatka and in the Commander Islands Russia; five in Southwestern Alaska (the Aleutian Islands, Alaska Peninsula, and Kodiak Island), and one remnant population in each of the following regions; Southcentral Alaska (Prince William Sound), Canada (Queen Charlotte Islands), central California, and Mexico (San Benito Islands) (Estes 1980). However, the Queen Charlotte, Canada and San Benito Island, Mexico remnant Sea Otter populations have presumably died out and likely did not contribute to the recolonization of the species following near extirpation (Kenyon 1969).


In north America, the Sea Ottes range is fairly continuous from the Aleutian Islands to Prince William Sound with population gaps along the Gulf of Alaska until Yakutat (which was a translocated population) with another gap in the population’s distribution until the outer islands of Southeast Alaska (also a translocated population form the Aleutian Islands and Prince William Sound). The next gap in the Sea Otter population distribution is between Southeast Alaska and British Columbia, Canada. Translocation efforts were successful in Washington State but not in Oregon thus there is a large population gap between the small Sea Otter population in Washington and that of central California.



Throughout their range, Sea Otters use a variety of near shore marine environments and 84% of foraging occurs in water ≤ 30m in depth (Bodkin et al. 2004) and throughout much of their range, foraging occurs within a kilometer of the shore. Their classic association is with rocky substrates supporting kelp beds, but they also frequent soft-sediment areas where kelp is absent (Riedman and Estes 1990, DeMaster et al. 1996, Burn and Doroff 2005). Kelp canopy is an important habitat component, used for foraging and resting (Riedman and Estes 1990). They are found most often in areas with protection from the most severe ocean winds, such as rocky coastlines, thick kelp forests, and barrier reefs. Although they are most strongly associated with rocky substrates, Sea Otters can also live in areas where the sea floor consists primarily of mud, sand, or silt. Individuals generally occupy a home range a few kilometers long, and remain there year-round. Sea Otters forage in rocky and soft-sediment communities on or near the ocean floor. The maximum confirmed depth of dive was 97 m (Newby 1975); however recent studies using time-depth recorders implanted in Sea Otters indicate average maximum forage depths of 54 m for female and 82m for male Sea Otters (Bodkin et al. 2004).

Sea Otters are weakly territorial (Kenyon 1969) with fighting and aggression rare (Loughlin 1980). Only adult male Sea Otters establish territories. Males patrol territorial boundaries and attempt to exclude other adult males from the area. Females move freely between and among male territories. Groups of male and female Sea Otters generally rest separately. Sea Otter annual home ranges can occupy up to 0.8 km² (80 ha) and extend along 16 km of coastline (Kenyon 1969, Loughlin 1980). Typically, female Sea Otter home ranges are about 1.5–2 times larger than resident adult males during the breeding season; however, females have smaller annual or lifetime home ranges than males (Riedman and Estes 1990). Jameson (1989) found that territorial adult males occupied a mean home range of 40.3 ha during the summer-fall period (when home range size was considered equal to territory size); and mean coastline length was 1.1 km. Winter-spring mean home range size of territorial adult males that remained in female areas was 78.0 ha, with a mean coastline length of 2.16 km.

The diet of Sea Otter consists almost exclusively of marine invertebrates, including sea urchins, a variety of bivalves such as clams and mussels, abalone, other molluscs, crustaceans, and snails. Its prey ranges in size from tiny limpets crabs and giant octopuses (Estes 1980). Sea urchins, abalones and rock crabs are the principal prey of Sea Otters in newly reoccupied habitats of central California (Vandevere, 1969) whereas clams and crab will make up the diet in soft-sediment habitats (Kvitek et al. 1992, Doroff and DeGange 1994). Where prey such as sea urchins, clams, and abalone are present in a range of sizes, Sea Otters tend to select larger items over smaller ones of similar type (Kvitek et al. 1992). In California, it has been noted that Sea Otters ignore Pismo clams smaller than 3 inches (7 cm) across. Only in the Aleutian archipelago were Sea Otters observed to regularly eat fish, which could comprise up to 50% of their diet. The fish species eaten were usually bottom dwelling and sedentary or sluggish forms, such as the Red Irish Lord and Globefish (Estes 1980). They also consume crab, clam, mussels, turban snails, sea cucumbers, squid, octopus, chitons, tubeworms, large barnacles, scallops, and sea stars (Wild and Ames 1974, Riedman and Estes 1990). Bivalve molluscs are excavated by digging in sand or mud bottoms and are the most common prey in soft-sediment communities (Calkins 1978, Kvitek et al. 1992, Doroff and DeGange 1994).

