Ocean climate changes and the Steller sea lion decline.(Report)

Steller sea lion populations declined by over 80% between the late 1970s and early 1990s in the western Gulf of Alaska and the Aleutian Islands. Concurrent declines also occurred farther west in the Russian coastal waters. Yet population trends were reversed along the coasts of southeast Alaska, British Columbia, Washington, and Oregon, where sea lions increased through the 1980s and 1990s. The cause or causes of these population changes have not been resolved and have been the subject of considerable debate and research because their preferred prey often coincides with economically important fisheries.

Much of the search for an explanation of the Steller sea lion decline in western Alaska has focused on trying to identify a single cause, rather than recognizing that many of the proposed theories are interrelated. The leading hypotheses of epidemic diseases, predation by killer whales, ocean climate change (regime shifts), and nutritional shifts in types of prey available to sea lions (the junk food hypothesis) may all be linked through bottom-up processes. Conceptually, changes in water temperatures, ocean currents, and other oceanographic variables can influence the survival and distribution of assemblages of species that are consumed by predators such as sea lions. This in turn will affect the quantity, quality, and accessibility of the prey that sea lions consume. Individuals that consume sufficient energy are typically fat and large and experience reduced levels of oxidative stress at a cellular level. In contrast, inadequate nutrition can increase oxidative stress (and susceptibility to disease), reduce body fat (and pregnancy rates), and increase rates of predation (as a function of reduced body size or increased vulnerability while spending longer times searching for prey). Such changes to the health of individuals ultimately translate into changes in numbers at a population level through decreased birth rates and increased death rates.

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A major change in both the physical state and the ecology of the North Pacific Ocean occurred during the mid-1970s, with basin-wide changes in temperature, mixed layer depth, primary productivity, fisheries, and other variables. This linkage between the physical climate and the oceanic ecosystem provided the impetus for the Cooperative Institute for Arctic Research (CIFAR) to fund a suite of studies that addressed the hypothesis that large-scale changes in the physical environment of the North Pacific Ocean influenced Steller sea lion populations directly or indirectly. The investigations covered a variety of topics, including physical and biological oceanographic data analysis, ocean modeling experiments, and archaeological evidence.

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CIFAR also sponsored a synthesis workshop in December 2004 that resulted in a detailed publication in Fisheries Oceanography, which is briefly summarized here. It had two primary goals. The first was to determine whether any spatial and temporal patterns in the physical and biological oceanographic data corresponded with observed differences in the diets and numbers of sea lions since the late 1950s. The second was to put the recent decline in context with similar changes that may have occurred over the past 4000 years.

Characteristics of Steller Sea Lions

Steller sea lions are restricted to the North Pacific Ocean and range along the Pacific Rim from California to northern Japan. Genetically there are two distinct population segments that are split at 144[degrees]W near Prince William Sound, Alaska. The sharp decline of the larger western population through the 1980s was mirrored by population growth in the smaller eastern populations in southeast Alaska, British Columbia, and Oregon.

Counts of Steller sea lions in Alaska began in 1956 and continued sporadically through the 1960s and 1970s. They suggest that sea lion numbers were relatively high and increased slightly through the 1960s and 1970s. Trouble was not noted until the mid-1970s, and it appeared to spread east and west from the eastern Aleutian Islands in following years. The frequency and thoroughness of sea lion censuses increased through the 1980s and 1990s and showed an overall rapid decline of sea lions through the 1980s, with an inflection point and slowing of the decline around 1989. Recent counts (2002) suggest the possibility that some breeding populations in the eastern Aleutian Islands and Gulf of Alaska may have increased slightly since 1999.

Steller sea lions regularly haul out on shore at breeding (rookeries) and nonbreeding (haulout) sites. They typically spend one to two days at sea followed by one day resting on shore. Principal prey species include Atka mackerel, walleye pollock, Pacific cod, squid, octopus, salmon, Pacific herring, sand lance, and arrowtooth flounder. The most complete set of dietary information for sea lions was collected during the 1990s. It also suggests distinct geographic clusterings, with the split points centered on major Aleutian passes (Samalga Pass and Unimak Pass during summer and Umnak Pass during winter).

Significant correlations between rates of population decline and the diversity of diets suggest that a relationship may exist between what sea lions eat and how their populations have fared. Sea lions living in regions with the highest rates of declines, such as the western Aleutian Islands, consumed the least diverse diets with the lowest energy prey. In contrast, the increasing populations of sea lions in southeast Alaska had the most energy-rich diet and the highest diversity of prey …