Steller sea lions are divided into a western distinct population segment, west of about 144 degrees west in Alaska, and an eastern segment to the south and east. Counts in the western segment declined steeply from the late 1970s, with reductions on the order of 80 per cent by the 1990s, and it was listed as endangered under the US Endangered Species Act. The eastern segment grew over much of the same period and was delisted in 2013.
Several explanations have been proposed: nutritional stress from a shift in the prey base toward lower-energy fish, competition with commercial fisheries, increased predation by killer whales, and residual effects of historical culling and shooting. Each is supported by some evidence and contradicted by other lines.
No single cause is agreed. The junk-food hypothesis, that a diet shift toward pollock and other lean fish reduced reproduction and survival, is widely discussed but not conclusively demonstrated, and the western population's recovery has been uneven across sub-regions in ways no one hypothesis explains cleanly.