The decline of Steller sea lions (Eumetopias jubatus) in the North Pacific: insights from indigenous people, ethnohistoric records and archaeological data

Abstract

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.

Assessment of Competition between Fisheries and Steller Sea Lions in Alaska Based on Estimated Prey Biomass, Fisheries Removals and Predator Foraging Behaviour

“…Concurrent with the decline of sea lions and expansion of groundfish fisheries in the Aleutians in the late 1970s, there was a substantial change in ocean climate and declines in the abundance of non-fished species such as capelin, skates and benthic invertebrates [ 88 ]. Thus while commercial fisheries might be evoked to partially explain the interannual fluctuations in the abundance of some species [ 89 ], the geographic and temporal coherence of the collapse of large numbers of taxa argues for a large-scale common cause such as changes in ocean climate [ 45 , 89 – 91 ].…”

The roles and impacts of human hunter-gatherers in North Pacific marine food webs

“…Most studies of the relationships of humans and ecosystems are presented in terms of human impacts on ecosystems 1 2 . However, our ability to understand and mitigate human impacts depends on research that elucidates the roles humans play in ecosystems including how they interact with other species 3 .…”

“…In modern marine ecosystems, humans are depleting many commercial fisheries, causing major disruptions to ecosystem function and the persistence of species 1 4 5 6 7 . As a result, regulators have curtailed fisheries and excluded local peoples from traditional harvesting territories 2 . This is a critical problem for the Aleut peoples of the western Gulf of Alaska, who depend strongly on biotic resources from the marine communities 8 9 10 .…”

Sustaining Sanak Island, Alaska: A Cultural Land Trust

“…It also showed that the Aleut were often required to respond directly to climate-driven changes in marine productivity and changes in the Aleut population correlated closely with changes in the overall productivity of the marine-scape [3,26,27]. Ecological surveys of the terrestrial and intertidal landscapes and near-shore food-web analyses show that certain components of the ecosystem, and certain species interactions, may have been a byproduct of human ecosystem engineering as the local peoples intensely harvested resources over thousands of years [3,[28][29][30]. Unexpectedly, there is no evidence that the local peoples put any harvesting pressure on key species over the last 4500 years, and no evidence of over-exploitation [31,32], although the Aleut did expand their diet breadth down the food chain during periods of decreasing marine productivity [33].…”