Species: Enhydra lutris
 Past distribution of the sea otter included Hokkaido Island of Japan north through the Kuril Islands and eastern coast of Kamchatka, east through the Commander Islands and Aleutian archipelago, the southern coast of Alaska, and the west coast of North America to Baja, Mexico. Sea ice limits their northern range to below 57 degrees N lattitude, and the distribution of kelp forests limits the southern range to about 22 degrees N lattitude. Hunting during the 18th and 19th centuries greatly reduced the distribution fo the sea otter. Three subspecies of Enhydra lutris are recognized today. E.l. lutris ranges from the Kuril Islands north to the commander islands in the western pacific. E.l. nereis is found off the coast of central California. E.l. kenyoni is distributed throughout the Aleutian Islands and southern Alaska, and has been reintroduced to various locations from south of Prince William Sound, Alaska to Oregon.
 Sea otters are marine mammals. They inhabit temperate coastal waters with rocky or soft sediment ocean bottoms less than 1 km from shore. Kelp forest ecosystems are characteristic of otter habitats.
 Range mass
14 to 45 kg
(30.84 to 99.12 lb)
1 to 1.5 m
(3.28 to 4.92 ft)
Average basal metabolic rate
Sea otters are the largest member of the family Mustelidae. Males weigh 22 to 45 kg and are 1.2 to 1.5 m in length. Females are slightly smaller, weighing 14 to 33 kg and measuring 1 to 1.4 m in length. The tail comprises less than a third of the body length. The pelage is brown or reddish brown. Sea otter fur is the densest of all mammals, with about 100,000 hairs per square centimeter. Since sea otters do not have any insulating fat, the fur is responsible for maintaining warmth. The hind legs are long and the paws are broad, flat, and webbed. The forelimbs are short and have retractable claws. Sea otters are the only carnivorans with just 4 lower incisors. Females have two mammae.
Breeding occurs year round.
Range number of offspring
Average number of offspring
Average number of offspring
Range gestation period
4 to 12 months
Average gestation period
Average birth mass
Range weaning age
2 to 11 months
Average weaning age
Range age at sexual or reproductive maturity (female)
4 to 6 years
Range age at sexual or reproductive maturity (male)
4 to 6 years
 Sea otters have a polygynous mating system. Many males activly defend territories. Disputes are usually settled with splashing and vocal displays, fighting is rare. Males will mate with females that inhabit their territory or seek out estrus females if no territory is established. Males and females bond for the duration of estrus, or 3 days. The male holds the female's head or nose with his jaws during copulation. Visible scars are often present on females from this behavior.
Sea otters can reproduce year round. There are peaks of birth in May-June in the Aleutian Islands and in January-March in the California population. Delayed implantation produces varied gestation times. Pregnancy has been reported to be 4-12 months. Females usually give birth about once a year. Orientation of the fetus may be either caudal or cephalic, although cephalic orientation is more common near birth. A single pup is born weighing 1.4-2.3 kg. Twins occur in 2% of births but only one pup can be raised successfully. Pups typically remain with their mother for 5 to 6 months after birth. Females that lose a pup will go into estrus sooner than if their pup had survived. Females reach sexual maturity at 4 years. Males reach sexual maturity at 5 to 6 years, but may not mate until much later.
 Females provide all of the parental care for the young. Pups will nurse until weaned, but start to eat solid foods shortly after birth. While the mother is foraging, young will remain on the surface. Pups start diving after two months. Pups learn from their mothers how to forage and what prey items to look for as well as swimming and grooming behaviors. The pup remains dependent on the mother for about 6 to 8 months, but there is considerable variation here also.
23 (high) years
The maximum estimated lifespan of sea otters is 23 years in the wild.
