The sea otter inhabits nearshore environments where it can quickly dive to the sea floor to forage. It preys mostly upon marine invertebrates such as sea urchins, various molluscs and crustaceans, and some species of fish. Its foraging and eating habits are noteworthy in several respects. First, its use of rocks to dislodge prey and to open shells makes it one of the few mammal species to use tools. In most of its range, it is a keystone species, controlling sea urchin populations which would otherwise inflict extensive damage to kelp forest ecosystems. Finally, its diet includes prey species that are also valued by humans as food, leading to conflicts between sea otters and fisheries.
Sea otters, whose numbers were once estimated at 150,000–300,000, were hunted extensively for their fur between 1741 and 1911, and the world population fell to 1,000–2,000 individuals in a fraction of their historic range. A subsequent international ban on hunting, conservation efforts, and reintroduction programs into previously populated areas have contributed to numbers rebounding, and the species now occupies about two-thirds of its former range. The recovery of the sea otter is considered an important success in marine conservation, although populations in the Aleutian Islands and California have recently declined or have plateaued at depressed levels. For these reasons (as well as its particular vulnerability to oil spills) the sea otter remains classified as an endangered species.
The sea otter is the heaviest member of the family Mustelidae, a diverse group that includes the thirteen otter species and terrestrial animals such as weasels, badgers, and minks. It is unique among the mustelids in not making dens or burrows, in having no functional anal scent glands, and in being able to live its entire life without leaving the water. The only member of the genus Enhydra, the sea otter is so different from other mustelid species that as recently as 1982, some scientists believed it was more closely related to the earless seals. Genetic analysis indicates that the sea otter and its closest extant relatives, which include the African speckle-throated otter, Eurasian otter, African clawless otter and oriental small-clawed otter, shared an ancestor approximately 5 million years ago (mya).
Fossil evidence indicates that the Enhydra lineage became isolated in the North Pacific approximately 2 mya, giving rise to the now-extinct Enhydra macrodonta and the modern sea otter, Enhydra lutris. The sea otter evolved initially in northern Hokkaidō and Russia, then spread east to the Aleutian Islands, mainland Alaska, and down the North American coast. In comparison to cetaceans, sirenians, and pinnipeds, which entered the water approximately 50 mya, 40 mya, and 20 mya, respectively, the sea otter is a relative newcomer to a marine existence. In some respects, however, the sea otter is more fully aquatically adapted than pinnipeds, which must haul out on land or ice to give birth.
The sea otter is one of the smallest marine mammal species. Male sea otters weigh 22 to 45 kg (49 to 99 lb) and are 1.2 to 1.5 m (4 to 5 ft) in length. Females are smaller, weighing 14 to 33 kg (30 to 73 lb) and measuring 1.0 to 1.4 m (3 ft 3 in to 4 ft 7 in) in length.
Unlike other marine mammals, the sea otter has no blubber and relies on its exceptionally thick fur to keep warm. With up to 150 thousand strands of hair per square centimeter (nearly one million per sq in), its fur is the most dense of any animal. The fur consists of long waterproof guard hairs and short underfur; the guard hairs keep the dense underfur layer dry. Cold water is thus kept completely away from the skin and heat loss is limited. The fur is thick year-round, as it is shed and replaced gradually rather than in a distinct molting season. As the ability of the guard hairs to repel water depends on utmost cleanliness, the sea otter has the ability to reach and groom the fur on any part of its body, taking advantage of its loose skin and an unusually supple skeleton. The coloration of the pelage is usually deep brown with sliver-gray speckles, however it can range from yellowish or grayish brown to almost black. In adults, the head, throat, and chest are lighter in color than the rest of the body.
The sea otter displays numerous adaptations to its marine environment. The nostrils and small ears can close. The hind feet, which provide most of its propulsion in swimming, are long, broadly flattened, and fully webbed. The fifth digit on each hind foot is longest, facilitating swimming while on its back, but making walking difficult. The tail is fairly short, thick, slightly flattened, and muscular. The front paws are short with retractable claws, with tough pads on the palms that enable gripping slippery prey.
