The Steller sea lion's range extends from the Kuril Islands and Sea of Okhotsk, Russia, to the Gulf of Alaska in the north and Año Nuevo Island, off the central coast of California, in the south, but on the latter he has not been sighted since the eighties.
The global Steller sea lion population was traditionally divided into two subpopulations, eastern and western, by an imaginary line running through longitude 144 degrees west, roughly midway down the Gulf of Alaska. In the summer, Steller's sea lions tend to move to the more southern regions of the range.
Thus, although there are no breeding colonies in Japan, it is easy to encounter large bachelor groups in the waters around the island of Hokkaido in winter and spring. Some sea lions are sometimes intentionally culled by fishermen, as they are seen as competitors and a threat to fish schools.
The killing of these animals is strictly prohibited in the USA, Canada and Russia, but in Japan a defined quota of animals are killed every year, ostensibly to protect fishing territories. Although Eastern and Asian populations appear stable, Western populations, particularly along the Aleutian Islands, have been estimated to have declined by 70-80% since the 1970s.
As a result, in 1990 the western population was classified under the Endangered Species Act as endangered and the eastern population as threatened. The species has since become the subject of intense study and much political and scientific debate in Alaska.
Overfishing of Alaskan cod, Pacific herring and other fish in the Gulf of Alaska is suspected to be one cause of this rapid decline. Other causes examined include increased predation by killer whales, indirect consequences of the variation of the species preyed on by climate change, the effects of disease or contamination, killings by fishermen and other reasons.
The decline is certainly due to a set of related factors that have yet to be defined by research. In recent years, Steller sea lions have begun entering the Columbia River estuary to feed on white sturgeon, some salmon species, and rainbow trout, including some species protected by the US Endangered Species Act. They enter the Columbia River mostly in late winter and spring, sometimes upstream as far as Bonneville Dam.