Sea Otter Natural History

Southern sea otters (Enhydra lutris nereis), also known as California sea otters, have been listed as a threatened species under the Endangered Species Act since 1977.  It belongs to the order Carnivora and the family Mustelidae. Two other otter subspecies are also recognized – E. lutris kenyoni, which is found from Oregon to Alaska, and E. lutris lutris, which inhabits parts of Russia and northern Japan. Sea otters are highly specialized marine mammals capable of living their entire lives without ever having to leave the ocean, have the densest fur of any mammal and are one of the few marine species to use tools. Sea otters are an apex predator of the nearshore ecosystem.  The species is considered a keystone species because of their critical importance to the health and stability of the nearshore marine ecosystem.  They are also considered a sentinel species because their health reflects that of California’s coastal oceans.



Sea Otter Skeleton and Skull Illustrations

Sea Otter Skeleton and Skull Illustrations

The sea otter is one of the smallest marine mammals, but one of the largest members of the family Mustelidae, a group that includes skunks and weasels among others.  Adult males reach an average length of 4.5 feet (1.4 m) with a typical weight between 50 and 100 lbs (23 to 45 kg), while adult females reach an average length of 4 feet (1.2 m) and typically weigh 45 lbs (20 kg).  It has a highly buoyant, elongated body, blunt snout and small, wide head. Sea otters have an acute sense of smell and taste and have good vision both above and below the water surface.  They also rely heavily on their sense of touch.

Sea otters exhibit numerous adaptations which help them survive in their challenging marine environment.  Long whiskers help them to detect vibrations in murky waters and sensitive forepaws, with retractable claws, help them to groom, locate and capture prey underwater, and use tools.  When underwater, they can close their nostrils and small ears. The sea otter’s hind feet are webbed and flipper-like, and are used in conjunction with its lower body to propel the animal through the water. It has a long, flattened tail which they use as a rudder and for added propulsion.  Hearing is one sense that is not yet fully understood, although studies suggest they are particularly sensitive to high-frequency sounds.  Their teeth are unique for a mammal in that they are blunt and designed for crushing, rather than being sharp for tearing like most marine mammals are equipped with.

With the exception of its nose and pads of its paws, the sea otter’s body is covered in dense fur.  The fur consists of two layers. The short, brown underfur can be as dense as 1 million hairs per square inch, making its fur the densest of any mammal.  By comparison, we only have about 100,000 hairs in total on our heads.  A top layer of long, waterproof guard hairs helps to keep the underfur layer dry by keeping cold water away from the skin. The pelage is typically deep brown in color with silver-gray highlights, with the coloration of the head and neck being lighter than the body.  Unlike other marine mammals, such as seals and sea lions, sea otters do not have any blubber, so they depend on this exceptionally thick, water-resistant fur to stay warm in the cold, coastal Pacific.


Range & Habitat

Historical and Current Sea Otter Range

Historically, southern sea otters were present in coastal marine habitats from northern California to Baja California in Mexico. This range decreased significantly during the fur trade during the 18th and 19th centuries, with excessive hunting nearly driving the species into extinction by the early 1900s. The current range extends along the California coast from Half Moon Bay in the north to Santa Barbara in the south, though individuals are occasionally seen outside these limits.  A small population of sea otters lives at San Nicolas Island as a result of translocation efforts initiated in 1987.

A subspecies of northern sea otter, E. lutris kenyoni, occurs on the Aleutian and Pribilof Islands of Alaska and the Alaska Peninsula. Translocated populations have also been established along the coasts of British Columbia in Canada and Washington and Oregon in the United States. The second subspecies of northern sea otter, E. lutris lutris, inhabits the Commander Islands of the Bering Sea, the coasts of the southeastern Kamchatka Peninsula and the Kuril Islands in Russia, and the northern coast of Japan.