The sea otter propels itself underwater by moving the rear end of its body, including its tail and hind feet, up and down, and is capable of speeds of up to 9 km/h (5.6 mph). When underwater, its body is long and streamlined, with the short forelimbs pressed closely against the chest. When at the surface, it usually floats on its back and moves by sculling its feet and tail from side to side. At rest, all four limbs can be folded onto the torso to conserve heat, whereas on particularly hot days the hind feet may be held underwater for cooling. The sea otter's body is highly buoyant because of its large lung capacity – about 2.5 times greater than that of similarly-sized land mammals – and the air trapped in its fur. The sea otter walks with a clumsy rolling gait on land, and can run in a bounding motion.
Long, highly sensitive whiskers and front paws help the sea otter find prey by touch when waters are dark or murky. Researchers have noted that when they approach in plain view, sea otters react more rapidly when the wind is blowing towards the animals, indicating that the sense of smell is more important than sight as a warning sense. Other observations indicate that the sea otter's sense of sight is useful above and below the water, although not as good as that of seals. Its hearing is neither particularly acute nor poor.
An adult's 32 teeth, particularly the molars, are flattened and rounded, designed to crush rather than cut food. Seals and sea otters are the only carnivores with two pairs of lower incisor teeth rather than three; the adult dental formula is: The sea otter has a metabolic rate two or three times that of comparatively sized terrestrial mammals. It must eat an estimated 25 to 38% of its own body weight in food each day in order to burn the calories necessary to counteract the loss of heat due to the cold water environment. Its digestive efficiency is estimated at 80 to 85%, and food is digested and passed in as little as three hours. Most of its need for water is met through food, although, in contrast to most other marine mammals, it also drinks seawater. Its relatively large kidneys enable it to derive fresh water from sea water and excrete concentrated urine.
The sea otter is diurnal. It has a period of foraging and eating in the morning, starting about an hour before sunrise, then rests or sleeps in mid-day. Foraging resumes for a few hours in the afternoon and subsides before sunset, and there may be a third foraging period around midnight. Females with pups appear to be more inclined to feed at night. Observations of the amount of time a sea otter must spend each day foraging range from 24 to 60%, apparently depending on the availability of food in the area.
The sea otter spends much of its time grooming, which 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 appears as if the animal is scratching, however sea otters are not known to have lice or other parasites in the fur. When eating, the sea otter rolls in the water frequently, apparently to wash food scraps from its fur.
Under each foreleg, the sea otter has a loose pouch of skin that extends across the chest. In this pouch (preferentially the left one), the animal stores collected food to bring to the surface. There, the sea otter eats while floating on its back, using its forepaws to tear food apart and bring it to its mouth. It can chew and swallow small mussels with their shells, whereas large mussel shells may be twisted apart. It uses its lower incisor teeth to access the meat in shellfish. To eat large sea urchins, which are mostly covered with spines, the sea otter bites through the underside where the spines are shortest, and licks the soft contents out of the urchin's shell.
The sea otter's use of rocks when hunting and feeding makes it one of the few mammal species to use tools. 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, it hammers the abalone shell using a large stone, with observed rates of 45 blows in 15 seconds. Releasing an abalone, which can cling to rock with a force equal to 4,000 times its own body weight, requires multiple dives.
A male sea otter is most likely to mate if he maintains a breeding territory in an area that is also favored by females. As autumn is the peak breeding season in most areas, males typically defend their territory only from spring to autumn. During this time, males patrol the boundaries of their territories to exclude other males, although actual fighting is rare. Adult females move freely between male territories, where they outnumber adult males by an average of five to one. Males who do not have territories tend to congregate in large male-only groups, and swim through female areas when searching for a mate.
The species exhibits a variety of vocal behaviors. The cry of a pup is often compared to that of a seagull. Females coo when they are apparently content; males may grunt instead. Distressed or frightened adults may whistle, hiss, or in extreme circumstances, scream.
Although sea otters can be playful and sociable, they are not considered to be truly social animals. They spend much time alone, and each adult can meet its own needs in terms of hunting, grooming, and defense.
