Sea otter (Enhydra lutris) COSEWIC assessment and status report 2007L chapter 2
The sea otter, Enhydra lutris, is the smallest marine mammal and the only member of the family Mustelidae to carry out all aspects of its life in the ocean. The sea otter relies on a layer of air trapped in its dense fur and a very high metabolic rate to survive in chilly seas. The integrity of the fur--and its ability to maintain an insulating layer of trapped air is maintained by intensive frequent grooming. Its powerful forelimbs are well adapted for foraging on benthic invertebrates and its hind limbs are modified to function as flippers for swimming. The genus has three recognized subspecies --Enhydra lutris kenyoni, Enhydra lutris nereis and Enhydra lutris lutris. Enhydra lutris kenyoni occurs in British Columbia, Canada.
Sea otters are found in shallow coastal areas in the North Pacific and historically ranged from northern Japan to central Baja California, Mexico. As a result of an intensive maritime fur trade that commenced in the 1700s, sea otters were extirpated throughout much of their global range by 1911. Today, the sea otter occupies about one half to two thirds of its historical range (but declines in Western Alaska make this difficult to estimate). Sea otters were reintroduced to Checleset Bay, British Columbia from Alaska between 1969 and 1972, and presently occur along much of the west coast of Vancouver Island and along a small section of the central British Columbia coast. Sea otters were also reintroduced to Oregon, Washington and Southeast Alaska between 1965 and 1971. Most of the outer British Columbia coast was likely occupied by sea otters historically, and thus the current population occupies 25-33% of its estimated historical distribution.
Sea otters occupy coastal areas from the intertidal to at least 50m depths. The extent of their habitat is defined by their ability to dive to the sea floor for food with most foraging dives to depths less than 40m. In British Columbia sea otters are typically most abundant in exposed coastal areas with shallow rocky reefs. During the winter months sea otters appear to move to more sheltered areas within their home ranges.
Females reach sexual maturity between 3 and 5 years of age, and all are reproductive by age 5. In contrast, males reproduce at 5 to 6 years of age when they become socially mature, although they may be sexually mature earlier. Females live to a maximum of about 20 years and males to a maximum of 15 years. Females give birth to a single pup at approximately 1-year intervals. Pups remain dependent on their mothers for the first 6 to 8 months. Sea otters prey on a variety of invertebrates, including species of bivalves, snails, urchins, chitons, crabs, and sea stars. In the Aleutian, Commander andKuril Islands, sea otters are also known to eat demersal fish species.
Population Sizes and Trends
A survey in 2001 resulted in a count of 2673 otters along the Vancouver Island coast and 507 on the central British Columbia coast for a total of 3180 otters. On Vancouver Island the population growth rate was estimated to be 15.6% per year (1977 to 2004) based on a simple log-linear regression. A piece-wise regression which allows for an inflection in the log-linear trend showed that the initial rapid growth of 19.1% per year from 1977 to 1995 slowed to 8.0% per year from 1995 to 2004. This decline in the growth rate likely reflects parts of the population near the centre of the range reaching equilibrium densities along Vancouver Island, but other sources of mortality cannot be ruled out as at least a partial explanation for this decline. On the central British Columbia coast, the population growth rate was estimated to be 12.4% per year between 1990 and 2004. The sea otter population in British Columbia represents approximately 3 to 4% of the total global population.
Limiting Factors and Threats
In the absence of significant density-independent factors (e.g. predation), sea otter populations are thought to be regulated by food through density-dependent factors that affect juvenile survival. Sea otters were hunted by native peoples prior to European contact but were extirpated from most of their range as a result of the maritime fur trade of the 18th and 19th centuries. Aside from humans, predators include killer whales, sharks (in California), and bald eagles (preying primarily on pups). Threats and limitations to sea otter population growth include environmental contamination (particularly oil spills), entanglement in fishing gear and collision with vessels, illegal killing, disease and possibly human disturbance.
Special Significance of the Species
An important predator, the sea otter is considered a keystone species exerting strong effects on the structure and composition of the nearshore communities in which it evolved. Sea otters have endeared themselves to the public because of their “teddy-bear-like” appearance, their near brush with extinction, and their vulnerability to oil spills. For these reasons they are also of increasing interest to the wildlife-viewing tourism industry in British Columbia. However, because sea otters feed on invertebrates and can control the abundance of many invertebrate species, conflicts have arisen where-ever sea otters and commercial and subsistence invertebrate fisheries exist in California, Alaska, and British Columbia.
In Canada sea otters were federally listed as Threatened in 2003 and under the Species at Risk Act are protected from killing, harming, capturing, and harassing. Sea otters and their habitat are also protected from being hunted, trapped or killed under the Fisheries Act and by provisions in the British Columbia Wildlife Act.
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