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Sea Otter
A sea otter wraps itself in kelp in Morro Bay, California
Conservation status
Endangered (IUCN)
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Mammalia Order: Carnivora Family: Mustelidae Subfamily: Lutrinae Genus: Enhydra Fleming, 1828 Species: E. lutris
Binomial name
Enhydra lutris (Linnaeus, 1758)
Modern and historical range

The sea otter or kalan (Enhydra lutris) is a marine mammal native to the coasts of the North Pacific, from northern Japan and Kamchatka east across the Aleutian Islands and south to California.

Adult sea otters typically weigh between 14 and 45 kg, making them the heaviest members of the weasel family, but one of the smallest marine mammal species. Unlike most marine mammals, the sea otter has a thick coat of fur, the densest of all mammals with up to 400,000 hairs per squared centimeter. It generally inhabits shallow coastlines, where it preys mostly upon small invertebrates such as sea urchins, shellfish, and octopuses, and on fish.

The sea otter is a classic example of a keystone species. It is an important predator of sea urchins, which would otherwise inflict extensive damage to kelp forests. Kelp forests provide crucial habitat and food for other marine animals, and prevent coastal erosion.

Sea otters were once hunted extensively for their luxurious fur. The population is thought to have once been 150,000 to 300,000. It is estimated that a half million to a million otters were killed between 1741 and 1911. By then, the world population was estimated to be just 1,000–2,000 individuals in 13 colonies, and the sea otter was gone from most of its historic range. Populations have since rebounded well through conservation efforts and re-introduction into previously populated areas. However, the species is considered endangered due to its vulnerability to oil spills, orca predation, and disease.

Contents 1 Taxonomy 2 Biology and behavior 2.1 Physical characteristics 2.2 Hunting and diet 2.3 Behavior 2.4 Reproduction and lifecycle 3 Ecology 3.1 Distribution and habitat 3.2 Ecological role 4 History 4.1 The "Great Hunt" 4.2 Recovery 4.2.1 California 4.2.2 Alaska, British Columbia, Washington, and Oregon 4.2.3 Russia 5 Conservation 5.1 Oil spills 5.2 Pollution and disease 6 Gallery 7 Bibliography 8 Notes 9 External links

The sea otter is the heaviest member of the weasel (Mustelidae) family.^[1] The giant otter is longer, but significantly slimmer. It is a member of the otter (Lutrinae) subfamily, and the only member of the genus Enhydra. There are three recognized subspecies:^[1][2]

• The northern sea otter, Enhydra lutris kenyoni (Wilson, 1991), is native to the Aleutian Islands, and southern Alaska. It has been re-introduced to various locations between Alaska and Oregon.^[1] It is also known as the Alaskan sea otter.
• The southern sea otter, Enhydra lutris nereis (Merriam, 1904), is found off the coast of central California.^[1] It is also known as the Californian sea otter.
• The common sea otter, Enhydra lutris lutris (Linnaeus, 1758), ranges from the Kuril Islands to the Commander Islands in the western Pacific Ocean.^[1] It is also known as the Asian sea otter.

It is commonly believed that the closest relatives of the sea otter are river otters.^[3] Recent genetic analysis indicates that the sea otter's closest relatives are the Eurasian otter, spotted-necked otter, cape clawless otter and small-clawed otter.^[4]

The sea otter is not to be confused with the marine otter, a rare marine mammal native to the west coast of South America.

