Watching sea otters in Monterey Bay or in the sheltered bays of south-east Alaska is one of the great pleasures of North Pacific travel: an animal floating on its back in the kelp, cracking a clam against a rock balanced on its chest, then rolling over and over to wash its fur. Few wild animals are so easy to watch, and few have a story with such high stakes. The sea otter was hunted almost to extinction for its fur, and its recovery has reshaped entire coastal ecosystems.

This page covers how the sea otter keeps warm without blubber, what it eats and how it uses tools, why it matters so much to kelp forests and seagrass, how it breeds and raises its young, where it lives today, and the threats that keep it on the endangered list.

How big is a sea otter?

Males usually weigh 22–45 kg and measure 1.2–1.5 m; females weigh 14–33 kg and measure 1.0–1.4 m. Males up to 54 kg have been recorded.

The sea otter (Enhydra lutris) is the heaviest member of the weasel family, the Mustelidae, which also includes badgers, minks and the 13 species of otter. The giant otter of South America is longer, but much slimmer. Among marine mammals, though, the sea otter is small. Most whales, seals and sea lions are far heavier, and only its South American relative, the marine otter, is smaller.

The body is built for life at sea. The hind feet are broad and webbed, almost flipper-like, and drive the otter through the water; the tail is short, thick and flattened and helps with steering. The forepaws are small, with retractable claws and tough pads that help grip slippery prey. Long, sensitive whiskers and the forepaws themselves are used to find food by touch on the seabed. Unlike most other otters, it has no functional anal scent glands, does not make dens and can live its whole life without coming ashore.

How does a sea otter stay warm without blubber?

With the densest fur of any animal: up to about 150,000 hairs per square centimetre, or roughly a million per square inch. Air trapped in the fur keeps cold water off the skin.

Whales, seals and sea lions rely on a thick layer of blubber. Sea otters have almost none. Instead, their coat combines long, waterproof guard hairs with a very dense underfur. The guard hairs shed water and keep the underfur dry, and a layer of air trapped next to the skin, warmed by the body, insulates the animal from the cold sea. The fur is shed and replaced gradually throughout the year, so there is no weak moulting season.

The system only works if the fur is spotless. Sea otters spend hours each day grooming: rubbing, combing with their claws, and blowing or whipping air into the underfur. Their skin is loose and their skeleton unusually flexible, so they can reach every part of the body. A mother grooms her pup for hours until it can manage alone.

Keeping warm is also a matter of fuel. A sea otter’s metabolic rate is two to three times that of a land mammal of the same size, and it must eat about a fifth to more than a third of its body weight in food every day. Estimates range from about 20% to 38%, depending on the animal and its circumstances. That appetite is one reason sea otters have such a large effect on the animals they eat.

What do sea otters eat?

Mostly bottom-living invertebrates: sea urchins, clams, mussels, abalone, crabs, snails, sea stars and fat innkeeper worms. In a few areas they also eat fish.

Sea otters dive to the seabed and search with their paws and whiskers, turning over rocks and digging in mud and sand. Under each foreleg is a loose pouch of skin, used to carry food back to the surface, along with a favourite stone. At the surface the otter floats on its back, using its chest as a table, and tears its food apart with its paws and strong, flattened back teeth. Eating so many sea urchins can stain an otter’s teeth and bones purple.

Diet varies a great deal from place to place and from one otter to the next. Where otters have lived for a long time and the richest prey is depleted, individuals often specialise: some eat mainly snails, others crabs or clams. At Amchitka Island in the Aleutians in the 1960s, where the population was at carrying capacity, about half the food found in stomachs was fish. Sea otters also drink seawater, which is unusual for a marine mammal; relatively large kidneys let them get rid of the extra salt.

How deep can sea otters dive?

Most foraging happens in less than about 30 m of water, but tagged otters in Alaska have been recorded foraging as deep as 100 m.

