Sea otters are the largest member of the weasel family and live primarily a marine life. They are able to spend their entire lives without needing to leave the ocean. They are a keystone species because they are critically important to the stability and health of the marine ecosystem.
Sea otters are well adapted to life on the water. Their hind limbs are webbed for swimming, but their front paws have separated digits with claws for handling prey, grooming and holding tools for opening shellfish. Their tail is long and oar-like.
They lack blubber but are insulated by air trapped in their thick fur which is the most dense amongst all mammals. There are up to 1 million hairs per square inch (humans have 20,000 on their entire head). The trapped air provides four times as much insulation as the same thickness of fat would provide.
In North America, there are two distinct sea otter subspecies:
Southern sea otters (Enhydra lutris nereis), also called California sea otters, are found along the California coastline.
Northern sea otters (Enhydra lutris kenyoni) are found in Oregon, Washington, British Columbia, and Alaska. Within Alaska there are 3 distinct populations:
The Southeast stock is in Southeast Alaska
The Southcentral stock stretches from Glacier Bay to Cook Inlet
The Southwest stock is found from Cook Inlet to the Aleutian Islands
Sea otters are one of the few marine species to use tools (octopus are also known to use tools).
An otter must consume approximately 25% of its bodyweight each day just to stay alive. That means an adult 100 pound male must eat 25 pounds of prey!
The Northern Pacific Rim from Asia to North America
Shallow coastal waters and offshore areas where there is an abundance of food and kelp canopy.
Urchins, abalone, mussels, crabs, other invertebrates (including sea cucumbers) and even fish
Transient orcas, sharks, sea lions, land predators (wolves and bears), and humans
Sea otters float on the surface primarily on their backs (whereas river otters generally swim or float on their bellies when in the water). They are excellent swimmers and can spend their entire lives in water. They rest, mate, give birth and suckle their young in the water. They occasionally haul out on land (or sometimes ice) to rest.
Sea otters are known for their acrobatics in the water and can often be observed somersaulting or twirling. This behavior is common during grooming, but it also serves an important purpose of trapping insulating air under the fur, which prevents water from reaching the skin. This form of waterproofing is a key element for thermoregulation and heat conservation in the cold water.
They are benthic feeders, which means they dive to the sea floor to catch their prey. Dives can last over 5 minutes and be as deep as 250 feet, but they usually are much shorter and shallower. Otters rise to the surface to eat their food. They lay on their backs and use their paws and tools (such a rock) to crack open shells. When not feeding, otters store their tools in the folds of their arm pits.
Sea otters are social animals. They may travel in groups or raft together to rest. As many as 1,000 animals can float together in rafts, oftentimes wrapping themselves in kelp, which serves as an anchor. While resting, they often hold their forepaws and hind feet up above the surface to conserve heat.
Sea otters do not migrate but maintain home ranges up to 10 miles. Individuals may travel farther when seeking new territory or food sources.
Sea otters are vocal. Pups squeal and cry when their mothers leave them alone to go foraging. Juveniles and adults also make grunts when content, whines when frustrated, growls, snarls and hisses when distressed.
Sea otters breed throughout the year, but in Alaska pups are generally born in late spring after 6 months of gestation. Females usually only give birth to one pup at a time.
Pups are ride on their mother’s chest as she swims on her back for the first 2 months where it is constantly groomed and protected from the cold water. She will only leave her pup to dive for food. Pups are weaned within 6 months, at which time it will weigh approximately 30 pounds and appear as large as its mother. The female will abruptly abandon her pup once it is weaned and mate again.
Sea otters are polygynous. Males are territorial and defend areas where females are concentrated. They will mate with several females within their territory. Females are left to raise the pup on her own. Nonbreeding males may form bachelor groups outside of the breeding territory.
Threats to sea otters include illegal harvest, predation, fishery interactions, disease and oil spills.
Historically, the worldwide population of sea otters numbered between 150,000 and 300,000 animals occurring along the Pacific Rim from northern Japan to the Baja Peninsula in Mexico. Human exploitation severely reduced their abundance.
In the mid-1700s Russian explorers and fur trappers began commercially harvesting sea otters. They were hunted nearly to extinction, and by 1911 otters were protected by the International Fur Seal Treaty. At that time, there were estimated to be fewer than 2,000 animals left in just 13 remnant colonies across their entire range. Some of these subpopulations died out while others began to recover. In the mid 1900s, translocation programs repopulated some areas of Alaska, British Columbia and Washington. However, expansion has not occurred as expected and some areas continue to have reduced population growth rates.
Northern sea otter
By the early 2000s, some subpopulations of the Southwest population in Alaska—particularly in the Aleutian Islands—had further decreased, which was largely attributed to increased predation by Orcas. In 2005, the US Fish and Wildlife Service (USFWS) listed the Southwest population of sea otters as threatened under the Endangered Species Act. The population trend has begun increasing but still remain well below carrying capacity.
Subpopulations of the Southcentral stock appear stable or increasing, according to the most recent Marine Mammal Protection Act Stock Assessment Report in 2014.
In Southeast Alaska, otter populations were depleted almost entirely before translocation in the mid-1900s. Population range expansion has not occurred as expected, and population growth rates are lower than predicted. Contributing factors are attributed to ongoing legal and illegal harvest amongst heightened conflicts with commercial dive fisheries in the region.
Southern sea otter
The Southern sea otter subspecies, which once numbered about 16,000 animals, is hovering around 3,000 today. They were listed as threatened under the Endangered Species Act in 1977 due to reduced range and population size and are recognized as depleted under the Marine Mammal Protection Act.
All sea otters are protected under the Marine Mammal Protection Act, which prohibits commercial harvest of sea otters but allows Alaska natives to hunt them for subsistence and handicrafts. They are further protected under the Endangered Species Act. Unlike most marine mammals—which are managed by National Marine Fisheries Service—the US Fish and Wildlife Service is responsible for managing sea otter populations (along with walruses, polar bears, dugongs and manatees).
