Sea otter (Enhydra lutris) COSEWIC assessment and status report 2007L chapter 5

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Species Information

• Name and Classification
• Taxonomy
• Morphological Description
• Genetic Description

Name and Classification

The sea otter, Enhydra lutris, is the second largest (by weight) member of the family Mustelidae. It is the only member of the genus Enhydra. Three subspecies in the genus are recognized based on skull measurements and mitochondrial DNA (Wilson et al. 1991; Cronin et al. 1996). Enhydra lutris kenyoni is thought to have ranged historically from the coast of Oregon to the Aleutian Islands; Enhydra lutris nereis, the southern sea otter, occurs along the California coast; and Enhydra lutris lutris ranges from the Kuril Islands to the Kamchatka Peninsula and the Commander Islands in Russia. Enhydra lutris kenyoni occurs in British Columbia.

Taxonomy

Morphological Description

Sea otters are sexually dimorphic. Adult males can reach weights of 46 kg and total lengths of 148 cm, whereas adult females can grow to 36 kg and reach lengths of 140 cm. At birth pups weigh 1.7-2.3 kg and are up to 60 cm in total length (Bodkin 2003). Sea otters in recently occupied habitat, where food is not limiting, are heavier (up to 28% for males and 16% for females) than animals in populations at or near equilibrium, where food is limiting (Bodkin 2003). Pelage in adults varies in shades of brown, although the fur may become progressively lighter with age creating a grizzled effect on the head, neck, chest and forelimbs (Estes 1980). Newborn pups have a light brown, or yellowish, woolly natal fur that is completely replaced by adult fur by 13 weeks (Payne and Jameson 1984).

Sea otters have hind feet that are flattened, with elongated digits, the fifth or outer digit being the longest. This adaptation allows an otter to swim efficiently while foraging underwater or while lying on its back on the surface. The dorso-ventrally flattened tail is used to scull and shift position while resting (Kenyon 1969) (Figure 1). Sea otters can attain surface swimming speeds of 1 to 1.5 km/h (Kenyon 1969). Rates of travel during foraging dives range up to >5 km/h (Bodkin et al. 2004). On land sea otters are clumsy and awkward (Estes 1980).

Figure 1: Sea Otter (Enhydra lutris)

Photo by B. Gisborne.

The sea otter’s powerful forelimbs are well adapted for grooming and obtaining invertebrate prey. They are not used for swimming (Kenyon 1969). A loose pouch of skin at the axilla (armpit) of each forelimb is used carry prey to the surface where it is consumed (Estes 1980). Rather than the shearing teeth typical of most carnivores, sea otters have bunodont molars adapted for crushing hard-shelled invertebrate prey (Riedman and Estes 1990).

Sea otters have little body fat. They maintain an exceptionally high metabolic rate and rely on a layer of air trapped in their dense fur for insulation. The fur consists of an outer layer of protective guard hairs and a fine dense underfur of approximately 100,000 hairs per cm² (Kenyon 1969). Sea otters groom frequently to maintain the integrity of their fur and its ability to hold a layer of trapped air for insulation (reviewed in Riedman and Estes 1990).

Genetic Description

Sea otters in British Columbia have suffered through at least two genetic bottlenecks, the initial global bottleneck brought about by the species’ near extinction as a result of the maritime fur trade of the 18^th and 19^th centuries, and a second bottleneck caused by introducing a small number of animals to British Columbia.

As a result of the fur trade, the total range-wide population had been reduced, by 1911, to less than 2000 animals, approximately 1-2% of its pre-exploitation size (Kenyon 1969). As a result of this bottleneck, genetic diversity among extant sea otter populations is significantly lower than pre-fur trade sea otters, with a loss in modern sea otters of at least 62% of the alleles and 43% of the heterozygosity compared to the pre-fur trade population (Larson et al. 2002a).

Sea otters are believed to have been extirpated from British Columbia by 1929 (Cowan and Guiguet 1960). They were reintroduced to British Columbia between 1969 and 1972, from Amchitka and Prince William Sound, Alaska, and were also reintroduced during the same period to unoccupied habitat in Southeast Alaska, Washington and Oregon. All reintroductions were successful except in Oregon (Jameson et al. 1982). With respect to this second bottleneck resulting from translocation of small numbers of animals, genetic studies suggest there is no significant difference in the amount of genetic variation between remnant (experienced one bottleneck) and translocated (experienced two bottlenecks) populations of sea otters (Bodkin et al. 1999; Larson et al. 2002b). Loss of genetic diversity among successfully reintroduced populations may have been largely avoided, at least among populations that arose from not less than 20 to 30 animals, because the bottleneck occurred for a relatively short time and there was rapid population growth (in the absence of food limitations) (Bodkin et al. 1999; Larson et al. 2002b).

In 1989, females with pups were first reported on the central British Columbia coast more than 235 km away from the reintroduced population on Vancouver Island (British Columbia Parks 1995). The origin of these otters was unknown (Watson et al. 1997), but recent genetic analysis of 18 sea otter samples from the central British Columbia coast in 2003 revealed 2 mtDNA haplotypes consistent with otters from Amchitka and Prince William Sound, suggesting otters on the central British Columbia coast are descendents of reintroduced Alaskan otters (Barrett-Lennard, pers. comm. 2003). Sea otters in Southeast Alaska and Washington State are of the same origin (Bodkin et al. 1999; Larson et al. 2002b).

There is only one population or designatable unit of sea otters in British Columbia.