Sea otter (Enhydra lutris) COSEWIC assessment and status report 2007L chapter 8
Female sea otters reach sexual maturity at 3 to 5 years (Bodkin et al. 1993; Jameson and Johnson 1993) with all females being reproductive by age 5 (Monson et al. 2000a). Males reproduce at 5 to 6 years of age, although they may be sexually mature earlier (Riedman and Estes 1990;: Bodkin et al. 1993). Females have a higher survival rate than males (Siniff and Ralls 1991) and live 15 to 20 years, whereas males live only 10 to 15 years (Riedman and Estes 1990). Although mating and pupping can occur year-round, distinct peaks in pupping in spring are noted in some populations including British Columbia (Watson 1993; Bodkin 2003). Sea otters are polygynous with males forming pair bonds consecutively with several females. Sea otters segregate by sex in spatially separate areas (Riedman and Estes 1990). Breeding-age males leave male areas and establish exclusive breeding territories in female areas during the summer and fall, after which they rejoin male rafts (Garshelis and Garshelis 1984; Jameson 1989). Females produce a single pup at approximately 1-year intervals (Siniff and Ralls 1991; Bodkin et al. 1993). Twins are rare. Birth occurs in the water (Kenyon 1969; Jameson and Bodkin 1986; Jameson and Johnson 1993). Annual estimates of birth rates (pups/adult) in British Columbia range from 0.12 to 0.33 and are similar to Alaska (0.30 pups/adult) and California (0.20 pups/adult) (Watson 1993; Watson et al. 1997).
A sea otter pup weighs 1.4 to 2.3 kg at birth (Riedman and Estes 1990). Pups remain dependent on their mothers for 6 to 8 months after which they are weaned (Payne and Jameson 1984; Jameson and Johnson 1993). Pre-weaning mortality is about 40% in California and 15 to 25% in Alaska (Siniff and Ralls 1991; Riedman et al. 1994). In general, pre-weaning mortality is higher in populations nearing equilibrium density than in growing populations (Monson et al. 2000a).
Sea otters feed on a wide variety of benthic invertebrates (e.g. clams, sea urchins etc.) with some variation in diet evident among individuals, among regions and according to population status (Estes et al. 1981; Estes et al. 2003a). In recently occupied rocky habitats, large easily-accessible prey such as sea urchins are consumed. As the abundance of preferred prey is reduced, the diet of the sea otter population diversifies to include a larger array of invertebrate species including various species of bivalves, snails, chitons, crabs, sea stars and even fish (Estes et al. 1981). Demersal fish are important prey in some parts of the Aleutian, Commander and Kuril Islands (Estes and VanBlaricom 1985; Watt et al. 2000). Prey species preference, at least in female otters, appears to be learned and is transmitted maternally (Estes et al. 2003a). Most foraging occurs in depths of 40 m or less, although otters are capable of foraging to depths of 100 m (Estes 1980; Riedman and Estes 1990; Bodkin et al. 2004). Foraging dives last from 50 seconds to more than 3 minutes (reviewed in Riedman and Estes 1990). Prey is consumed at the surface. Sea ottersuse rocks or other hard objects to break open hard-shelled prey.
The sea otter has little body fat and relies on a layer of air trapped in its dense fur and metabolic heat production to stay warm. The sea otter metabolic rate is 2.4 to 3.2 times higher than those of similar-sized terrestrial carnivores (Costa 1978; Costa and Kooyman 1982). To fuel internal heat production, free-ranging sea otters consume the equivalent of more than 20%of their body weight in prey per day (Costa 1978; Costa 1982).
Sea otters are non migratory and show great site fidelity, although seasonal movements and occasional long-distance movements of individuals may occur (Garshelis 1983; Jameson 1989). Sea otters occupy relatively small overlapping home ranges varying in size from a few to 10s of kilometres of coastline (Loughlin 1980; Garshelis et al. 1984; Jameson 1989). Population range expansion typically occurs when males move en masse from the periphery of the occupied range into previously unoccupied habitat. Females gradually occupy the areas vacated by males (Loughlin 1980; Garshelis et al. 1984; Wendell et al. 1986; Jameson 1989).
Sea otters in British Columbia are typically wary of humans, and rafts of sea otters are difficult to approach and are easily disturbed by boat traffic. Females with pups are most sensitive to disturbance. However, where sea otters are routinely exposed to boats or are adjacent to inhabited areas they seem to habituate to disturbance (e.g. Woolfenden 1995). In terms of foraging, sea otters demonstrate a range of techniques for obtaining and feeding on a wide variety of species that may be available within their physiological dive depth limit (reviewed in Riedman and Estes 1990).
Sea otters are adapted to a wide range of water temperatures. The northern range limit of otters appears to be the southern extent of pack ice, which excludes otters from foraging areas. The southern range limits are poorly understood, but appear to be associated with the southern extent of coastal upwelling and the 20-22 °C isotherm (Estes 1980, Bodkin 2003). Changes in water temperature may thus affect the future global range of sea otters.
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