A study by James Bodkin and colleagues, published in 2004, fitted 14 sea otters in south-east Alaska with time-depth recorders and classified more than 180,000 dives. Most foraging, about 84%, took place between 2 and 30 m; the rest was between 30 and 100 m. Males often dived deeper than females, with maximum foraging depths averaging 82 m for males and 54 m for females, and the deepest recorded foraging dive was about 100 m. Fewer than 2% of foraging dives went beyond 55 m. Dives typically last about a minute and rarely more than four, and sea otters usually stay within about a kilometre of shore.

How do sea otters use tools?

They use rocks as hammers and anvils to open shellfish and knock abalone off the rock, making them one of the few mammals other than primates to use tools routinely.

The classic image is an otter floating on its back with a flat stone on its chest, bringing a clam or mussel down on it again and again until the shell breaks. Otters also hammer at prey with a rock held in both paws, pound shells against hard surfaces such as boat hulls and rocks on shore, and use stones underwater to strike abalone loose. Individuals often keep a favourite stone in their armpit pouch between dives.

Tool use is not equally common in all otters. A 2024 study by Chris Law and colleagues, based on 196 radio-tagged southern sea otters in California, found that tool users, particularly females, could get at larger and harder-shelled prey. They also suffered less tooth damage, which matters for an animal whose teeth wear down over its life. The authors concluded that tool use may be essential for some otters to survive where their preferred, easy-to-open prey has been depleted.

Why are sea otters a keystone species?

By eating sea urchins, sea otters stop urchins from overgrazing kelp. Where otters are present, kelp forests thrive; where they are lost, urchins can turn the forest into bare rock.

In 1974 James Estes and John Palmisano compared islands in the western Aleutians with and without sea otters. Around islands with otters, urchins were small and scarce and kelp grew in dense forests. Around islands where otters had been wiped out, urchins were large and abundant and had grazed the kelp down to “urchin barrens”. The study became a textbook example of a keystone species and a trophic cascade: a predator whose effects ripple down through the food web far beyond its own numbers.

The story took a new turn in the 1990s. Sea otter numbers in the Aleutians fell sharply, and in a 1998 paper in Science, Estes and colleagues argued that killer whales had begun eating sea otters, possibly because their usual prey, seals and sea lions, had declined. As otters disappeared, urchins increased and kelp declined again. The explanation remains debated, but the Aleutian decline was severe: by 2000, numbers there had fallen by about 90%.

Sea otters help other habitats too. In Elkhorn Slough, a Californian estuary, research published in 2013 found that recovering sea otters ate large numbers of crabs. With fewer crabs, the small grazers that clean algae off eelgrass leaves survived, and the seagrass beds recovered despite heavy nutrient pollution. In kelp forests and seagrass meadows alike, otters help keep plant-based habitats that shelter fish and store carbon.

How do sea otters reproduce?

Females give birth to a single pup, usually in the water, after a pregnancy that can last from about four to twelve months because of delayed implantation.

Sea otters mate in the water; males hold the female’s nose in their teeth during mating, often leaving scars. Breeding happens year-round, with seasonal peaks that differ between north and south. After fertilisation, the embryo may not implant in the womb for some time, so the total length of pregnancy varies, followed by about four months of active development. In California, females usually breed every year, about twice as often as in Alaska.

A newborn pup weighs 1.4–2.3 kg and is born with its eyes open and a thick, fluffy coat that holds so much air that it floats like a cork and cannot dive. Twins occur in about 2% of births, but usually only one survives. The pup rides on its mother’s chest while she floats on her back. When she dives to feed, she may leave it floating at the surface, sometimes wrapped in kelp. The fluffy natal fur is replaced by adult fur after about 13 weeks. Pups nurse for six to eight months in California and four to twelve months in Alaska, learning to dive and forage alongside their mothers.

How long do sea otters live?

In the wild, males typically live 10–15 years and females 15–20 years, with a recorded maximum of 23 years. Worn teeth may help explain the shorter wild lifespans. In aquariums, several sea otters have lived past 20, and a female named Etika at the Seattle Aquarium died aged 28.

Sea otters forage alone, but they rest together in single-sex groups called rafts, usually of 10 to 100 animals, with male rafts generally larger. The largest raft ever recorded held more than 2,000 otters. To avoid drifting while they sleep, otters often wrap themselves in strands of kelp; captive otters have been filmed holding paws, which may serve the same purpose.

