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Sea Otter

(Enhydra lutris)

The biology of sea animals that have been traditionally hunted is usually well known. Throughout the eighteenth and nineteenth centuries, the sea otter , prized universally for its exceptionally warm, sheik and tremendously valuable pelt, was heavily exploited by man. Subsequently, in order to maximize harvest yield, the human predatory machine studied sea otter biology intently. This cute, furry, charismatic species was so well hunted that it was extirpated along most of its historical range in North America. A second wave of study in sea otter biology was initiated at the beginning of the twentieth century in efforts to prevent the animal's extinction. Recent studies of Sea Otter appear to emerge from all areas of science. Currently, numerous scientific papers are being published on many different scales in the disciplines of physiology, ecology, behavior, pathology, and conservation of this animal. The resources utilized in the synthesis of this page represent a very minute fraction of the myriad of past and current publications dedicated to Sea Otter.

Sea Otter - Classification and Systematic
The sea otter is the only member of the genus Enhydra . It is the largest member of the family Mustelidae that includes approximately 70 species; the otter is also the smallest marine mammal and is the only one in the order Carnivora. Based on morphological differences in color, body and skull sizes, three sub-species of sea otters have been proposed historically: E. l. lutris Linn., "the Commander-Aleutian North American sea otter"; E. l. gracilis Bechstein, "the Kuril-Kamchatka sea otter"; and E. l. nereis Merriam, "the southern California sea otter". However, these sub-species determinations were based on limited data that were collected before the advent of modern molecular phylogenetic analysis. A recent study of skull characteristics has yielded claims that a new subspecies, E. l. kenyoni , is alive and kicking. More recent genetic analysis of otter mitochondrial DNA suggests that E. l. nereis has monophyletic mtDNA while E. l. lutris and E. l. kenyoni do not. The phylogenetic study of E. lutris is currently still in progress and the focus of heated debate between traditional and modern systemists. Despite recent advances in the field, some taxonomists still subscribe to the three original subspecies while others suggest that even those racial distinctions are unsubstantiated.

Sea Otter - Physical characteristics

Sea Otter is the heaviest of the otters, reaching a maximum weight of 45 kg. The body of the sea otter is relatively long and heavy, making progress on land clumsy and slow. At birth, the sea otter is covered with dense brownish fur and long silky yellowish-tipped guard hair; over a period of several weeks, the guard hair grows out, often giving the pup a distinct yellowish appearance. The sea otter has eight main physical characteristics that help to distinguish the animal from other mustelids: a coat of darkly colored (shades of brown), sparse guard hair and dense insulating fur that traps air and prevents water from contacting the skin; flattened hind feet or flippers for propulsion; retractile claws on the front feet; a loose flap or pouch of skin under each foreleg which is used to hold food items gathered from the sea bottom; flattened, rounded molar teeth with no cutting cusps; a horizontally flattened tail that aids in propulsion; a manner of swimming underwater by means of vertical undulations of the hind flippers and tail; and an external ear that resembles the ear of an otariid more than that of its closest relative, the river otter.

Sea Otter - Distribution
Historically, the sea otter was found on the coasts and islands around much of the north Pacific from northern Japan in the west to the islands off southern California in the east. The populations were drastically reduced by human exploitation in the eighteenth and nineteenth centuries. Commercial hunting of Sea Otter for its valuable pelt began in 1741, eventually eliminating it from most areas by the late 19th century. By the early 1900s, only about a dozen colonies containing greater than one thousand otters remained. Sea otters were exterminated from Near Island, westernmost of the Aleutian archipelago, where records from the fur trade indicated that they were once abundant. Legal protection and reintroduction efforts have reestablished stable sea otter colonies along much of its historical range. Recent studies of otter populations at Amchitka Island reported estimated population sizes ranging from 5500-8500. Non-migratory, resident populations of Sea Otter have now been reported to extend from the Commander Islands east along the Aleutian Islands and from Prince William Sound south to southern California. Otters have recently been observed as far south as Mexico.

