Walruses are large semi-aquatic mammals that live in the cold Arctic seas of the Northern Hemisphere. Walruses spend about half their time in the water and half their time on beaches or ice floes, where they gather in large herds. They may spend several days at a time either on land or in the sea. Diving to depths of 90 m (300 ft), they sometimes stay under for as long as a half hour. In the sea they sometimes catch fish, but generally graze along the sea bottom for clams which they suck from the shell. Abrasion patterns of the tusks show that the tusks are dragged through the sediment but are not used to dig up prey. Walruses can also spit jets of water to look for clams. Clams and mollusks form a large part of their diet. Large male walruses have been observed to attack seals if they cannot find any other food source.