The sea otter is one of the smallest marine mammal species. Male sea otters weigh 22 to 45 kg (nearly 100 pounds) and are 1.2 to 1.5 m (nearly 5 feet) in length. Females are smaller, weighing 14 to 33 kg and measuring 1 to 1.4 m in length.^[5]

The species has exceptionally thick brown or reddish-brown fur with densities of 100,000 to 400,000 hairs/cm². The head, throat, and chest are lighter in color than the rest of the body. The fur consists of long guard hairs and a layer of dense underfur. When clean, the guard hairs are fully waterproof, allowing the underfur to retain air and remain dry and keeping cold water completely away from the skin.^[5]Unlike other marine mamals, the sea otter has no blubber and it relies on its fur to keep warm. It can reach and groom the fur on any part of its body, as it has loose skin and an unusually supple skeleton.^[6]

The sea otter displays numerous adaptations to its marine environment. The ears and nostrils can close.^[7] The hind feet, which provide most of its propulsion in swimming, are long, broadly flattened, and fully webbed.^[8] The fifth digit on each hind foot is the longest, which facilitates swimming while the animal is on is back but makes walking difficult.^[9] The tail is fairly short, thick, slightly flattened, and muscular. The sea otter swims underwater by moving the rear end of its body, including its tail and hind feet, up and down.^[10] When at the surface, it lies on its back and moves by sculling its feet and tail from side to side.^[11] The front paws are short with retractable claws, with tough pads on the palms that enable gripping slippery prey.^[12] On land, the sea otter moves with a lumbering gait because of its long body and the awkward shape of its hind feet.

The sea otter's body is highly buoyant because of its large lung capacity and the air trapped in its fur. Its lung capacity is 2.5 times greater than that of similarly-sized land mammals.^[13] The teeth, particularly the molars, are flattened and rounded, designed to crush rather than cut food.^[14] The sea otter is the only carnivore with two pairs of lower incisors rather than three.^[15] Unlike all other mustelids, the sea otter has no functional anal scent glands.^[16]

The sea otter hunts in short dives, often to the sea floor. Dives typically last about a minute and no more than 4 minutes.^[5] It is the only marine animal capable of lifting and turning over boulders, which it often does with its front paws when searching for prey.^[17] The sea otter may also pluck snails and other organisms from kelp, dig deep into mud for clams,^[17], and catch fish with its paws. Sea otters eat nearly any type of seafood available, including sea urchins, crabs, clams, octopus, and squid.

The sea otter has, under each foreleg, a pouch of skin that extends across the chest. In this pouch (typically the left one), the animal stores collected food to bring to the surface.^[15] There, the sea otter eats while floating on its back. It uses its lower incisor teeth to access the meat in shellfish.^[18]

The sea otter's use of rocks when hunting and feeding makes it one of the few mammal species to use tools.^[19] To open hard shells, it may pound its prey with both paws against a rock on its chest. To pry an abalone off its rock, a sea otter hammers the abalone shell using a large stone, at a rate of 45 blows in 15 seconds.^[5]

Feeding is crepuscular, peaking in the early morning and in the evening.^[5] As mothers teach hunting skills to their pups, sea otters often maintain matrilineal differences in food preferences.^[17]

Wild sea otters must eat 20–30% of their body weight in food each day (captives a little less), and may forage for prey as often as every 5 hours. A sea otter's metabolic rate is two or three times that of comparatively sized terrestrial mammals.^[20] They have the lowest assimilation efficiency of carnivores (82%), and their gut transit time is only 3 hours. Sea otters lose heat easily to their cold water environment, and therefore must maintain a high metabolism to stay warm.

The sea otter obtains most of its water from food. Unlike most other marine mammals it also drinks sea water, and has relatively large kidneys that enable it to derive fresh water from sea water and eliminate wastes as a concentrated urine.^[21]

Sea otters can live their entire lives in water, even giving birth there, but sometimes haul out to land to rest.^[17] They are generally sociable and often float together in large groups, called rafts. The largest raft of sea otters ever seen was over 2000 otters.^[22] To keep from drifting out to sea, they may also wrap themselves in kelp.

Females avoid males outside of breeding periods, and the otters segregate into male and female areas.^[5] Females may be seen in matrilineal groups with several generations sharing an area. Males sometimes concentrate in "bachelor groups" near the female areas. Conflicting data regarding home range and migration of sea otters suggests their movements are dependent on availability of resources. Home ranges may be as large as 5.4 square kilometers, with most animals traveling found within 1 or 2 kilometers of the previous day's location.