Sea Otter - Habitat

Sea otters are generally found inhabiting near shore coastal waters of less than 54 m in depth. Sea Otter can be found in association with both rocky and soft-bottom habitats. Precipitous rocky shores, barrier reefs, tidewater stones, and dense kelp forests characterize the typical haunts of the sea otter.

Sea Otter - Range
Sea Otters have been found to vary enormously among different individuals and are believed to be a function of resource distribution and availability. One estimate of the area used by individual otters during a 24h period is 6.9-1166.4 ha. Annual home ranges have been observed as small as 35 ha - 540 ha. Regardless of the discrepancies between these numbers, otters of all age and sex classes are most often found within 1-2 km of their locations on the previous day. However, individuals often remain within a small area for an extended period and then suddenly move a much greater distance within a short period of time.

Sea Otter - Behaviour
When resting, sea otters often lie on their backs among kelp or in quiet water; the most common position is with the head up, and with folded paws and chin resting on the chest. Otters are usually found sleeping in kelp beds, and many have been observed to wrap kelp around their bodies to prevent them from drifting during resting periods. Sea otters spend a large proportion of its time grooming its fur. Sea otters are usually found in groups known as rafts or pods ranging in size from a few to several hundred animals. The animals are long-lived, interact frequently and typically remain in the same area for years. Olfaction is apparently well developed in the sea otter and is important in chemical communication during close-range social and reproductive interactions. The sexes tend to segregate into separate groups and to some extent, in separate areas also. In larger groups of otters, there is little evidence of avoidance or territorial behavior. Some independent males have been observed to exhibit territorial behavior.

Recent spectrographic analysis has been performed on the vocalizations of young and adult sea otters, both in captivity and in the wild. Ten basic vocal categories have been recognized and it is believed that the vocal patterns have characteristics that are most suitable for short-range communication among familiar individuals. The otter's 'whine' and 'squeal' that is commonly heard during courtship has been found to consist of graded signals that vary over a continuum. This degree of complexity and richness of communication patters is believed to have evolved as a result of complex social relationships.

Sea Otter - Feeding
The sea otter has been described as a resourceful, voracious and gregarious, generalist predator. Recent studies have revealed that otters do not forage cooperatively, and rarely exhibit other types of coordinated group movement. Small feeding groups are common, and although large aggregations have been observed, they are rare. Individuals dive in search of prey and return to the surface to breath and consume their catch. Sea otters have been observed to forage at night as well as during the day, and there is a great deal of individual variation in diet and foraging patterns. Diets are known to primarily consist of deep sea fish and marine invertebrates, including various species of mussels, tunicates, sea stars, bivalves, crabs, abalone and octopus. When quality prey abundance is limited, otters have been known to feed on relatively low caloric value prey items such as sand dollars. Occasionally, otters have been observed to feed on some kelp species, but algae are not believed to be a main part of their regular diet.

Dive duration has been observed to vary with prey type, with mean dive lengths of about 74s, and longer dives approaching 246s. Studies have revealed that otters prefer to feed in shallow (<10m), inshore waters and tend to forage in deeper waters (>20m) when food availability in the shallow waters is diminished.

Sea otter demonstrates significant selectivity and intelligence when foraging and consuming its prey. It has been shown that otters can detect and avoid toxic shellfish if suitable low toxicity preys are available. Otters are often observed using rocks to pound small, hard bodied prey items to gain access to the edible fleshy interior.

Sea Otter - Breeding/Reproduction

Among many mustelids, mating tends to be prolonged and aggressive. In sea otters, mating is always aquatic and often involves violent and prolonged copulations during which the male approaches the female from behind and grasps her face and nose with his teeth, sometimes pulling her head underwater while attempting to subdue her. Some females may form pair bonds with a single male while others may mate with up to three different males during a single estrous period. Groups of females periodically bring their young pups ashore to rest in a process known as hauling. Female reproductive rates and pup survivorship are generally higher in undisturbed areas with abundant food resources.

