What is the difference between river otter and sea otter?

September 1997

River and sea otters are both members of the Mustelidae family that includes badgers, weasels, minks, wolverines and skunks. The primary difference between the two species has more to do with where they live than actual physical differences.

River otters, like this one, are better suited for life on land than their cousins, sea otters. North Carolina river otters are found primarily in the coastal and central sections of the state.

Sea otters are true marine mammals, found only in the ocean and rarely seen on land. They are really quite clumsy on land and are perfectly capable of spending their entire lives at sea. River otters are land mammals that inhabit fresh water lakes, rivers and streams.

River otters are amphibious and well adapted to life in water or on land. Their strong legs enable them to move quickly, and on land they can outrun a man over a short distance.

In North America, sea otters are found only in shallow waters of the Pacific Ocean along the coast of California and Alaska. River otters can be found in waterways throughout North America (except extreme northern Canada and the desert areas of the southwest). In North Carolina, river otters are found chiefly in the coastal and central portions of the state. A relocation program instituted by the North Carolina Wildlife Resources Commission in the early ’90s reintroduced otters to the river systems of western Carolina.

Physically, the two species are quite similar. Both have streamlined bodies well suited to swimming and diving; dark, dense fur coats consisting of two layers that serve to insulate and waterproof; webbed feet; and long, muscular tails. The river otter is smaller than the sea otter, weighing from 10 to 30 pounds and measuring 3 to 4 feet in length from the tip of its nose to the end of its tail. Its tail is longer than its cousin’s, approximately two-thirds the length of its head and body. Sea otters average 50 to 90 pounds and reach lengths of up 5 feet. Their tails are roughly a third the length of their head and body.

Seeing an otter in the water is one of the best ways to tell the difference between the two species. At the surface, river otters swim belly down and expose very little of their back, while sea otters swim belly up and float high in the water due to their air-filled fur. Both are strong swimmers and divers; river otters can swim at a rate of up to 12 mph, while sea otters can dive to depths of up to 180 feet.

Because sea otters are true ocean-dwelling creatures, their hind limbs are paddle-shaped and are webbed to the tips of their toes to aid in swimming. River otters have smaller, more circular-shaped webbed paws because they are adapted to land travel.

Both species are known for their playful behavior and acrobatic water stunts. Sea otters, however, are more social, living in large groups called rafts. River otters are rarely seen in groups larger than a single family consisting of mother and three to four young.

Whether ocean or river dwellers, otters in North America are threatened by pollution, habitat degradation, hunting and trapping. Wildlife protection legislation and programs are in existence in many states to protect and preserve these aquatic acrobats.