The sea otter is diurnal, and most of its day is spent foraging and grooming. Grooming consists of cleaning the fur, untangling knots, removing loose fur, rubbing the fur to squeeze out water and introduce air, and blowing air into the fur. To an observer it looks like the animal is scratching.^[23] When eating, the sea otter rolls in the water frequently, apparently to wash food scraps from its fur.^[17]

Males reach sexual maturity around 5 years of age, but often do not successfully breed until a few years later.^[25]. They have been known to still sire offspring at 19. Females become sexually mature at around 3 or 4 years of age.^[26] Sea otters are polygynous: that is, males have multiple female partners. Females in captivity are seen to be polyestrous, coming into estrus in late winter/spring and then again in late summer/autumn. Bonding does occur between the sexes during the female's estrus, which lasts 3–5 days. During these periods males defend their territories; there is very rarely actual fighting involved, with most disputes being settled by raucous posturing.

A courting pair swims and dives together, with the male twisting and doing corkscrews in the water to let the female know he is interested. The male may swim facedown and swim more quickly than usual. Mating takes place in the water and can be rough, the male biting the female on the side of the face or on the muzzle and sometimes holding her head under water. Females often have characteristic scars on the nose from this,^[1] and at least one female is known to have died from an infected nose.^[27]

No set breeding season has been observed, but peaks occur from May to June in northern populations and January to March in southern populations.^[28] The species is capable of delayed implantation. California otters this lasts 2–3 months with gestation taking 6 months or so. In Alaska gestation can be longer and around 7–8 months, with 3.5–4.5 months of this in an unimplanted phase.

Gestation usually ends with the birth of a single pup weighing 1.4 to 2.3 kg. Twins occur in 2% of births, however usually only one of them survives.^[1] Pups are born with a brownish yellow fur which is fluffier than adult fur and lasts for over two months. Sea otter baby fur retains so much air that it makes the pup unable to dive.^[19] The mother gives her pup almost constant attention, cradling it on her chest away from the cold water and attentively grooming its fur. When searching for food, she leaves her pup floating on the water, sometimes wrapped in kelp to keep it from floating away.^[29] If the pup is not sleeping, it cries loudly until she returns.^[30] The species is known for the depth of the affectionate bond between mother and pup; mothers have been known to carry their pup for days after the pup's death.^[31]

Nursing lasts six to eight months for Californian sea otters, and four to twelve months for Alaskan sea otters.^[32] The milk is more similar in composition to other marine mammals than to other mustelids, with 23% fat, 13% protein, and only 1% lactose. The mother begins to offer bits of prey after one to two months.^[33] The mother teaches the pup how to hunt, dive, and groom. A pup practices swimming and diving for several weeks before it is able to reach the bottom. Initially the objects it retrieves are of little food value, such as brightly colored starfish and pebbles.^[15] Pups stay with their mothers for 5–15 months (averaging 6) and many times only 30% of pups survive their first year. Experienced mothers have the best success. Occasionally adoptions have been seen.

In California, sea otters usually breed every year, about twice as often as Alaskan sea otters.^[34] Male sea otters do not help to raise pups.^[35]

In the wild, sea otters live to a maximum age of 23 years, and about 10 to 11 years on average.^[5] They can live longer than 20 years in captivity.

Sea otters typically live in coastal waters 15 to 23 meters (50 to 75 feet) deep.^[36] They are found most often in areas with protection from the most severe ocean winds, such as rocky coastlines, thick kelp forests, and barrier reefs.^[37] The sea otter's historic range is a wide arc across the North Pacific, from northern Japan and the Kuril Islands north to the Commander Islands of Russia, and down the coast of North America to the Baja Peninsula of Mexico.^[38] They are currently found in significant numbers in about two thirds of their original range.