Sea otters are found in a variety of coastal marine habitats, including rocky shores and sea-bottoms, sandy sea-bottoms, as well as coastal wetlands.  Sea otters naturally inhabit offshore areas with an abundance of food and kelp canopy.  They tend to live in ocean depths shallower than 130 feet (40 m) with water temperatures ranging between 35°F and 60°F.



Most of a sea otter’s life is spent at sea, though they do occasionally haul out on land, where they appear clumsy and walk with a rather awkward gait.  They eat, sleep, mate and give birth in the water.  Sea otters spend most of their time floating on their backs at the surface grooming, eating, resting, and diving for food on the seafloor.  Sea otters are relatively slow swimmers, generally traveling at 3-5 mph (5-8 km/h).  They typically swim belly-up on their backs, propelling themselves through the water using their webbed hind feet.  If a faster speed is required, for instance when a male is patrolling it’s territory for competing males or when in hot pursuit of a sexually receptive female, it turns over onto its stomach and in addition to using its webbed hind feet, it undulates its entire body for greater propulsion and acceleration.

Sea otters groom themselves almost continuously while at the surface, a practice critical for maintaining the insulating and water repellant properties of their fur.  Its pliable skeleton and loosely-fitted skin allow the animal the flexibility to reach any part of its body.  During a grooming bout, which generally occurs directly after a foraging bout (a period of time in which diving and eating takes place) or resting bout, the animal can be seen somersaulting, twisting and turning, and meticulously rubbing its fur at the water surface. This behavior not only cleans the fur, but also traps air bubbles against the skin within the millions of hairs of its pelage.  This layer of entrapped air creates an insulating barrier (similar to that of a double-paned window) which prevents water from reaching the skin. Constant grooming is absolutely critical for their survival.  If cold ocean water reaches their skin, it will immediately begin to draw heat out of the animal, which disrupts the animal’s ability to thermoregulate and will ultimately lead to hypothermia and death.

Sea otters often rest together in single-sex groups called rafts.  They are known to wrap themselves up in kelp to keep from drifting out to sea.  While resting at the surface, a sea otter will often times hold its forepaws above the water surface and fold its hind feet up onto to its torso to help conserve heat.

With the exception of territorial males, who have the privilege of living among females, males and females tend to live in separate groups.  The center of the sea otter range is predominately occupied by females (of all ages) and territorial males, as well as some dependent pups and recently weaned juvenile males. The northern and southern edges of the range are largely male dominated areas, consisting of juvenile, subadult and adult males.  Numbers in these male areas tend to increase in winter and spring because there are fewer mating opportunities with sexually receptive females during this time of the year.

Females generally have small home territories while many adult males hold larger aquatic territories consisting of several adult females.  Bachelor males (animals who are either to young or too old to defend their own territories) reside in the large male-only groups at either end of the range.  Males travel much greater distances throughout the range than females, typically making seasonal treks of up to 200 miles between the months of June and November when the highest proportion of females are in estrous.  On any given day though, males tend to remain in the same general location, moving only a mile or two along the coastline.  Females, on the other hand, are sedentary by nature, generally staying within 10 – 20 miles of their home ranges.  Their home ranges are smaller because they have higher metabolic costs while pregnant and raising their pup.

Sea otters are equally active both night and day.  A foraging bout occurs for several hours in the morning, typically starting just before sunrise.  A second foraging bout begins in the afternoon, usually lasting for several hours until sunset.  A grooming bout occurs before and after each foraging bout. and resting bout follow at mid day. , followed again by another grooming and resting bout.  A third foraging bout may also occur around midnight.

Although difficult to hear from shore, sea otters exhibit a variety of vocal behaviors.  Pups are the most vocal.  A pup can be heard squealing when its mother leaves it to dive for food and often times when a male approaches.  Their cry is similar to that of a gull.  Other vocalizations include:  coos and grunts, which occur when an animal is eating or when content, as in the case of a pair-bonded couple during courtship;  whines occur when an animal is frustrated, as in the case of an older pup wanting to suckle or an adult male attempting to mate with an uninterested female;  growls, snarls, whistles and hisses can be heard when an animal is frightened or distressed, as in the case of a captured otter.


