The northern sea otter, Enhydra lutris kenyoni, is a fascinating marine mammal found in the northern Pacific Ocean. This article delves into the dietary habits of this subspecies, exploring what they eat, how they hunt, and the ecological significance of their diet.
Distribution and Subspecies
The sea otter (Enhydra lutris) is native to the coasts of the northern and eastern North Pacific Ocean. Three subspecies are recognized with distinct geographical distributions: Enhydra lutris lutris (Asian sea otter), E. l. kenyoni (northern sea otter), and E. l. nereis (southern sea otter). The northern sea otter (E. l. kenyoni) is found from Alaska's Aleutian Islands to Oregon. Within Alaska, there are three stocks: the Southeast stock, the South central population, and the Aleutian Islands population.
Physical Characteristics and Adaptations
Adult sea otters typically weigh between 14 and 45 kg (30-100 lb), making them the heaviest members of the weasel family, yet among the smallest marine mammals. Alaskan sea otters are slightly larger than Californian otters. Male otters weigh 27 to 39 kg, while females weigh 16 to 27 kg. Sea otters rely on their dense fur for insulation instead of blubber like other marine mammals. Unlike most marine mammals, the sea otter's primary form of insulation is an exceptionally thick coat of fur, the densest in the animal kingdom, with around 160,000 hairs per square centimeter (1,000,000 per square inch). The pelage is brown or reddish brown. They also have long whiskers that assist in foraging for food. Their hind feet are broad, flat, and webbed, providing propulsion in swimming.
The sea otter displays numerous adaptations to its marine environment. The nostrils and small ears can close. The tail is fairly short, thick, slightly flattened, and muscular.
Foraging Behavior and Diet
Sea otters are carnivorous and eat a variety of marine invertebrates and some species of fish. They dive to the sea floor to forage, preying mostly on marine invertebrates such as sea urchins, various mollusks and crustaceans. Their main prey species include sea urchins, crabs, clams, mussels, octopus, fish, and other marine invertebrates. They consume 20 to 25% of their body weight each day. Sea otters commonly feed in small groups.
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Hunting occurs on the sea floor. Sea otters use their sensitive whiskers and paws to locate prey. They pull apart prey, store it in their armpits, and eat at the surface. Feeding, cleaning their fur after a meal, lasts 2 to 3 hours. Sea otters break open prey items with hard shells or exoskeletons with a rock. Otters hold the rock on their chest and drive the prey into the rocks, or they place the prey on their chests and hit the prey with the rocks. They may make many dives. Males steal from females if they get a chance.
Tool Use
The sea otter's use of rocks when hunting and feeding makes it one of the few mammal species to use tools. They may pound their prey with both paws against a rock on their chest to open hard shells. Under each foreleg, the sea otter has a loose pouch of skin that extends across the chest. In this pouch (preferentially the left one), the animal stores collected food to bring to the surface, and sometimes a rock that is used to break open shellfish and clams. At the surface, the sea otter eats while floating on its back, using its forepaws to tear food apart and bring it to its mouth.
Nutritional Ecology
The nutritional ecology of marine predators is poorly understood compared to terrestrial predators. Consumers may target lower energy prey for various reasons, including the predator's sex and reproductive status. Sea otters are a dynamic species in which to study nutritional ecology because of their unique adaptations to the marine environment. Unlike most marine mammals, sea otters do not have blubber to keep them warm. Instead, sea otters use a combination of dense fur for insulation and maintain very high metabolic rates. Various studies estimate sea otters consume anywhere from 19% to 39% of their body weight in food per day to sustain these elevated metabolic costs.
Diet Composition in Southeast Alaska
A study in southern Southeast Alaska examined the energy content of sea otter diets through direct foraging observations and prey collection. The study area, Prince of Wales Island, exhibits a gradient of sea otter recolonization, thus providing a natural experiment to test diet change in regions with different recolonization histories. Sea otter prey items were collected in three seasons (spring, summer, and winter) to measure caloric value and lipid and protein content. A majority of the sea otter diet consisted of clams. Sea otters in newly recolonized areas had lower diet diversity, higher energetic intake rates (EIR, kcal/min), and prey had higher energy content (kcal/g). Females with pups had the highest diet diversity and the lowest EIR. Sea otter EIR were higher in the fall and winter vs. spring and summer. Sea cucumber energy and lipid content appeared to correspond with times when sea otters consumed the highest proportion of sea cucumbers.
Importance of Sea Otters in the Ecosystem
Sea otters are considered a keystone species, playing a fundamental role in the natural food web. They help maintain kelp by preying on sea urchins, which can clear-cut kelp forests when left unchecked. Sea otters enhance seagrass by preying on crabs, which eat sea slugs and isopods. By controlling the predators of these mid-size underwater grazers, sea otters allow the sea slugs and isopods to graze the algal epiphytes that can coat the seagrass blades, allowing sunlight to penetrate and the seagrass to flourish.
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The presence of kelp forests contributes to an increase in marine diversity. Kelp forests provide numerous benefits, including habitat for hundreds of invertebrate and fish species, reductions in coastal erosion, and carbon storage that can moderate climate change. Seagrasses also provide important benefits, like nursery habitat for many other species, shoreline protection, and carbon sequestration.
Threats and Conservation
Sea otters, whose numbers were once estimated at 150,000-300,000, were hunted extensively for their fur between 1741 and 1911, and the world population fell to 1,000-2,000 individuals living in a fraction of their historic range. A subsequent international ban on hunting, sea otter conservation efforts, and reintroduction programs into previously populated areas have contributed to numbers rebounding, and the species occupies about two-thirds of its former range.
The recovery of the sea otter is considered an important success in marine conservation, although populations in the Aleutian Islands, in California, and in Russia have recently declined or have plateaued at depressed levels. The population in Japan likewise remains small and precarious.
Oil spills are the greatest potential threat to the population. Limited information is available regarding the response of sea otters to climate change, but as ocean temperature and other conditions vary, it seems likely they will be impacted to some extent. Their sensitivity will be primarily due to changes in prey abundance (e.g., red urchins, clams, bivalves), particularly since sea otters require large amounts of prey (~30% of their body mass per day) to meet their metabolic requirements. A variety of sea otter prey may be sensitive to increasing ocean acidity and possibly increasing water temperatures, and declines in prey abundance could impact sea otters, though their sensitivity may not be as high due to their ability to switch between prey species. Additionally, increasing temperatures could promote survival of marine bacterial pathogens that infect sea otters and cause mortality, though there are high levels of uncertainty regarding the level of increase in and potential effects of bacterial pathogens on sea otters.
Reproduction and Life Cycle
Sea otters can reproduce year round. They breed throughout the year, but in Alaska most pups are born in the late spring. Female otters give birth to one pup at a time. Pups are 3-5 pounds at birth and light brown in color. Breeding males are territorial and defend areas where females are concentrated. They will drive other males out of their territory.
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Female sea otters reach sexual maturity at 2-5 years of age. Males become sexually mature at 4-6 years of age.
Females typically give birth to a single pup, and solely provide care to the pup for approximately six months until weaning. Pup rearing and provisioning impose high energetic costs on females, which requires them to increase the amount of effort that they put into foraging. During this period, female parents are highly susceptible to stressors, like infections and aggression by males, that they may encounter when they come into estrus after weaning.