Sea otters are polygynous: males have multiple female partners. However, temporary pair-bonding occurs for a few days between a female in estrus and her mate. Mating takes place in the water and can be rough, the male biting the female on the muzzle – which often leaves scars on the nose – and sometimes holding her head under water.
Births occur year-round, with peaks between May and June in northern populations and between January and March in southern populations. Gestation appears to vary from four to twelve months, as the species is capable of delayed implantation followed by four months of pregnancy. In California, sea otters usually breed every year, about twice as often as sea otters in Alaska.
Birth usually takes place in the water and typically produces a single pup weighing 1.4 to 2.3 kg (3 to 5 lb). Twins occur in 2% of births; however, usually only one pup survives. At birth, the eyes are open, ten teeth are visible, and the pup has a thick coat of baby fur. Mothers have been observed to lick and fluff a newborn for hours; after grooming, the pup's fur retains so much air that the pup floats like a cork and cannot dive. The fluffy baby fur is replaced by adult fur after about thirteen weeks.
Nursing lasts six to eight months in California populations and four to twelve months in Alaska, with the mother beginning to offer bits of prey at one to two months. The milk from a sea otter's two abdominal nipples is rich in fat and more similar to the milk of other marine mammals than to that of other mustelids. A pup, with guidance from its mother, practices swimming and diving for several weeks before it is able to reach the sea floor. Initially the objects it retrieves are of little food value, such as brightly colored starfish and pebbles. Juveniles are typically independent at six to eight months, however a mother may be forced to abandon a pup if she cannot find enough food for it and at the other extreme, a pup may nurse until it is almost adult size. Pup mortality is high, particularly during an individual's first winter – by one estimate, only 25% of pups survive their first year. Pups born to experienced mothers have the highest survival rates.
Females perform all tasks of feeding and raising offspring, and have occasionally been observed caring for orphaned pups. Much has been written about the level of devotion of sea otter mothers for their pups – a mother gives her infant almost constant attention, cradling it on her chest away from the cold water and attentively grooming its fur. When foraging, she leaves her pup floating on the water, sometimes wrapped in kelp to keep it from floating away; if the pup is not sleeping, it cries loudly until she returns. Mothers have been known to carry their pup for days after the pup's death.
Females become sexually mature at around three or four years of age and males at around five; however, males often do not successfully breed until a few years later. A captive male sired offspring at age 19. In the wild, sea otters live to a maximum age of 23 years, with average lifespans of 10–15 years for males and 15–20 years for females. Several captive individuals have lived past 20 years, and a female at the Seattle Aquarium died at the age of 28 years. Sea otters in the wild often develop worn teeth, which may account for their apparently shorter lifespans.
Sea otters live in coastal waters 15 to 23 meters (50 to 75 ft) deep, and usually stay within a kilometer (⅔ mi) of the shore. 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. Although they are most strongly associated with rocky substrates, sea otters can also live in areas where the sea floor consists primarily of mud, sand, or silt. Their northern range is limited by ice, as sea otters can survive amidst drift ice but not land-fast ice. Individuals generally occupy a home range a few kilometers long, and remain there year-round.
The sea otter population is thought to have once been 150,000 to 300,000, stretching in an arc across the North Pacific from northern Japan to the central Baja Peninsula in Mexico. The fur trade that began in the 1740s reduced the sea otter's numbers to an estimated 1,000 to 2,000 members in thirteen colonies. In about two-thirds of its former range, the species is at varying levels of recovery, with high population densities in some areas and threatened populations in others. Sea otters currently have stable populations in parts of the Russian east coast, Alaska, British Columbia, Washington, and California, and there have been reports of recolonizations in Mexico and Japan. Population estimates made between 2004 and 2007 give a worldwide total of approximately 107,000 sea otters.
In 1969 and 1970, 59 sea otters were translocated from Amchitka Island to Washington. Annual surveys between 2000 and 2004 have recorded between 504 and 743 individuals, and their range is in the Olympic Peninsula from just south of Destruction Island to Pillar Point.