Food & Foraging

An otter must consume approximately 25% of its bodyweight in prey each day just to stay alive (for a 75 pound kid, that would amount to eating 75 quarter pound hamburgers every day!).  To meet it’s high energetic and thermoregulation demands, a sea otter’s metabolic rate is 2 to 3 times that of comparatively sized mammals.  Sea otters consume a wide variety of benthic invertebrates.  Prey items include sea urchins, abalone, crabs, mussels, clams, marine snails, marine worms, sea stars, and squid.  In total, otters eat at least 50 species of benthic (bottom-dwelling) invertebrates, although individuals tend to specialize on only a few main prey types.  Prey specialization and feeding preferences are passed on from mother to pup.

The strong forelegs paws are used to locate and capture prey.  Pockets of loose skin under each foreleg are used to store prey it has gathered on the seafloor for the ascent to the surface.  Rocks are often used as tools to dislodge prey on the sea floor and to break open the hard outer shells of some prey items upon returning to the surface.  Floating belly-up in the water, they place rocks on their chests and repeatedly pound hard-shelled prey against them to gain access the meat inside.  While eating, an otter will roll repeatedly in the water to wash away food scraps from its chest.  Unlike most other marine mammals, sea otters commonly drink sea water.  Although most of the animal’s water needs are met through the consumption of prey, it’s large kidneys allow it to extract fresh water from sea water.

Sea otters generally forage close to shore in depths shallower than 60 feet (18 m) but are capable of diving to depths of 300 feet (90 m) or more. With a relatively large lung capacity for it’s size, an otter can hold its breath for 5 minutes, but most dives are two minutes or less in duration.



Sea otters are polygynous, meaning that males have multiple female partners.  Males are usually territorial, typically mating with several females in an area that they defend. Males and females form breeding pairs for several days, after which time the male moves on to look for other receptive females and the female is left to ultimately raise the pup on her own. Females often sport bloody noses during the breeding season, as males will bite them during mating.

Female otters can give birth at any time of the year.  Gestation lasts approximately 6 months.  It takes 4 months for the fetus to fully develop but delayed implantation (of approximately 2 months) almost always occurs. Scientists suggest that this adaptation may help mothers give birth at the most opportune time of year—when food is most abundant, for example.

At birth, pups are approximately 2 feet (.6 m) in length and weigh 4-5 lbs (2-2.3 kg).  Pups are born with a thick coat of baby fur and are so buoyant they cannot immediately dive for food.  The mother carries the pup on its chest for the first two months, where it is constantly groomed and protected from the cold water.  When diving for food, the female leaves her pup floating in the water, often times wrapping kelp fronds around the pup to keep it from floating away.  The mother-pup bond is strong.  Mothers have have been known to carry their pup for several days after the pup’s death.

Pups begin to dive and forage at about 2 months, although nursing continues until weaning.  They have high-pitched calls, which they use to communicate with their mothers.  Females typically nurse their pups for 6 to 12 months, before abruptly weaning and abandoning them.  Within a few days of weaning her pup, a female will come into estrous and mate again. Pup mortality is high – only 25% of pups survive their first year.  Females mature at age 3 and males at age 5. Wild otters typically have a lifespan between 15 and 20 years.


Status & Conservation

Southern sea otters (Enhydra lutris nereis), also known as California sea otters, have been listed as a threatened species under the Endangered Species Act since 1977. The species is also protected under the federal Marine Mammal Protection Act, and has “fully protected” status under California state law.  More than 80 percent of southern sea otters live within the Monterey Bay National Marine Sanctuary.  Currently the population is estimated at just above 3,000 animals.

