Sea Otter vs River Otter: Key Differences Explained

Key differences at a glance

Both animals are otters, members of the weasel family Mustelidae, but they belong to different genera and live very different lives. The sea otter (Enhydra lutris) is a marine mammal of the North Pacific that can spend its entire life in the water. In North America, “river otter” usually means the North American river otter (Lontra canadensis), a semiaquatic animal of rivers, lakes, marshes and coasts that hunts in water but rests on land.

In Europe, the only wild otter is the Eurasian otter (Lutra lutra), which plays the same role as the North American river otter. Most of the differences below apply to it too.

Size and shape: which is bigger?

The sea otter is much heavier. Males usually weigh 22–45 kg (49–99 lb) and females 14–33 kg (31–73 lb), making it the heaviest member of the weasel family. A North American river otter weighs 5–14 kg (11–31 lb), and a Eurasian otter typically 7–12 kg (15–26 lb).

Total length is closer than weight suggests. A sea otter measures about 1–1.5 m (3.3–4.9 ft), and a river otter about 0.9–1.3 m (3–4.3 ft), but the river otter’s tail makes up about a third of that. The sea otter has a stocky, rounded body, a broad head and a short, flattened tail of 25–35 cm (10–14 in). The river otter is long, slim and snake-like, with a thick, muscular tail of 30–50 cm (12–20 in) that tapers to a point.

Look at the feet, too. A sea otter’s hind feet are broad and flipper-like, and its front paws are small mittens with retractable claws. A river otter has four similar webbed feet with claws, built for running as well as swimming.

Swimming: back-floaters vs belly-swimmers

A sea otter at the surface usually lies on its back, paddling with its hind flippers and using its chest as a table for eating, a cradle for its pup and a workbench for cracking shells. Underwater it swims with up-and-down strokes of its hind body and tail, reaching about 9 km/h (5.6 mph).

A river otter swims belly-down with its head held up, and when it travels it often rolls through the surface in smooth arches. It may float briefly on its back, but it does not live that way. On land the difference is even clearer: river otters run, bound and slide easily, while sea otters walk with a slow, clumsy gait and rarely go far from the water.

Saltwater vs freshwater life

Sea otters live in salt water along rocky coasts, kelp forests, bays and estuaries, usually within about 1 km (0.6 mi) of shore. They eat, sleep, mate and give birth in the sea, although in places such as Alaska they sometimes haul out on rocks. Because they have no blubber, they depend on the densest fur of any animal and spend hours grooming it; see How do sea otters stay warm?

River otters are freshwater animals that also use the coast. They live along rivers, streams, lakes and marshes, and in the Pacific Northwest and Alaska many hunt in the sea. Even coastal river otters rest, groom and raise their young on land, in dens under tree roots, rock piles or old burrows, and they usually need fresh water nearby to rinse their fur.

Diet and tool use

Sea otters feed mainly on seabed invertebrates: sea urchins, crabs, clams, mussels, abalone, snails and octopuses, plus fish in some northern areas. They eat about a quarter of their body weight a day, and many crack hard shells against a stone held on the chest. Their urchin-eating protects kelp forests, which makes them a keystone species.

River otters are mainly fish-eaters, taking slow-moving fish such as suckers, sunfish and catfish, along with crayfish, frogs, crabs and the occasional bird. An adult eats roughly 1–1.5 kg (2.2–3.3 lb) a day. They catch prey with their mouths and are not known to use tools. Sea otters, by contrast, grab prey with their front paws; they are the only marine mammal known to catch fish with the paws rather than the teeth.

Family life

Sea otter mothers give birth to a single pup in the water. The pup rides on its mother’s belly for weeks and stays with her for about six to eight months. Rafts of resting sea otters, sometimes hundreds strong, are a famous sight on the Pacific coast.

North American river otters have litters of one to six pups, usually two or three, born in a den on land. The pups are born blind and first swim at about two months. Family groups of a mother and her young often travel and hunt together, and some coastal males form groups too.

Where each lives in North America and Europe

Sea otters live only in the North Pacific: from northern Japan and eastern Russia through the Aleutian Islands and Alaska to British Columbia, Washington’s outer coast and central California. Oregon gets only occasional wanderers, and there are no wild sea otters on the Atlantic coast or in Europe.

North American river otters live across most of Canada and Alaska and much of the United States, including the Pacific coast. Where both species share the coast, from Alaska to Washington, confusion is common. In Washington’s Puget Sound and San Juan Islands, most reported “sea otters” turn out to be river otters.

In Europe, coastal Eurasian otters in Scotland, Ireland and Norway are often called sea otters, but they are the same species as river otters inland; see Are there otters in the UK?

Conservation status and interbreeding

The sea otter is listed as Endangered by the IUCN. Hunted for its fur to perhaps 1,000–2,000 animals by 1911, it has recovered in parts of its range but remains vulnerable to oil spills, disease and predation. The North American river otter is Least Concern and has returned to many US states through reintroductions, while the Eurasian otter is Near Threatened.

Sea otters and river otters cannot interbreed. No hybrids are known, and their lineages split millions of years ago; the sea otter’s closest living relatives are actually Old World otters such as the Eurasian otter, not the North American river otter.