Marine Mammals
IV. Sea otter

Though once plentiful along the west coast from central Baja California to Alaska, sea otters were hunted nearly to extinction between 1741 and 1911 (Kenyon 1975). Estimated to have numbered from 16,000 to 20,000 animals before commercial exploitation (California Department of Fish and Game 1976, Riedman and Estes 1990), the entire 1914 California population is believed to have consisted of 50 animals inhabiting the Point Sur area (Riedman and Estes 1990). Under international protection since 1911 and state protection since 1913, the California otter population has increased at a 6% to 7% annual rate since 1914, which represents 1/3 to 1/2 the rate of increase recorded for the Alaska otter population (Riedman and Estes 1990). The 1995 California otter population was approximately 2400 individuals (J. Ames pers. comm.). In 1977, the California sea otter was listed as a threatened species under the Endangered Species Act.

Sea otter distribution in California currently extends from near Año Nuevo Point, San Mateo County, southward to the Santa Maria River, San Luis Obispo County (Riedman and Estes 1990), i.e. overlapping broadly with the MBNMS . A small number of otters (< 20) originally from this population are also present around San Nicolas Island in the Channel Islands as a result of a U. S. Fish and Wildlife Service translocation study (U.S. Fish and Wildlife Service 1990). Sea otters exhibit a high degree of sexual segregation that varies somewhat seasonally, with the southern and northern peripheries of the California range occupied primarily by males, and the central portion of the range occupied primarily by females and pups (Riedman and Estes 1990).

Sea otters inhabit shallow coastal waters, seldom ranging more than 1 to 2 km from shore (Riedman and Estes 1990). In California, otters generally forage at depths of less than 25 m (Wild and Ames 1974), although along the northern Monterey peninsula males forage to depths of 30-40 m (Riedman and Estes 1990). Foraging studies, many conducted in the Monterey area, have found that in recently re-colonized areas with rocky substrates, otters forage almost exclusively on abalones and sea urchins; as the abundance of these preferred prey animals decreases, otters forage on other species including snails, mussels, chitons, and clams (Costa 1978, Fisher 1939, Hall and Schaller 1964). Food habits studies conducted in soft-bottom habitats, including areas in Monterey Bay and Elkhorn Slough, have revealed that otters in those areas feed primarily on clams (Miller et al. 1975, Wendell et al. 1986, Kvitek et al. 1988). Sea otter predation on certain species including abalone, sea urchins and Pismo clams appears to have markedly depleted the abundance of those species within otter foraging areas (Ebert 1968, Wild and Ames 1974, Miller et al. 1975, Wendell et al. 1986). The sea otter has been described as a "keystone species" for its role in structuring nearshore rocky communities (see discussions in Riedman and Estes 1990 and Kelp Forest section).