The Fur Trade

Sea otters were hunted extensively for their luxurious pelts during the 18th and 19th centuries, significantly reducing the original population whose numbers were estimated by historians to have been around 16,000 animals.  The current population of southern sea otters descended from a small surviving group of approximately 50 animals discovered off the Big Sur coastline in the early 1900s.  Numbers climbed from the 1930s until the 1980s, when gill net and trammel fisheries caused their numbers to decline again. After these fisheries were controlled, otter numbers began to increase once again, until the mid 1990s, when increased mortality in adults caused the population to again decrease. The southern sea otter population has grown slowly since that time, but it has exhibited high levels of mortality in recent years.

Sea Otter Mortality Today

Causes of Mortality in Southern Sea Otters

Causes of Mortality in Southern Sea Otters

Recent studies have shown that fatal white shark bites have increasingly become the leading cause of sea otter mortality in California, a concerning trend that is likely impacting range expansion and population recovery. Scientists also attribute a substantial percentage of southern sea otter mortality to infectious diseases, many of which are known to have anthropogenic causes and land-to-sea linkages. White shark bites, pathogens and parasites, food availability, nutritional deficiencies, habitat degradation, coastal pollutants and contaminant exposure are among many of the contributing factors threatening the recovery of the species. The single greatest threat to the sea otter is an oil spill. One large oil spill in central California could be catastrophic, with the potential of driving the entire southern sea otter population into extinction. Determining precisely how these factors are impacting the overall health of the southern sea otter population and the nearshore marine ecosystem on which they and other species depend is critical to the development and implementation of effective, long-term management and mitigation strategies, and ultimately to the recovery of the species.

California Sea Otter Strandings, 1985-2017

California Sea Otter Strandings, 1985-2017

Sea Otter Strandings

Since 1985, biologists and veterinarians at the California Department of Fish and Wildlife and the USGS Western Ecological Research Center have collected and examined all reported sea otter strandings — counting the number of dead, sick or injured sea otters recovered along California each year — in an effort to understand the population trends of the southern sea otter and to inform resource management on recovery and conservation of the species.

Note: Stranding numbers only account for sea otters that people find, including any dead animal, or stranded live animal that would have died without intervention. Past research indicates that possibly fewer than 50% of sea otters that die in the wild wash ashore, so the data presented here at best provide only an index of trends in population mortality.


California Sea Otter Population Annual Survey: Every Otter Counts

A range-wide census is conducted each spring to monitor trends in abundance and distribution of the southern sea otter, providing State and Federal agencies with the information they need for effective management. Utilizing shore-based and aerial observations, survey teams use binoculars and spotting scopes to count individual animals from accessible stretches of coastline and from fixed-wing aircraft in the remaining areas.  The ” spring survey” records the total number of sea otters, the number of dependent pups, and the number of independents (adults and subadults) observed. These counts provide a critical measure of the current overall status of the population.

Spring survey results are used as an indicator of the population trend of California sea otters. No single year’s survey result is indicative of a population change, however. Factors that can influence the count include viewing conditions, abundance and species composition of surface canopy kelp, observer experience, and distribution and movements of the animals.

To reduce the influence of anomalously high or low counts during any particular year, three-year running averages — commonly called a “population index” — of the survey results are also used to assess whether the population is growing or declining. So for example, values for 2017 are based on the averages of the 2016, 2017, and 2018 spring surveys. The population index is also used to determine the species’ official listing status under the Endangered Species Act and what threshold has to be met for the species to be “delisted.” For southern sea otters to be considered for removal from threatened species listing, the population index would have to exceed 3,090 for three consecutive years, according to the threshold established under the Southern Sea Otter Recovery Plan by the U.S. Fish and Wildlife Service.

The annual southern sea otter census is a cooperative effort between USGS-Western Ecological Research Center, California Department of Fish and Wildlife–Marine Wildlife Veterinary Care and Research Center, Monterey Bay Aquarium, University of California-Santa Cruz, the U.S. Fish and Wildlife Service, and many experienced and dedicated volunteers.