Reported sightings of sea otters in the Strait of Georgia and Puget Sound invariably turn out to be river otters, which are commonly seen along the seashore.^[15] It is unlikely that sea otters ever existed in significant numbers in these inner coastal waters.

Sea otters are a keystone species; their presence affects the ecosystem more profoundly than their size and numbers would suggest. Sea otters keep the population of certain invertebrates, particularly the sea urchin, in check. Sea urchins graze on the roots of kelp, causing the kelp to drift away and die. Loss of the habitat and nutrients provided by kelp forests leads to profound cascade effects on the marine ecosystem. Coastal areas that do not have sea otters often turn into urchin barrens, with abundant sea urchins and no kelp forest.^[1] Reintroduction of sea otters to British Columbia has led to a dramatic improvement in the health of coastal ecosystems.^[39]

The sea otter population the Aleutian Islands of Alaska has plummeted in recent decades. In the 1980s, the area was home to an estimated 55,000 to 100,000 sea otters, but the population fell to around 6,000 animals by 2000.^[40] One controversial hypothesis is that orcas have been eating the otters. The evidence in support of this explanation is circumstantial: It is unlikely that the otters have been dying from disease or starvation, as few bodies of sick or emaciated otters have been recovered.^[40] Also, populations have declined in areas of open water frequented by orcas, but not in nearby lagoons where orcas are absent.^[41]

Some Alaska orcas specialize in preying on marine mammals and others on fish. The orcas that prefer marine mammal prey usually attack seals, sea lions, and small cetaceans, and have occasionally been seen preying on grey whale calves. The small, furry sea otter offers little nutrition for an orca, however orcas may have started eating otters due to shortages of their usual, much larger, prey. The region's population of large whales was decimated by commercial whaling in the 1960s. Then stocks of harbor seals and Steller sea lions experienced massive declines in the 1970s and 1980s, respectively, which may have forced orcas to seek smaller prey.^[42] The theory that orca predation has been responsible for this cascade of declines remains controversial, and so far there has been no direct evidence that orcas prey on sea otters to any significant extent.^[41]

Other predators of sea otters include sharks and sea lions.^[20] Bald eagles prey on sea otter pups, snatching them from the water surface.^[19]

In some areas, the sea otter competes with humans for food, particularly some species of abalone and crab.

Archaeological evidence indicates that for thousands of years, indigenous peoples have hunted sea otters for food and fur.^[43] They took sea otters in moderation; large-scale hunting began in the 1700s when hunters began to arrive from all over the world to meet foreign demand for otter pelts, which were one of the world's most valuable types of fur.^[44] The sea otter population is thought to have once been 150,000 to 300,000. It is estimated that approximately one million sea otters were killed between 1741 and 1911.^[45]

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In the 1600s to the 1700s, Russia was heavily involved in the sable fur trade. Peter the Great declared that Russia should have the monopoly in sable furs, and that they should find new populations of sable to hunt. The Russians conquered Kamchatka, in the eastern edge of Russia, where they could harvest sable and sea otter fur. The Russians continued to explore the north Pacific, looking for sea routes to Japan and America. Vitus Bering and Alexi I. Chirikov, two Navy officers, were given the task of mapping the Arctic coast and finding a way to America. Chirikov managed to return to Kamchatka in 1741, after much difficulty. Bering's sailors suffered from scurvy, and were marooned on Bering Island, in the Commander Islands, where Bering died. There, the surviving crew spent the winter hunting sea otters and gambling with otter pelts. They finally returned home in 1742 with 900 sea otter pelts, enough to pay for the entire expedition. It was Bering's expedition that set off what's called The Great Hunt which would continue for another hundred years.

