The male harbour seal is a serial polygamist, meaning that each male will mate with several females during the breeding season. Competition for mates, and copulation usually occur in the water. The Pacific harbour seal pup is born on tidal reefs or on beaches between late June and September, peaking in July and August. A harbour seal pup can swim and dive immediately after birth but requires practice to increase its skill and endurance. Following birth, the pup is nursed and protected by its mother for only three to four weeks.
Nursing time corresponds to the ocean tide cycles, and usually occurs at low tide.
The harbour seal pup grows rapidly during its first month of life, gaining much extra fat to help it survive while developing its hunting skills. Hearing and sight are highly developed in harbour seals. The harbour seal may use echolocation. The harbour seal's sense of smell can be used to locate a lost pup. Steller sea lions are larger and have longer flippers.