The Steller sea lion (Eumetopias jubatus) is the largest member of the Otariidae, the family of pinnipeds sometimes referred to as the "eared seals". The males of this species, also known as the northern sea lion, are substantially larger than females. Weighing as much as 1,120kg, adult males dwarf adult females, which weigh up to 350kg (Loughlin, 2002). This extreme sexual dimorphism suggests a highly polygynous mating system where only a few of the big territorial males father most of the pups in a given season.
Steller sea lions occur throughout the north Pacific rim from Japan to southern California. Individuals, particularly juveniles, can range widely across the north Pacific ocean but return to traditional breeding sites (rookeries) in the Russian Far East, Alaska, British Columbia, Oregon and California.
In recent decades, Steller sea lions have declined dramatically in the Gulf of Alaska and Aleutian Islands, areas that historically were centers of abundance. By contrast, numbers of this species have increased in Southeast Alaska. The reasons for these differing trends are not clear but may be related to disease, reduced food availability and the combined effects of commercial fishing and climate change (Loughlin, 2002). The declines in much of the species' Alaska range prompted the investigation of patterns of variation within molecular genetic markers as a way of resolving stock structure. Early research divided the species range into an eastern (increasing) stock and a western (declining) stock (Loughlin, 1997). The western stock is currently listed as "endangered" and the eastern stock as "threatened" under the Endangered Species Act.
The Southwest Fisheries Science Center (SWFSC) is collaborating with the National Marine Mammal Lab (NMML), the Alaska Department of Fish and Game (ADF&G) and Texas A&M University in conducting ongoing molecular genetic research on Steller sea lions in order to estimate dispersal between Steller sea lion rookeries. The estimation of dispersal rates is important to understanding both the decline of Steller sea lions in the western part of their range and the effects of management actions. Patterns of genetic variation at selectively neutral genetic markers can be influenced greatly by patterns of dispersal. Research to date has focused on analyzing large numbers of pups from adjacent rookeries for variation within the mitochondrial genome. This work is still at a preliminary stage and is set to expand to include more rookeries and more genetic markers.
Loughlin, T.R. (2002) Steller's Sea Lion. pp. 1181-1185 In Encyclopedia of Marine Mammals. (W.F. Perrin, B. Wursig and J.G.M. Thewissen, eds.), Academic Press. San Diego.
Loughlin, T.R. (1997) Using the phylogeographic method to identify Steller sea lion stocks. pp.159-171. In Molecular Genetics of Marine Mammals. (A.E. Dizon, S.J. Chivers and W.F. Perrin, eds.), Special Publication #3. The Society of Marine Mammalogy.