The Russians sent many ships to harvest otter furs, and soon depleted all of the otter populations in the Commander Islands. In 1745, they began to move on to the Aleutian Islands, which were inhabited by the Aleuts.^[46] The Russians initially traded with the Aleuts for otter pelts, but later enslaved the Aleuts, taking women and children hostage and forcing the men to hunt. Many Aleuts were either murdered by the Russians or died from diseases that the hunters had introduced.^[47] The Aleut population was reduced, by the Russian's own estimate, from 20,000 to 2,000.^[48] It wasn't only otters that were hunted; the Russians wiped out the local fox population, along with the Aleuts. It wasn't until 1776, when Captain James Cook reached the north Pacific that other nations joined in the hunt. Captain Cook was killed in Hawaii by natives, but his crew sailed on to the lively trading port of Guangzhou in China, where they found merchants offering outrageous amounts of money for even the smallest scrap of otter fur. The sailors almost mutinied in their desire to return for more otters. It was then that the English discovered the value of these skins. A pelt could be worth as much as $1,125.^[1]

The Great Hunt continued, with Russians, Americans, and other Europeans competing amongst each other for that prized commodity. Alaska was almost completely depleted of sea otters, causing the Russians to sell it to the United States in 1867. Now, the sea otters were being slaughtered by the Americans.

In 1911, the International Fur Seal Treaty was signed, bringing an end to the large-scalehunting of sea otters.^[1] So few remained, perhaps only 1,000–2,000 individuals in the wild, that many assumed they would become extinct.

The pre-fur trade population in California is believed to have been 16,000.^[49] In 1938, biologists discovered a group of about 50 southern sea otters in a remote part of the coast south of Carmel, California. With conservation efforts, this group has since grown and expanded its range.^[50] However, numbers have not increased significantly since the late 1980s.^[49] A survey taken in the spring of 2007 counted over 3,000 sea otters in California.^[49]

In the 1930s, sea otter colonies were discovered in Alaska's Aleutian Islands and Prince William Sound. A sanctuary was created in Amchitka Island, whose sea otter population grew to outstrip its supply of prey.^[51] By the mid-1960s, Amchitka Island was being used a site for nuclear testing, which would kill many sea otters in the area. In advance of a major test in 1968, the U.S. Atomic Energy Commission agreed to move hundreds of the animals to other parts of the coast. Seven hundred sea otters were transplanted in the 1960s and 1970s, with survival rates improving as scientists became more knowledgeable about how to safely transport the animals.^[52]

Between 1969 and 1972, 89 sea otters were flown or shipped from Alaska to the west coast of Vancouver Island, British Columbia. They established a healthy population, estimated to be over 3,000 as of 2004.^[53] Their range is now from Tofino to Cape Scott.^[53] In 1989, a separate colony was discovered in the central B.C. coast. It is not known if this colony, which had a size of about 300 animals in 2004, was founded by transplanted otters or by survivors of the fur trade.^[53]

In 1969 and 1970, 59 sea otters were transplanted from Amchitka Island to Washington State. Between 2000 and 2004, annual surveys have counted between 504 and 743 sea otters.^[54] Attempts were made to transplant 93 sea otters to the Oregon coast in the 1970s, however none have been seen since the early 1980s.^[55] It is not known if they died or moved away.^[56]

Before the 19th century there were around 20,000 to 25,000 sea otters in the Kuril Islands, with more on Kamchatka and the Commander Islands. After the years of the Great Hunt, the population in these areas, currently part of Russia, was only 750.^[57] As of 2004, sea otters have repopulated all of their former habitat in these areas, with an estimated total population of about 27,000. Of these, about 19,000 are in the Kurils, 2000 to 3500 on Kamchatka and another 5000 to 5500 on the Commander Islands.^[57] Growth has slowed slightly, suggesting that the numbers are reaching carrying capacity.^[57] The success of the sea otter's recovery in Russia has been credited to large-scale and long-term protection, enlargement of the species's range, and human emigration from the islands.^[57]

