Seals enter breeding rookeries in May. Generally, older males (10 years and older) return first and compete for prime breeding spots on the rookeries. They remain on the rookery, fasting throughout the duration of the breeding season. The females come somewhat later, and give birth shortly thereafter. Like all other otariids, northern fur seals are polygynous, with some males breeding with up to 50 females in a single breeding season. Unlike Steller sea lions, with which they share habitat and some breeding sites, northern fur seals are possessive of individual females in their harem, often aggressively competing with neighboring males for females. Deaths of females as a consequence of these conflicts have been recorded, though the males themselves are rarely seriously injured. Young males unable to acquire and maintain a territory of a harem typically aggregate in neighboring "haulouts", occasionally making incursions into the reproductive sections of the rookery in an attempt to displace an older male. After remaining with their pups for the first eight to ten days of their lives, females begin foraging trips lasting up to a week. These trips last for about four months before weaning, which happens abruptly, typically in October. Most of the animals on a rookery enter the water and disperse towards the end of November, typically migrating southward. Breeding site fidelity is generally high for fur seal females, though young males might disperse to other existing rookeries, or occasionally find new haulouts. Peak mating occurs somewhat later than peak birthing from late June to late July. As with many other otariids, the fertilized egg undergoes delayed implantation: after the blastocyst stage occurs, development halts and implantation occurs four months after fertilization. In total, gestation lasts around a year, such that the pups born in a given summer are the product of the previous year's breeding cycle. Status Recently, concern about the status of fur seal populations has increased, particularly in the Pribilof Islands, where pup production has decreased about 50% since the 1970s, with a continuing drop of about 6–7% per year. This has caused them to be listed as "vulnerable" under the U.S. Endangered Species Act, and has led to an intensified research program into their behavioral and foraging ecology. Possible causes are increased predation by killer whales, competition with fisheries, and climate change effects, but to date, no scientific consensus has been reached. The IUCN (2008) lists the species as