Sea Otter - Diseases and Pathology
There are a variety of pathogens and parasites that inflict otter populations. Most studies have focused on the more problematic southern populations. One of the main causes of death in California populations is Acanthocephalan peritonitis. This is caused by small worm-like parasites that enter the body with food, perforate the intestinal wall, resulting in infections of the abdominal cavity. Protozoal encephalitis is a disease that is also relatively common in otters. This disease involves swelling of the brain, which has been found to be caused by several genera of protozoa. Coccidioidomycosis is an infection derived from a soil-borne fungus that is common in dry interior valleys. Cases of this type of infection are limited to populations in San Louis Obispo County. The nature of propagation of the disease is not well understood. Bacterial infection is another significant cause of otter mortality. The types of bacteria involved vary significant between the individuals examined. In the final stages of some infections, significant interstitial fibrosis has been documented. Recent examinations of otters imported to Japan from Alaska have revealed intestinal parasites. Four species of acanthocephalan of the genus Corynosoma have been recovered from these sea otters.

Sea Otter - Predation and Danger
Sea otter predation has a predictable and broadly generalized influence on the structure of Alaskan kelp forests. The animals are well documented as 'keystone' predators in rocky marine communities. Otters have also been found to exert a strong influence on in faunal prey communities of soft -bottom sediments. In some areas of Alaska, sea urchin biomass has been found to decline by nearly 100% following the spread of sea otters into previously unoccupied habitats. Kelp forests are broadly dependent on sea otter predation for protection against destructive grazing.

Sea Otter - Conservation
At present, the sea otter is considered endangered along much of its range, and is legally protected in the United States under the Marine Mammal Protection Act and the Endangered Species Act. Gill nets used to be a major source of mortality to California populations; recent laws prohibiting gill nets at depths above 54m have resulted in 5-7% increases in populations that were previously stable or declining. Currently, industrial pollution is believed to be a major problem for southern populations of Sea otter.

The large-scale mortality of sea otters that resulted from this incident led to many studies (lots of post-mortem work) of otter reproductive biology and effects of water pollution on otter physiology. For Northern populations of sea otters, salmon gill nets are still a major cause of mortality. Presently in Alaska, the illegal shooting by fishermen and the legal hunting of otters by Indigenous Peoples are believed to be the main causes of mortality. Brown bear predation and eagle predation of pups have been observed, but their contributions to mortality are considered to be minimal.

In a recent conservation publication, criteria for delisting endangered species has been examined. It has been suggested that sea otter threats are currently minimal and populations have stabilized to the extent that the animal might no longer merit protection under the U.S. Endangered Species Act. Presently in Canada, there is no legislation protecting E. lutris . However, the Committee On the Staus of Endangered Wildlife in Canada (COSEWIC) lists the animal as threatened along the Canadian spans of its range.

Sea Otter - Interesting facts

  • Two separate populations of sea otters live in North America: Alaska sea otters and California sea otters.
  • The sea otter's brown to black fur is the finest and densest of any animal fur. On a large animal, they’re an estimated 650,000 hairs per square inch. A sea otter relies on its fur to keep it warm it doesn't have blubber as other marine mammals do. Natural oils in a sea otter's fur repel water and trap tiny air bubbles, providing a layer of warm air between the otter's skin and the harsh elements of its environment.
  • Sea otters spend up to 48% of the daylight hours grooming their fur. They groom by rubbing fur with their forepaws. Their strong claws comb and rake the fur. Then they roll and whirl in the water to smooth their fur.
  • Sea otters sleep, rest, and usually swim on their backs. California sea otters spend almost all of their time in the water. Alaska sea otters often sleep, groom, and nurse on land.
  • Because they rely on their dense fur for insulation from the chilly ocean water, sea otters are particularly vulnerable to the detrimental effects of an oil spill. If a sea otter swims into an oil spill, the fur becomes soiled and loses its insulating qualities, allowing water to penetrate to the skin and causing hypothermia and ultimately, death.