Charismatic in every sense of the word, sea otters (Enhydra lutris) have entertained peoples’ interests for decades making them one of today’s great marine icons of the North Pacific. Since the mid 1970s, sea otters have become popular with wildlife viewers, researchers, and resource managers—so much so that they are now a political symbol for environmental awareness, a key for non-profit advocacy, and anthropomorphized (personified) by the American culture. Cute, cuddly, and popular, however, does not hide the fact that the otter story in the United States is mired with declining populations and eventual threatened species status under the Endangered Species Act of 1973.
Currently, there are three recognized subspecies of sea otters: the Russian northern sea otter (Enhyrda lutris lutris); the northern sea otter (Enhydra lutris kenyoni); and the southern sea otter (E. l. nereis). E. l. lutris occurs in the Kuril Islands, Kamchatka Peninsula, and Commander Islands; E. l. kenyoni ranges from the Aleutian Island archipelago to Washington State; E. l. nereis is found in southern California. E. l. lutris and E. l. kenyoni are separated by approximately 320 km (200 mi) of open water, and the southernmost population of E. l. kenyoni is 965 km (600 mi) away from the southern sea otter.
Historically, sea otters were abundantly found all along the North Pacific Rim from southern California to northern Japan. During the fur trade (1750-early 1900s), otters were heavily exploited and hunted to near extinction. Only 13 isolated colonies, representing a total population of about 1,000-2,000 individuals, remained after the fur trade. By 1911, the International Fur Seal Treaty was formed, offering the first legal protection for sea otters to recover from their depleted status. After 20 years of commercial harvesting protection, 11 of the 13 colonies began to grow in numbers, some reaching historical population levels, and they recolonized much of their former ranges. During the 1960-1970s, small groups of otters from Amchitka Island and Prince William Sound were relocated to areas where populations might not have otherwise recovered.
By 1976, it was becoming apparent that the three subspecies of sea otters were showing different trends of recovery. The southern sea otter did not recover as fast as other populations—only an average growth rate of 5% from 1914-1984. The lack of rapid recovery and potential threat of oil spills prompted the U.S. Fish and Wildlife Service (USFWS) to list the southern sea otter as threatened in 1977. Since their listing, the population has fluctuated, but there have been recent declines from 1995-1999. The Russian otter populations in the Kuril and Commander Islands (E. l. lutris) had recovered to historical levels by the 1970s and 1980s, and by 2005 were reported to have the highest numbers ever recorded. Similarly, the northern sea otter (E. l. kenyoni) was presumed to have reached historical numbers by the mid 1980s, and was believed to contain more than half of the world’s sea otters. In 1989, however, the northern sea otter started to show signs of vulnerability. Approximately 2,000 otters died resulting from the 1989 Exxon Valdez oil spill in Prince William Sound. During the 1990s, populations in the Aleutian Archipelago declined by at least 55-67% and as much as 90% in some areas. The cause(s) of the decline remains unclear but compelling evidence supports predation by killer whales (Orcinus orca) as a major factor. It was the latter decline that drove the USFWS to list the southwest population of the northern sea otter (E. l. kenyoni) as threatened in 2005. This listing includes individuals found in the Aleutian Archipelago, Alaskan Peninsula coast, and Kodiak Archipeligo.
Being the smallest marine mammal in the world and living in some of nature’s harshest environments, sea otters have adapted to marine life much differently than pinnipeds and cetaceans. Sea otters lack a blubber (composed of lipid or fat) layer that most marine mammals use to store energy and keep them insulated. Instead, sea otters have the densest fur of any mammal averaging 100,000 hairs per square centimeter (645,000 hairs per square inch). To keep the insulation properties of the fur, sea otters must constantly groom it thereby adding air between the hairs. Grooming can occupy approximately 10% of their daily activities. They also have very high metabolic rates, which maintain their consistent level of heat production. Sea otters will eat between 23-33% of their body weight a day. Because of their extraordinary food requirements, sea otters are primarily limited to the coastal environment where food is more abundant. Traveling large distances away from coastal environments is extremely rare given their metabolic requirements and diet.
Sea otters prey upon many marine invertebrates—more than 150 different prey items, such as urchins, mollusks, and clams. These invertebrates are herbivores feeding upon the kelp forests that dominate the algal communities along the North Pacific coastline. Their abilities to use tools such as sharp rocks to crack open shells highlights their tactile abilites. Due to sea otters’ strong relationship to the coastal environment, they are often considered the classic example of a keystone predator—a predator that regulates the effects of other species in the community. In the Aleutian Islands for example, the presence of sea otters preying upon sea urchins promoted a healthier, more diverse ecosystem. The absence of sea otters in the same location allowed the urchins to heavily graze upon the kelp forests stripping the area barren. This phenomenon has been referred to as a trophic cascade—that is kelp forests are regulated by the presence or absence of sea otters due to their predation on the herbivores, which in turn, forage on the kelp.
Unlike other otter species but similar to other marine mammals, sea otters give birth to only one pup, although twins have been reported. Birth and mating typically occur in the water whereas phocids and otariids (seals and sea lions) birth at land sites called rookeries. Pups are born at any time of the year. Females reach sexual maturity at about 3 years and will enter estrus within hours of parturition also known as giving birth. The reproductive cycle is about 1 year—6 months from conception to birth, and another 6 months from birth to the pup being weaned from its mother. The latter stage is critical in the pup’s development as it learns to forage, groom, and use tools to open shells of prey. Thus, it is conceivable to have a biannual peak in pups—that is adult females giving birth to two pups in a single year.
Sea otters are moderately sexually dimorphic with adult males weighing 38% more than females and only slight longer (8%). Adult males weigh approximately 64 lbs (29 kg) and adult females weigh approximately 44 lbs (20 kg). Adult females are slightly smaller than 4 ft (120 cm) and adult males are slightly over 4 ft (129 cm).