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The Steller sea lion (Eumetopias jubatus) also known as the northern sea lion, is a threatened species of sea lion in the northern Pacific. It is the sole member of the genus Eumetopias and the largest of the eared seals (Otariidae). Among pinnipeds, it is inferior in size only to the walrus and the two elephant seals. The species is named for the naturalist Georg Wilhelm Steller who first described them in 1741. The Steller sea lion has attracted considerable attention in recent decades due to significant, unexplained declines in their numbers over a large portion of their range in Alaska.
Steller sea lions are skilled and opportunistic marine predators feeding on a wide range of fish and cephalopod species. Important diet components include walleye pollock (Theragra chalcogramma) , Atka mackerel (Pleurogrammus spp.), Pacific salmon (Oncorhynchus spp.), herring (Clupea spp.), capelin, Pacific cod (Gadus macrocephalus), rockfish (Sebastes spp.), sculpins (family Cottoidae), and invertebrates such as squid and octopus. They seem to prefer schooling fish and remain primarily in between inter-tidal zones and continental shelves. They are also known to enter estuarine environments and feed on some semi-freshwater fish like sturgeon (Asipenser spp.). Very occasionally, they have been known to predate on Northern fur seal, harbor seal and sea otter pups. They are near the top of the marine food chain but are susceptible to predation by orcas.
Steller sea lions are sometimes killed intentionally by fishermen as they are seen as competitors and a threat to fish stocks. Killing sea lions is strictly prohibited in the U.S, Canada and Russia, but in Japan, a fixed number are still removed annually ostensibly to protect their fisheries.
While the populations of the eastern and Asian stocks appear stable, the population in the western stock, particularly along the Aleutian Islands, was estimated to have fallen by 70-80% since the 1970s. As a consequence, in 1990 the western stock of Steller sea lions was listed as endangered and the eastern stock was listed as threatened under the Endangered Species Act. They have since been the object of intense study and the focus of much political and scientific debate in Alaska.
One suspected cause of their precipitous decline is overfishing of Alaska pollock, herring, and other fish stocks in the Gulf of Alaska. Other hypotheses include increased predation by orcas, indirect effects of prey species composition shifts due to changes in climate, effects of disease or contaminants, shooting by fishermen, and others. The decline is certainly due to a complex of interrelated factors which have yet to be definitively teased apart by the research effort.