A number of hypotheses have been proposed to explain the most recent decline (1977–2012) of Steller sea lions (SSL; Eumetopias jubatus) in the Gulf of Alaska and Aleutian Islands. We examined hypotheses about fisheries competition, environmental change, predation, anthropogenic effects and disease using observations of modern Aleut and archaeological, ethnohistoric and ethnographic data from the western Gulf of Alaska and Aleutian Islands. These data indicate that Steller sea lion numbers have declined and recovered repeatedly over the past 4500 years and were last at critically low numbers during the 1870s–1930s. Steller sea lions appear to have been more abundant during the cool periods – and lower during the warmer periods. Observations by local peoples, explorers, early government surveyors and biologists since the late 1800s suggest that low populations of SSL have been associated with high populations of Gadidae fishes (Pacific cod – Gadus macrocephalus and walleye pollock – Theragra chalcogramma) and are consistent with the ocean climate hypothesis to explain the decline of sea lions. They suggest that removals by people and killer whales (Orcinus orca) did not cause the sea lion declines, but could have compounded the magnitude of the decline as sea lion numbers approached low densities. Archaeological, anthropological and ethnohistorical analyses demonstrate that fluctuations have occurred in the North Pacific over hundreds to thousands of years and provide context for understanding the changes that occur today and the changes that will continue to occur in the future.