 Sea otters are solitary for the most part. Males congregate in groups when resting. Females tend to stay away from males except when mating. Sea otters can spend their whole life in the ocean but will rest on land when the population density is high. Swimming is performed using the hind limbs, tail, and vertical undulations of the body while the forelimbs are tucked into the chest. Otters can swim as fast as 9 km per hour under water. Sea otters are diurnal with crepuscular peaks in foraging activity. Foraging dives usually last 50-90 seconds, but otters can remain submerged for nearly 6 minutes. Foraging takes up 15-55% of an otter's time; time is dependent on availability of food. Prey is located using vision and touch and is captured with the forepaws. It is then brought to the surface where feeding takes place as the otter floats on its back using its chest as a table. Sea otters are one of few mammals that exhibit tool use. Prey items with hard shells or exoskeletons are broken open with a rock. Some otters will hold the rock on their chests and drive the prey into the rocks. Others will leave the prey on their chests and hit the prey with the rocks. The same rock is kept for many dives. Otters will often wash their prey by holding it against their body and turning in the water. Males steal from females if they get a chance. For this reason, females tend to forage in separate areas. When resting or sleeping, sea otters float on their backs and wrap themselves in kelp to keep from drifting. Their hind limbs stick up out of the water and forelimbs are either folded on their chest or used to cover their eyes. Grooming and cleaning their fur is important for maintaining its insulating ability.
 Sea otters are carnivorous. They will eat nearly any seafood they can find in their kelp forest foraging grounds. Their diet consists of marine invertebrate herbivores and filter feeders such as mussels, sea urchins (Strongylocentrotus sp.), snails, and abalone. Otters also are known to eat crabs, octopus, squid, sea stars, and fish. Individuals tend to be specialized in their choice of prey: one otter may consume only urchins and crabs while another will eat mostly fish, all depending on the abilities of the individual otter and what is available in the area. Otters need to consume 20-25% of their body weight each day. They obtain most of their water from prey but will drink seawater to satisfy thirst also.
- great white sharks (Carcharodon carcharias)
- killer whales (Orcinus orca)
- California sea lions (Zalophus californianus)
- bald eagles (Haliaeetus leucocephalus)
- humans (Homo sapiens)
Predation by killer whales is thought to be responsible for recent population declines. Vigilance and avoidance protects otters from predators other than humans.
 Sea otters are a keystone species. They play a major role in the community by controling of herbivorous invertebrates, mainly the sea urchin (Strongylocentrotus sp.), inhabiting kelp forests. Sea urchins graze on kelp. In coastal areas where otters are absent, sea urchins are abundant and the area is devoid of kelp forests. Where sea otters are present, the urchins are limited by otter predation and kelp forests are abundant. Kelp forests are dependent on sea otters for protection from grazers. The diversity of the sea otter diet reduces competition between benthic grazers and supports greater diversity in those species. The presence of sea otters is believed to be important in the evolution of kelp forest ecosystems.
Economic Importance for Humans: Positive
The fur of sea otters was of great importance in the fur trade from the mid 1700s to 1911. Pelts would bring as much as $1,125 each.
Economic Importance for Humans: Negative
Sea otters compete with commercial fisheries for shellfish and crab.
The fur trade depleted sea otters to about 1000-2000 individuals worldwide by 1911 when the International Fur Seal Treaty between the U.S., Russia, Japan, and Great Britain was established. The U.S. Marine Mammal Protection Act in 1972 provided more support for sea otters. Reintroduction along the west coast of North America and protection from hunting has brought the world population up to 100,000-150,000.  The population of California sea otters (Enhydra lutris nereis) is listed as threatened under the Endangered Species Act and will remain so until a population of 2650 is recorded for three consecutive years. The current population is about 2200.  There are concerns that a single oil spill could wipe out the California sea otter. In 1989, the Exxon Valdez flooded Prince William Sound, Alaska with oil. An estimated 5,000 sea otters were killed as a result. E.l. nereis is listed in appendix 1 of CITES, the other subspecies are in appendix 2.  The IUCN lists Enhydra lutris as endangered.