The boundaries of these territories are vigorously patrolled, and intruding males are repulsed, but serious fighting is rare. The owner seeks to mate with any female that enters, though sometimes a pair bond is formed for a few days or weeks. Male territories are usually about 20-50 ha. Breeding occurs throughout the year, but births peak in late May and June in the Aleutians and from December to February off California. Males may mate with more than one female during the season.
Females are capable of giving birth every year but usually do so at greater intervals.
If a litter does not survive, the female may experience a postpartum estrus. Females are known to adopt and nurse orphaned pups. Reports of the period of pregnancy range from 6.5 to 9 months, and delayed implantation is probably involved. Births probably occur most often in the water. There is normally a single offspring.