Male Sea Otters reach sexual maturity around age five or six, but probably do not become territorial or reproductively successful for two or three subsequent years (Riedman and Estes 1990). Most female Sea Otters are sexually mature at age four or five (Kenyon 1969, Jameson and Johnson 1993, Monson et al. 2000, Monson and DeGange 1995, von Biela 2007). Sea Otters apparently are polygynous, although the exact nature of the mating system may vary. Females normally give birth to a single pup that weighs 1.4 to 2.3 kg at birth (Riedman and Estes 1990). Twinning has been documented in Sea Otters (Williams et al. 1980); however, litters larger than one are rare, and when they occur, neither pup is likely to survive (Jameson and Bodkin 1986). Pups remain dependent upon their mothers for about six months (Jameson and Johnson 1993). Longevity in Sea Otters is estimated to be 15 to 20 years for females and 10 to 15 years for males (Riedman and Estes 1990).



The Sea Otter is considered to be Endangered due its vulnerability to large-scale population declines. The species is believed to have undergone a decline exceeding 50% over the past 30 years (approximately three generations). The world-wide population of Sea Otters decreased to approximately 2,000 animals by the end of the commercial fur trade in 1911 (Kenyon 1969). The population recovered from 11 remnant populations located in Russia (Bering Island, Kamchatka Peninsula, and Kuril Islands) and in the United States (in Alaska (Aleutian Islands, Alaska Peninsula, Kodiak archipelago, and Prince William Sound) and California). The remnant populations were small and widely dispersed, as a result, this species has low genetic diversity (Ralls et al. 1983). Since the 1980s, the species had been recovering in many areas thanks to intensive management and regulatory efforts by several governments. However contemporary issues (oil spills, potential fisheries interactions, predation, and disease events), have either prevented Sea Otter populations from thriving or have caused population declines throughout much of the species range. In the United States, two subspecies of Sea Otters are listed as threatened (E. lutris kenyoni in SW Alaska and E. lutris nereis in California) due to precipitous population declines in Alaska and slow growth (and vulnerability to anthropogenic factors) of a small population in California.

In Alaska, precipitous population declines occurred in the Aleutian Islands beginning in the late 1980s–2005. By 2000, counts of Sea Otters had decreased by 90% with a declining trend through 2005 (Doroff et al. 2003, Estes et al. 2005, Burn et al. 2003). The probable cause of the decline was increased predation by killer whales (Orcinus orca) (Estes et al. 1998). More recent Sea Otters surveys indicate the population trend has increased since 2005, however, counts remain well below carrying capacity for this region (D.M. Burn pers. comm. 2010). Population counts also remain low for the Alaska Peninsula (Burn and Doroff 2005, U.S. Fish and Wildlife Service Stock Assessment Reports). The population in the Kodiak archipelago and lower Cook Inlet appeared stable or increasing during the same period that population declines were documented in the Aleutian Islands and Alaska Peninsula (Kodiak and lower Cook Inlet are part of the Southwest population stock), however, this habitat has not been surveyed since 2004.

Recent studies have found infectious disease to be an important mortality factor in California Sea Otter populations (Conrad et al. 2005, Johnson et al. 2009). Information collected from forensic-level necropsies of dead Sea Otters and sampling of free-ranging Sea Otters indicate a strong link to protozoan parasites, Toxoplasma gondii and Sacrocystis neurona, that are known to breed in cats and opossums (Thomas and Cole 1996, Conrad et al. 2005) thus sources of mortality for the Sea Otter population include land-based factors. Other factors identified as causing significant mortality include acanthocephalan peritonitis, protozoal encephalitis, bacterial and fungal infections (Thomas and Cole 1996).

