Pre-mating behavior is exhibited by both males and females, such as rolling, bubble-blowing, and mouthing each other's necks. This behavior ends once mating begins. Males initiate behavior by chasing, playfully biting, and embracing females. Females respond, and the act of copulation usually takes place in the water. One male may mate with multiple females. Harbor seals return to the same breeding grounds every year.

Mating System: polygynous

It is believed that males become sexually mature once a weight of around 75 kg is achieved; females mature at about 50 kg. This occurs between 3 and 7 years of age for males and at 2 to 6 years for females. While the mating season varies between the different subspecies, it generally occurs from late spring through fall. About 6 weeks after they give birth to their previous year's pups, the females come into estrus. The gestation period lasts between 9 and 11 months, and usually only 1 pup is born each year.

Breeding season: While the mating season varies between the different subspecies, it generally occurs from late spring through fall.

Average number of offspring: 1.

Range gestation period: 9 to 11 months.

Range age at sexual or reproductive maturity (female): 2 to 6 years.

Range age at sexual or reproductive maturity (male): 3 to 7 years.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); viviparous ; delayed implantation

Average birth mass: 11000 g.

Average number of offspring: 1.