In British Columbia and Washington, sea otters are found almost exclusively on the outer coasts. Reported sightings of sea otters in the San Juan Islands and Puget Sound almost always turn out to be northern river otters which are commonly seen along the seashore. However, biologists have confirmed isolated sightings of sea otters in these areas since the mid-1990s.
When the Fish and Wildlife Service implemented the translocation program, it also attempted to implement "zonal management" of the California population. To manage the competition between sea otters and fisheries, it declared an "otter-free zone" stretching from Point Conception to the Mexican border. In this zone, only San Nicolas Island was designated as sea otter habitat, and sea otters found elsewhere in the area were supposed to be captured and relocated. These plans were abandoned after it proved impractical to capture the hundreds of otters which ignored regulations and swam into the zone. However, after engaging in a period of public commentary in 2005, the Fish and Wildlife Service has yet to release a formal decision on the issue.
Sea otters consume over 100 different prey species. In most of its range, the sea otter's diet consists almost exclusively of marine invertebrates, including sea urchins, a variety of bivalves such as clams and mussels, abalone, other mollusks, crustaceans, and snails. Its prey ranges in size from tiny limpets crabs to giant octopuses. Where prey such as sea urchins, clams, and abalone are present in a range of sizes, sea otters tend to select larger items over smaller ones of similar type. In California, it has been noted that sea otters ignore Pismo clams smaller than 3 inches (7 cm) across.
In a few northern areas, fish are also eaten. In studies performed at Amchitka Island in the 1960s, where the sea otter population was at carrying capacity, 50% of food found in sea otter stomachs was fish. The fish species were usually bottom-dwelling and sedentary or sluggish forms, such as the red Irish lord and globefish. However, south of Alaska on the North American coast, fish are a negligible or extremely minor part of the sea otter's diet. Contrary to popular depictions, sea otters rarely eat starfish, and any kelp that is consumed apparently passes through the sea otter's system undigested.
The individuals within a particular area often differ in their foraging methods and their prey types, and tend to follow the same patterns as their mothers. The diet of local populations also changes over time, as sea otters can significantly deplete populations of highly preferred prey such as large sea urchins, and prey availability is also affected by other factors such as fishing by humans. Sea otters can thoroughly remove abalone from an area except for specimens in deep rock crevices, however, they never completely wipe out a prey species from an area. A 2007 California study demonstrated that in areas where food was relatively scarce, a wider variety of prey was consumed. However, surprisingly, the diets of individuals were more specialized in these areas than in areas where food was plentiful.
Reintroduction of sea otters to British Columbia has led to a dramatic improvement in the health of coastal ecosystems, and similar changes have been observed as sea otter populations recovered in the Aleutian and Commander Islands and the Big Sur coast of California However, some kelp forest ecosystems in California have also thrived without sea otters, with sea urchin populations apparently controlled by other factors. The role of sea otters in maintaining kelp forests has been observed to be more important in areas of open coast than in more protected bays and estuaries.
In addition to promoting growth of kelp forests, sea otters can also have a profound effect in rocky areas that tend to be dominated by mussel beds. They remove mussels from rocks, liberating space for competitive species and thereby increasing the diversity of species in the area.
Archaeological evidence indicates that for thousands of years, indigenous peoples have hunted sea otters in moderation for food and fur. Large-scale hunting, which would eventually kill approximately one million sea otters, began in the 1700s when hunters and traders 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.
In the early 1700s, Russians began to hunt sea otters in the Kuril Islands and sold them to China. Russia was also exploring the far northern Pacific at this time, and sent Vitus Bering to map the Arctic coast and find routes from Siberia to North America. In 1741, on his second North Pacific voyage, Bering was shipwrecked off Bering Island in the Commander Islands, where Bering and many of his crew died. The surviving crew members, which included naturalist Georg Steller, discovered sea otters on the beaches of the island and spent the winter hunting sea otters and gambling with otter pelts. They returned to Siberia having killed nearly 1000 sea otters, and were able to command high prices for the pelts. Thus began what is sometimes called the "Great Hunt", which would continue for another hundred years.
