Sea Otter

Small sea mammal with thick fur in ocean.

Tim Hauf, timhaufphotography.com

Scientific Name
Enhydra lutris nereis

Introduction
Today in California, the sea otter is a beloved and almost an iconic marine mammal that plays an important role in influencing and bringing about public policy for the protection of all marine mammals. An occasional visitor to the Channel Islands National Park, it is well-represented in coastal areas from Santa Barbara to just south of San Francisco.

Quick and Cool Facts

  • Sea otters are members of the weasel or mustelid family. 1
  • Sea otters have the thickest fur of any mammal, at 850,000 to one million hairs per square inch. 1
  • Adult sea otters can eat 25%-30% of their body weight in one day. 1
  • Sea otters are one of the few animals to use tools. They eat animals with shells, like clams and abalone, and use a stone to break open the shells. 1
  • Sea otters are the smallest marine mammal. 1
  • The maximum estimated lifespan of sea otters is 23 years in the wild. 4
  • Great white sharks are one of the primary predators of sea otters. 4
Appearance
As described by the FAO Species Identification Guide: Marine Mammals of the World, “The sea otter is the most changed of its weasel family origins. The muzzle has a set of thick vibrissae (whiskers). The large head has a blunt snout, and is connected to the body by a short, stocky neck. The forelimbs are short and similar to those of other otters, with a loose flap of skin under each that is used to store food. The hind limbs are large and flattened like flippers; they are oriented backwards. Although the short tail is not noticeably tapered, it is flattened top to bottom into a paddle-like structure.

The pelage (fur) of sea otters is the densest of any mammal (850,000 to one million hairs per square inch). A layer of sparse guard hairs overlays the dense underfur. Sea otters are completely covered with fur, except for the nose pad, inside of the ear flaps, and the pads on the bottom of the feet. The color of the fur is dark brown to reddish brown. Older individuals become grizzled, with the fur around the head, neck, and shoulders becoming almost white.

Male sea otters reach lengths of nearly 5 feet and weigh up to 100 pounds. Females can be up to 4 and ½ feet and weigh up to 70 pounds. Newborn sea otter pups weight ranges from about 2 to 4 and ½ pounds.”

Range
Historically, the California sea otter’s ranged from Punta Abreojos, Baja California, Mexico to northern California and the offshore islands that are now Channel Islands National Park. Due to a period of extensive hunting for sea otter pelts, known as "the Great Hunt" between 1741 and 1911, the world population plummeted to as little as 1000 individuals. In 1938, the population in California was only about 50 animals. Presently, through restoration efforts, the sea otter has been increasing in the nearshore area waters along the mainland coastline of California from San Mateo County to Santa Barbara County. 2

Habitat
Sea otters inhabit temperate coastal waters with rocky or soft sediment ocean bottom. They live in offshore forests of giant kelp and spend most of their active time foraging below the canopy. They eat, rest, and groom themselves at the water surface. While sea otters are capable of diving to depths of at least 45 meters, they prefer coastal waters up to 30 meters deep. The shallower the water, the less time is spent diving to reach food.

Feeding
An According to information provided by the Animal Diversity Web, through the University of Michigan Zoology Department, “the sea otters are carnivorous. They will eat nearly any fish or marine invertebrate they can find in their kelp forest foraging grounds. Their diet consists of marine invertebrate herbivores and filter feeders such as sea urchins, sea stars, limpets, coast mussels, chitons, and purple-hinged rock scallops. Otters also eat crabs, octopus, squid, and fish. Individuals tend to be specialized in their choice of prey; one otter may consume only urchins and crabs while another may eat mostly fish, depending on the abilities of the individual and local food availability. Otters consume 20 to 25% of their body weight each day. They obtain most of their water from prey but also drink seawater to satisfy thirst

Sea otters commonly feed in small groups. Hunting occurs on the sea floor. They use their sensitive whiskers to locate small creatures in the dense kelp beds and crevices. They use their small, agile forepaws to capture prey and to rub, roll, twist, and pull apart prey. Sea otters collect invertebrates in loose folds of skin under their armpits and eat at the surface. The feeding process, including foraging, eating, and cleaning their fur after a meal, lasts 2 to 3 hours. Sea otters usually eat 3 to 4 times a day.

Sea otters break open prey items with hard shells or exoskeletons with a rock. Some otters hold the rock on their chest and drive the prey into the rocks. Others leave the prey on their chests and hit the prey with the rocks. The same rock is kept for many dives. Otters 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.” 4

Reproduction
Sea otters can reproduce year round. The peaks of birth in California are from January to March. Sea otters are one of several species of mammals that undergo delayed implantation in which the embryo does not implant during the immediate period following fertilization, but remains in a state of suspended growth allowing for birth to occur under favorable conditions. Females usually give birth about once a year, though many females experience longer breeding intervals, giving birth every 2 years. A single pup is born weighing from 3 to 5 pounds. Twins occur in 2% of births, but only one pup can be raised successfully.

Male sea otters do not provide any care to their offspring. Pups are weaned at around 6 months of age but start to eat solid foods shortly after birth. Females carry their pups on their bellies while they nurse. Their milk is 20 to 25% fat. While a mother is foraging, she wraps her pup in kelp at the water surface to keep it from drifting away. At any sign of a predator, the female clamps onto her pup’s neck with her mouth and dives. Females groom their pups extensively for 3 months as their coat develops. A pup’s coat traps air, which keeps the animal afloat. Pups start diving at 2 months of age. The pup remains dependent on the mother for about 6 to 8 months. 4

Conservation Status
The southern sea otter is designated a fully protected mammal under California state law (California Fish and Game Code §4700) and was listed as a threatened species in 1977 (42 FR2965) pursuant to the federal Endangered Species Act, as amended (16 U.S.C. 1531 et seq.). As a consequence of its threatened status, the southern sea otter is considered by default to be a “strategic stock” and “depleted” under the Marine Mammal Protection Act of 1972, amended (16 U.S.C. 1361 et seq.). 2

Based upon a 2010 assessment, the International Union for Conservation of Nature and Natural Resources, (IUCN) list of threatened species considers the sea otter to be endangered due its vulnerability to large-scale population declines. The species is believed to have undergone a decline exceeding 50% over the past 30 years (approximately three generations). Recent studies have found infectious disease to be an important mortality factor in California Sea Otter populations. Around 280 Sea Otters found dead have been linked “to a pair of protozoan parasites, Toxoplasma gondii and Sacrocystis neurona.

References and Additional Information

  1. Marine Mammal Center
  2. Ventura Fish and Wildlife Office
  3. FAO Species Identification Guide: Marine Mammals of the World
  4. Animal Diversity Web
  5. The IUCN List of Threatened Species


Last updated: November 18, 2018

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