Steller sea lions are distributed throughout most of the North Pacific from the coast of California to eastern Russia and northern Japan. Major rookeries and haulouts in the Alaskan part of the Steller sea lion range are shown in Figure 4.1. As indicated in Box 4.1, Steller sea lions are polygynous: males come ashore to establish breeding territories and guard harems of females. Immature animals occupy haulout sites, usually in the vicinity of the breeding and pupping rookeries. Thus, the sex and age structure observed on the breeding rookeries does not really represent the sex and age structure of the population. Existing data suggest sex ratios at birth are near 50-50; thus, there is little reason to believe any bias exists in population sex ratios except possibly in older age groups when males may show higher mortality rates than females. Higher mortality rates for older males are suggested because the competition for breeding among males creates wounds that might contribute to shorter lives, but there are very little data on survival rates at older ages.

During the period of rapid decline of Steller sea lions in western Alaska, the population in southeastern Alaska showed a slow but steady increase. This prompted consideration of the western stock as a distinct population segment as defined by the Endangered Species Act. In 1995 the National Marine Fisheries Service (NMFS) proposed listing the western stock as an endangered population separate from the eastern stock. This consideration was based on genetic studies, tagging and branding studies, and telemetry studies (60 Federal Register 192, Oct. 4, 1995, pp. 51968-51978).

With respect to behavior at the colony level that supports separation of the two stocks, Raum-Suryan et al. (2002) carried out a complete analysis of branded and tagged individuals from past marking studies. They found that the observed breeding fidelity to the rookery of birth for females branded in 1987-1988 was 67% for the western stock and 81% for females branded in 1994-1995 in the eastern stock. Sighting of marked females showed that they generally remain within 500 km of the natal rookery. In general, some straying occurred, but homing by females to their natal areas was quite strong. Males tended to use more haulout sites and rookeries and to disperse greater distances. However, there was little evidence of exchange of individuals between stocks. No adults were observed breeding in the opposite stock, but some adults of breeding age did move between stocks.

Genetic studies have mostly used mitochondrial DNA (mtDNA) sequences to characterize the stock structure of sea lions throughout their range. The mitochondrial genome is maternally inherited as a discrete unit and can be used to analyze female lineages. The distribution of female