Russian fur-hunting expeditions soon depleted the sea otter populations in the Commander Islands, and by 1745 they began to move on to the Aleutian Islands. The Russians initially traded with the Aleuts inhabitants of these islands for otter pelts, but later enslaved the Aleuts, taking women and children hostage and torturing and killing Aleut men to force them to hunt. Many Aleuts were either murdered by the Russians or died from diseases that the hunters had introduced. The Aleut population was reduced, by the Russians' own estimate, from 20,000 to 2,000. By the 1760s, the Russians had reached Alaska. Other nations joined in the hunt in the south. Along the coasts of what is now Mexico and California, Spanish explorers bought sea otter pelts from Native Americans and sold them in Asia. In 1778, British explorer Captain James Cook reached Vancouver Island and bought sea otter furs from the First Nations people. When Cook's ship later stopped at a Chinese port, the pelts rapidly sold at high prices, and were soon known as "soft gold". As word spread, people from all over Europe and North America began to arrive in the Pacific Northwest to trade for sea otter furs.
Russian hunting expanded to the south, in what is now Washington, Oregon, and California, and the Russians founded what is now the Fort Ross settlement in northern California as their southern headquarters. In the next 29 years, they would kill 50,000 California sea otters.
Eventually, sea otter populations became so depleted that commercial hunting was no longer viable. In the Aleutian Islands, commercial hunting had stopped by 1808. When Russia sold Alaska to the United States in 1867, the Alaska population had recovered to over 100,000, but Americans resumed hunting and quickly extirpated the sea otter again. Prices rose as the species became rare: During the 1880s, a pelt brought $105 to $165 in the London market, however by 1903 a pelt could be worth as much as $1,125. In 1911, Russia, Japan, Great Britain (for Canada) and the United States signed the Treaty for the Preservation and Protection of Fur Seals, imposing a moratorium on the harvesting of sea otters. So few remained, perhaps only 1,000–2,000 individuals in the wild, that many believed the species would become extinct.
During the 20th century, sea otter numbers rebounded in about two-thirds of their historic range, a recovery that is considered one of the greatest successes in marine conservation. However, the IUCN lists the 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 – sea otters can drown if entangled in fishing gear. The hunting of sea otters is no longer legal except for limited harvests by indigenous peoples in the United States. Poaching was a serious concern in the Russian Far East immediately after the collapse of the Soviet Union in 1991, however it has declined significantly with stricter law enforcement and better economic conditions.
The most significant threat to sea otters is oil spills. 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. The liver, kidneys, and lungs of sea otters also become damaged after they inhale oil or ingest it when grooming. The Exxon Valdez oil spill of 24 March 1989 killed thousands of sea otters in Prince William Sound, and as of 2006 the lingering oil in the area continues to affect the population. Describing the public sympathy for sea otters that developed from media coverage of the event, a U.S. Fish and Wildlife Service spokesperson wrote:
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. Prevention of oil spills and preparation for the rescue of otters in the event of one are major areas of focus for conservation efforts. Increasing the size and the range of sea otter populations would also reduce the risk of an oil spill wiping out a population. However, because of the species' reputation for depleting shellfish resources, advocates for commercial, recreational, and subsistence shellfish harvesting have often opposed allowing the sea otter's range to increase, and there have even been instances of fishermen and others illegally killing them.
In the Aleutian Islands, a massive and unexpected disappearance of sea otters has occurred 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. The most widely-accepted, but still controversial, hypothesis is that orcas have been eating the otters. The pattern of sea otter disappearances is consistent with a rise in orca predation, however there has been no direct evidence that orcas prey on sea otters to any significant extent.
Another area of concern is California, where recovery began to fluctuate or decline in the late 1990s. Unusually high mortality rates amongst adult and sub-adult otters, particularly females, have been reported. Necropsies of dead sea otters indicate that diseases, particularly Toxoplasma gondii infection and acanthocephalan parasite infection, are a major cause of sea otter mortality in California. The Toxoplasma gondii parasite, which is often fatal to sea otters, is carried by wild and domestic cats and by opossums, and may be transmitted by domestic cat droppings flushed into the ocean via the sewage system. Although it is clear that disease has contributed to the deaths of many of California's sea otters, it is not known why the California population is apparently more affected by disease than populations in other areas.
