In the eastern North Pacific Ocean, the harbor porpoise ranges from Point Barrow, along the Alaska coast, and down the west coast of North America to Point Conception, California. They are found primarily in coastal waters and occur most frequently in waters less than 100 m in depth. They are widely distributed and locally abundant. Harbor porpoise are small, most under 2 m in length and 90 kg in weight. They are most often seen singly, in pairs, or groups of up to 10, although there are reports of aggregations of up to 200 harbor porpoise. Most seasonal movements appear to be inshore-offshore and may be influenced by prey availability or the presence of ice-free waters. They are non-descript in coloration and behavior; they infrequently breach or porpoise clear of the water. Unlike most other porpoises, they do not approach boats to bow ride. Little is known of their reproductive biology: most mating occurs in summer; the gestation period is about 11 months; most births occur between May and July.