Sea Otter Facts

Sea otters (Enhydra lutris) are an easily recognized and beloved marine mammal. They have furry bodies, whiskered faces, and a propensity to lay on their backs and float on the water, a behavior that humans perceive as evidence of fun-loving. They are native to the northern coastlines of the Pacific Ocean, from northern Japan to Baja, Mexico. Most critically, they are a keystone species, meaning that their continued existence is required for several other species to survive.

Description

Sea otters are carnivores in the family Mustelidae—a group of animals that also includes terrestrial and semi-aquatic forms such as weasels, badgers, skunks, fishers, minks, and river otters. Sea otters are the only fully aquatic form of otters, but they share features with the others such as thick fur and short ears. This thick fur keeps the animals warm ​but unfortunately has led to over-hunting by humans of many of these mustelid species. 

Sea otters are the smallest fully marine mammal in the world: Males range in length between 3.9–4.9 feet, while females range between 3.3–4.6 feet. The average body mass for males is about 88 pounds, with a range of 49–99 pounds; females range from 31–73 pounds. 

Temperature balance is a significant challenge for sea otters, who lack the blubber of other marine mammals such as seals and walruses. Otters have a dense fur made up of a combination of undercoat and longer guard hairs that provides insulation, but it must be almost continuously maintained. Fully 10 percent of a sea otter's day is spent grooming its fur. However, fur is an inflexible insulation, so, when necessary, sea otters cool off by flapping their nearly-hairless rear flippers.

Habitat and Distribution

Unlike some marine mammals like whales that would die if they were on land for too long, sea otters can go up onto land to rest, groom, or nurse. However, they do spend most if not all of their lives in the water—Sea otters even give birth in the water.

Although there is just one species of sea otter, there are three subspecies:

• The Russian northern sea otter (Enhyrda lutris lutris), which lives in the Kuril Islands, Kamchatka Peninsula, and Commander Islands off Russia,
• The northern sea otter (Enhyrda lutris kenyoni), which lives from the Aleutian Islands off Alaska, down to Washington state, and
• The southern sea otter (Enhyrda lutris nereis), which lives in southern California.

Diet

Sea otters eat fish and marine invertebrates like crabs, urchins, sea stars, and abalone, as well as squid and octopuses. Some of these animals have hard shells, which protect them from predators. But that isn't an issue for the talented sea otter, which cracks open the shells by banging them with rocks.

To hunt prey, sea otters have been known to dive as deep as 320 feet; however, males mostly forage at depths of around 260 feet and females about 180 feet.

Sea otters have a baggy patch of skin under their forelimbs which is used for storage. They can keep extra food in this spot, and also store a favorite rock for cracking the shell of their prey.

Behavior

Sea otters are social, and hang out together in groups called rafts. Sea otter rafts are segregated: Groups of between two and 1,000 otters are either all males or females and their young. Only adult males establish territories, which they patrol during mating season to keep out other adult males. Females rove freely between and among male territories.

Reproduction and Offspring

Sea otters reproduce sexually and that only occurs when the females are in estrus. Mating is polygynous—one male breeds with all the females in its breeding territory. The gestation period lasts for six months, and females nearly always give birth to a single live pup, although twinning does occur.

Young sea otters have a form of extremely woolly fur that makes an otter pup so buoyant that it can't dive underwater and can float off if not tended carefully. Before a mother otter leaves to forage for her pup, she wraps the pup in a piece of kelp to keep it anchored in one spot. It takes 8–10 weeks for the pup to shed its initial fur and learn to dive and the pup stays with the mother for up to six months after birth. The females enter estrus again within several days to weeks after weaning. 

Female sea otters become sexually mature at about 3 or 4 years of age; males do so at 5 or 6 although most males don't establish a territory until they are 7 or 8. Female otters live 15–20 years and can have pups every year from the first estrus; males live for 10–15 years.

Keystone Species

Sea otters are a keystone species and play a critical role in the food web of the kelp forest, so much so that even terrestrial species are influenced by sea otter activity. When sea otter populations are healthy, urchin populations are kept in check, and kelp is abundant. Kelp provides shelter for sea otters and their pups and a variety of other marine organisms. If there is a decline in sea otters due to natural predation or other factors such as an oil spill, urchin populations explode. As a result, kelp abundance decreases and other marine species have less habitat.

Kelp forests absorb carbon dioxide from the atmosphere, and a healthy forest can absorb as much as 12 times the amount of CO₂ from the atmosphere than if it were subject to sea urchin predation. 

When sea otter populations are abundant, bald eagles prey primarily on fish and sea otter pups, but when sea otter populations declined in early the 2000s due to predation by an increased population of orcas, bald eagles preyed more on marine birds and had more offspring because of the higher caloric content of a seabird diet.

Threats

Because they are dependent on their fur for warmth, sea otters are heavily affected by oil spills. When oil coats a sea otter's fur, air can't get through and the sea otter can't clean it out. The infamous Exxon Valdez spill killed at least several hundred sea otters and affected the sea otter population in Prince William Sound for well over a decade, according to the Exxon Valdez Oil Spill Trustee Council. 

While sea otter populations increased after legal protections were put in place, there have been recent declines in sea otters in the Aleutian Islands (thought to be from orca predation) and a decline or plateau in the populations in California.

Other than natural predators, threats to sea otters include pollution, diseases, parasites, entanglement in marine debris, and boat strikes.

Conservation Status

Sea otters first became protected from the fur trade by the International Fur Seal Treaty in 1911, after the population had decreased to about 2,000 as a result of unrestrained hunting for furs. Since then, sea otter populations have rebounded, but the International Union for Conservation of Nature (IUCN) lists the species as a whole as Endangered. The ECOS Environmental Conservation Online System lists both northern and southern sea otters as threatened.

Sea otters in the U.S. today are protected under the Marine Mammal Protection Act.

Sources

• Anthony, Robert G., et al. "Bald Eagles and Sea Otters in the Aleutian Archipelago: Indirect Effects of Trophic Cascades." Ecology 89.10 (2008): 2725–35. Print
• Doroff, A. and A. Burdin. "Enhydra lutris." The IUCN Red List of Threatened Species: e.T7750A21939518, 2015. 
• "Northern Sea Otter (Enhydra lutris kenyoni)." ECOS Environmental Conservation Online System, 2005.
• "Southern sea otter (Enhydra lutris nereis)." ECOS Environmental Conservation Online System, 2016.
• Tinker, M. T., et al. "Otters: Enhydra Lutris and Lontra Felina." Encyclopedia of Marine Mammals (Third Edition). Eds. Würsig, Bernd, J. G. M. Thewissen and Kit M. Kovacs: Academic Press, 2018. 664–71. Print.
• Wilmers, Christopher C, et al. "Do Trophic Cascades Affect the Storage and Flux of Atmospheric Carbon? An Analysis of Sea Otters and Kelp Forests." Frontiers in Ecology and the Environment 10.8 (2012): 409–15. Print.