The IUCN lists sea otter as an endangered species, and describes the significant threats to sea otters as oil pollution, predation by orcas, poaching, and conflicts with fisheries. There have been reports that sea otters have been poached in Russia. Sea otters can drown if entangled in fishing gear.^[58]

It is unclear why sea otter populations in some areas are growing slowly or declining. In California, unusually high mortality rates amongst adult and young adult otters, particularly females, have been reported, with disease a possible but not certain cause.^[49]

In Canada, the sea otter is considered a threatened species and is protected under the federal Species at Risk Act.^[39] In Washington State, it has been listed as an endangered species since 1981.^[54] In California, it is considered a threatened species.^[49]

The most significant threat to sea otters is oil spills.^[19] Sea otters are particularly vulnerable, as they rely on their fur to keep warm. When their fur is soaked with oil, it loses its ability to retain air, and the animal quickly dies from hypothermia.^[19] Also, sea otters groom themselves constantly, and when doing so inhale and ingest the oil. This can cause damage to the liver, kidneys, and lungs.^[19]

Prevention of oil spills, and preparation for rescue of otters in the event of a spill, is a major focus of conservation efforts. The small geographic ranges of the sea otter populations in California, Washington, and British Columbia mean that a single major spill could be catastrophic for that state or province.^[17][20][54]

The 1989 Exxon Valdez oil spill devastated the sea otter population in Prince William Sound, Alaska. Over 1,000 oiled sea otter carcasses were recovered,^[50] and the actual death toll was estimated to be several times that number.^[17][59] Approximately 350 oiled sea otters were rescued, and over the next five months given intensive rehabilitation.^[60] Each otter was tranquilized and thoroughly washed and dried. Those which had swallowed a lot of oil were treated with activated charcoal. The effort saved about 200 of the 350 rescued sea otters, although many later died after being released.^[61] A 2006 report from the Exxon Valdez Oil Spill Trustee Council named the sea otter as one of several species still being affected by the lingering oil in the area.^[62]

Parasites and infectious diseases cause 40% of sea otter deaths in California.^[50] In recent years, pathogens from urban cats may have been responsible for sea otter deaths. According to a 2002 study in California, otters near heavy freshwater flows — storm drains and river mouths — are three times more likely to have been infected by the often-fatal Toxoplasma gondii parasite than otters from areas where runoff is light.^[63]

In a survey of 233 live and dead otters from Santa Barbara to Half Moon Bay, California, 76 percent of sea otters near heavy freshwater outflows had antibodies to Toxoplasma gondii. In the general sea otter population, 42 percent of live otters surveyed had antibodies to the parasite, an almost certain sign of infection. Domestic and wild cats are the only animals known to shed the parasite's eggs in their droppings.^[63] The droppings can then be washed into streams and storm drains by rain, or after being flushed down the toilet. Cat owners are encouraged to dispose of droppings in the trash rather than flushing them.^[64]

Sea otter with pup in Morro Bay	An Alaskan sea otter	A sea otter cradling her pup	Sea otter in Morro Bay
Sea otters are social, but do not form stable pair bonds.	Sea otters at Moss Landing Harbor.	Sea otter at Moss Landing Harbor.	Sea otter at the Vancouver Aquarium. The unusual shape of the animal's hind feet, in which the outermost digit is the longest, can be seen.

• Kenyon, Karl W. (1969). The Sea Otter in the Eastern Pacific Ocean. Washington, D.C.: U.S. Bureau of Sport Fisheries and Wildlife. 
• Love, John A. (1992). Sea Otters. Golden, Colorado: Fulcrum Publishing. ISBN 1-55591-123-4. 
• Silverstein, Alvin; Silverstein, Virginia and Robert (1995). The Sea Otter. Brookfield, Connecticut: The Millbrook Press, Inc.. ISBN 1-56294-418-5. 

• Animal Diversity Web: Enhydra lutris
• OceanInfo: Sea otters
• The Otter Project
• Save The Otters