Where do sea otters live?

In shallow coastal waters of the North Pacific, usually within about a kilometre of shore: from the Kuril Islands and Kamchatka through the Commander and Aleutian islands, Alaska and British Columbia to Washington and central California.

Three subspecies are recognised. The Asian sea otter (E. l. lutris) lives around the Kuril Islands, Kamchatka and the Commander Islands, with a few animals reaching northern Japan. The northern sea otter (E. l. kenyoni) ranges from the Aleutians through Alaska to British Columbia and Washington, and accounts for most of the world’s sea otters. The southern or California sea otter (E. l. nereis) lives along the central California coast. Sea otters favour rocky coasts with kelp beds, but also live in sheltered bays and estuaries with soft bottoms. They usually stay in water about 15–23 m deep or less.

The figures in the table come from the IUCN Red List assessment published in 2020, which put the world population at roughly 129,000 based on counts from 2000 to 2018. Numbers in Alaska are estimates over huge, remote coastlines, and some regional figures are years old, so the total should be read as an approximation.

What happened to sea otters during the fur trade?

Commercial hunting from 1741 to 1911 reduced a population once estimated at 150,000–300,000 to perhaps 1,000–2,000 animals in about 13 scattered colonies.

The first scientific account of the sea otter came from Georg Steller, the naturalist on Vitus Bering’s second voyage, who was shipwrecked on what became Bering Island in 1741. The survivors returned with sea otter pelts, and the value of the fur, especially in trade with China, launched a rush of Russian, then British, American and Japanese hunters across the North Pacific. Sea otter fur was among the most valuable in the world. Over the next century and a half, the animals disappeared from most of their range, which once stretched from northern Japan to central Baja California in Mexico.

In 1911 the International Fur Seal Treaty between the United States, Russia, Japan and Great Britain banned commercial hunting of sea otters at sea. In California, the species was widely believed extinct until a group of about 50 was found near Bixby Creek on the Big Sur coast in 1938. All of today’s southern sea otters descend from that colony.

Recovery in many places has been helped by moving otters. In the late 1960s and early 1970s, otters from Amchitka Island in the Aleutians were moved to south-east Alaska, British Columbia, Washington and Oregon. The Oregon releases failed, but the others succeeded: 59 otters released in Washington in 1969–1970 have grown to more than 2,000.

Why are sea otters still endangered?

The IUCN lists the sea otter as Endangered because of large declines in parts of its range, especially the Aleutians and Russia, and the ongoing risk of oil spills.

Oil is the greatest human threat. Oiled fur loses its insulation, and otters swallow oil while grooming, damaging the liver, kidneys and lungs. The Exxon Valdez spill in Prince William Sound, Alaska, in March 1989 killed an estimated 2,650 to 3,905 sea otters, according to figures in the IUCN assessment, and lingering oil affected the population for many years. Because sea otters live close to shore and near shipping lanes, a single large spill could devastate a small population such as California’s.

Southern sea otters face particular problems. Shark bites, almost all from white sharks that bite but do not eat otters, are the most common primary cause of death found in stranded animals in California. Infectious diseases also kill many otters, including toxoplasmosis caused by Toxoplasma gondii, a parasite whose eggs are shed by wild and domestic cats and washed into the sea in runoff and sewage. Other threats include harmful algal blooms, drowning in fishing nets, illegal killing and conflict with shellfish fisheries. Under the US Endangered Species Act, the southern sea otter is listed as Threatened.

Where can you see sea otters?

In California, the best places include Monterey Bay, Moss Landing and Elkhorn Slough, and Morro Bay, where otters often rest in the kelp or in harbours close to shore. In Alaska, Kachemak Bay, Prince William Sound and Kenai Fjords are reliable; in British Columbia, the west coast of Vancouver Island; in Washington, the outer Olympic coast. Use binoculars and keep your distance. In the United States the Marine Mammal Protection Act makes it illegal to disturb or harass sea otters, and if an otter changes its behaviour because of you, you are too close.

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