At the start of the breeding season, males begin arriving at their traditional sites about a month ahead of the females and start competing for breeding territories (4). These fights can be extremely violent, with each mature bull aiming to slash an opponent's neck with his sharp canine teeth. Only the largest and heaviest bulls can hope to claim the title of 'beachmaster'; the smaller younger males, who have no chance of competing with the fully-grown animals, occupy the fringes of the breeding territories (3).
The females arrive in mid-June and give birth to the pups, conceived the year before, some two days after their arrival. Within a week of the birth, the females will mate again (4). Males compete to secure as many females as he can within his harem, although it is thought that females are influenced by the presence of other females and the characteristics of the territory rather than the mere size and power of the male (2). The beachmasters will continue to squabble and fight over females right through the breeding season, usually because these colonies are crowded, and wandering females sometimes stray into another male's territory (3). Occasionally, younger males will attempt to steal a mating with a female and, if spotted by one of the beachmasters, he will be chased off (2). In order to ensure that his females are not claimed by another male, bull northern fur seals do not feed throughout the breeding period and may eventually loose 20% of their body weight (4).
Mothers suckle their pups for up to 10 days before returning to feed at sea, usually during the night. She will stay at sea from between four and ten days feeding, returning to feed her pup for one or two days. She will do this for four months before leaving her youngster and migrating south, usually in late October (4). Fur seals feed on a variety of prey, including squid and pollack (4) and have also been recorded taking seabirds (2).
The fertilised egg within the female fur seal undergoes a four month period of delayed implantation. This ensures that that the developing pup will be born at the right time the following year when the animals return to their breeding grounds. The pups will spend as long as 22 months at sea before returning to the beach where they were born (4). Fur seals mature between the ages of three and six, but males will probably not begin to breed for an additional three years (2).
The principal natural predators of fur seals are orca (killer whales), great white sharks and the much larger Steller sea lion Eumetopias jubatus. On land, pups can fall prey to foxes (4).