Sea otter habitat is preserved through several protected areas in the United States, Russia and Canada. In marine protected areas, polluting activities such as dumping of waste and oil drilling are typically prohibited. There are estimated to be more than 1,200 sea otters within the Monterey Bay National Marine Sanctuary, and more than 500 within the Olympic Coast National Marine Sanctuary.
Some of the sea otter's preferred prey species, particularly abalone, clams, and crabs, are also food sources for humans. In some areas, massive declines in shellfish harvests have been blamed on the sea otter, and intense public debate has taken place over how to manage the competition between sea otters and humans for seafood.
The debate is complicated by the fact that sea otters have sometimes been held responsible for declines of shellfish stocks that were more likely caused by overfishing by humans, disease, pollution, and seismic activity. Shellfish declines have also occurred in many parts of the North American Pacific coast that do not have sea otters, and conservationists sometimes note that the existence of large concentrations of shellfish on the coast is a recent development resulting from the fur trade's near-extirpation of the sea otter. Although many factors affect shellfish stocks, sea otter predation can deplete a fishery to the point that it is no longer commercially viable. There is a consensus among scientists that sea otters and abalone fisheries cannot co-exist in the same area, and the same is likely true for certain other types of shellfish as well.
There are many facets to the interaction between sea otters and the human economy that are not as immediately felt. Sea otters have been credited with contributing to the kelp harvesting industry via their well-known role in controlling sea urchin populations; kelp is used in the production of diverse food and pharmaceutical products. Although human divers harvest red sea urchins both for food and to protect the kelp, sea otters hunt more sea urchin species and are more consistently effective in controlling these populations. The health of the kelp forest ecosystem is significant in nurturing populations of fish, including commercially important fish species. In some areas, sea otters are a popular tourist attraction, bringing visitors to local hotels, restaurants, and sea otter-watching expeditions.
|Left: Aleut sea otter amulet in the form of a mother with pup. Above: Aleut carving of a sea otter hunt on a whalebone spear. Both items are on display at the St. Petersburg Museum of Anthropology and Ethnography. Articles depicting sea otters were considered to have magical properties.|
For many maritime indigenous cultures throughout the North Pacific, especially the Ainu in the Kuril Islands, the Koryaks and Itelmen of Kamchatka, the Aleut in the Aleutian Islands and a host of tribes on the Pacific coast of North America, the sea otter has played an important role as a cultural as well as material resource. In these cultures, many of which have strongly animist traditions full of legends and stories in which many aspects of the natural world are associated with spirits, the sea otter was considered particularly kin to humans. The Nuu-chah-nulth, Haida, and other First Nations of coastal British Columbia used the warm and luxurious pelts as chiefs' regalia. Sea otter pelts were given in potlatches to mark coming-of-age ceremonies, weddings, and funerals. The Aleuts carved sea otter bones for use as ornaments and in games, and used powdered sea otter baculum as a medicine for fever.
Among the Ainu, the otter is portrayed as an occasional messenger between humans and the creator. Versions of a widespread Aleut legend tell of lovers or despairing women who plunge into the sea and become otters. These links have been associated with the many human-like behavioral features of the sea otter, including apparent playfulness, strong mother-pup bonds and tool use, yielding to ready anthropomorphism. The beginning of commercial exploitation had a great impact on the human as well as animal populations – the Ainu and Aleuts have been displaced or their numbers are dwindling, while the coastal tribes of North America, where the otter is in any case greatly depleted, no longer rely as intimately on sea mammals for survival.
Since the mid-1970s, the beauty and charisma of the species have gained wide appreciation, and the sea otter has become an icon of environmental conservation. The round, expressive face and soft furry body of the sea otter are depicted in a wide variety of souvenirs, postcards, clothing, and stuffed toys.