Sea otter (Enhydra lutris), marine mammal of the weasel family that lives near shores in the North Pacific Ocean. The animal averages 4–5 ft (1.2–1.5 m) and 60–85 lb (27–39 kg), and is covered with a thick, soft brown fur that insulates it against cold. It swims on its back, paddling with its hind feet, the pup carried on the chest of the female, sleeping in masses of kelp. Sea otters dive to a depth of 180 ft (55 m) for food, consisting of shellfish and other sea animals. They eat on their backs, breaking open shells by pounding them together. Sea otters have been hunted for their fur since the 1700s; a 1911 treaty saved them from extinction.