The situation in the Russian Federation is clearer now. The Sea Otter number on the Commander Islands reached maximum since last 150 years period (A. Burdin and S.V. Zagrebelny pers. comm. 2006). In 2007, the direct count revealed around 8,000 otters in both Bering and Medny Islands. The Commanders Island population of Sea Otter was never so abundant, but in 2008, it was found that the population was on decline. In 2004 the Kuril Islands population of Sea Otter was estimated around 19,000 (Kornev and Korneva 2004), but later count have shown sever decline (up to 40–50% in different locations). Though the causes of such decline are not very clear, the threat due to poaching can’t be ruled out.




  • 2008
    (IUCN 2008)
  • 2000
  • 1996
    Lower Risk/least concern
    (Baillie and Groombridge 1996)




Oil spills are the greatest anthropogenic threat to Sea Otter (Geraci and Williams 1990). Sea Otters become hypothermic when oiled because oiled Sea Otter fur loses its insulative property and Sea Otters have no blubber layer, oil can be ingested while grooming, leading to gastrointestinal disorders, other ailments and death and volatile components of oil inhaled by Sea Otters can cause lung damage. Estimates of sea otter mortality following the Exxon Valdez spill in Prince William Sound ranged from 2,650 (Garrott et al. 1993) to 3,905 (DeGange et al. 1994).

Significant numbers of Sea Otters drowned in gill and trammel nets in California from the mid-1970s to the early 1980s (Estes 1990). Recent population declines in California’s Sea Otters may be incidental to summer commercial fisheries. Estes et al. (2003) found that Sea Otter mortality was elevated in the summer months and that commercial fin fish landings in the coastal live trap fishery increased. Recent analyses indicated annual Sea Otter carcass recoveries and reported fishery landings were significantly correlated.

Thomas and Cole (1996) found 10% of southern Sea Otters they examined to be emaciated without specific cause of mortality. Severe weather and periodic climatic events such as El Niño can disrupt foraging behaviour and food availability, and increase pup loss. Under these circumstances, Sea Otters may find it difficult to meet their high metabolic needs, leading to malnutrition or starvation. Serious tooth wear in older Sea Otters may also contribute to mortality (Riedman and Estes 1990). Recent studies have found infectious disease to be an important mortality factor in California Sea Otter populations. Around 280 Sea Otters found dead have been linked “to a pair of protozoan parasites, Toxoplasma gondii and Sacrocystis neurona, that are known to breed in cats and opossums (Conrad et al. 2005, Johnson et al. 2009). In Alaska, Streptococcal endocarditis, encephalitis and/or septicemia, referred to as Strep. syndrome has been identified in forensic-level necropies of northern sea otters (Unusual mortality event working group) as well as trauma from boat strikes. Goldstein et al. (2009) found northern sea otters from the Alaska Peninsula, Kodiak and Kachemak Bay area infected with phocine distemper.

Killer Whales (Orcinus orca), Great White Sharks (Carcharodon carcharias), Bald Eagles (Haliaeetus leucocephalus), Coyotes (Canis latrans), and Brown Bears (Ursus arctos) have been documented as predators of Sea Otters (Riedman and Estes 1990). Predation by Killer Whales is one factor that is believed to have caused Sea Otter population declines across the Western Gulf of Alaska and Aleutian Islands (Doroff et al. 2003, Estes et al. 1998, Hatfield et al. 1998). Significant declines in preferred prey species populations - Northern Fur Seals (Callorhinus ursinus), Harbour Seals (Phoca vitulina), and Steller Sea Lions (Eumetopias jubatus) are believed to have caused Killer Whales to prey switch and consume Sea Otters (Estes et al. 1998).

Studies in Alaska and Washington and elsewhere have shown that Sea Otter predation on sea urchins may indirectly enhance the growth of kelp and kelp-associated communities. Shellfish are important to commercial, recreational, and tribal fisheries throughout the species range and predation by Sea Otters can be significant and result in localized fisheries and economic issues.


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