We hypothesized that: (1) Steller sea lion Eumetopias jubatus diet choice is a function of prey availability, (2) sea lions move to take advantage of times and locations of seasonal prey concentrations and (3) the number present depends on the amount of prey available (numerical response). Over 3 yr, typically on a quarterly basis, in Frederick Sound, SE Alaska, multiple measurements were taken of Steller sea lion abundance (aerial surveys), diet (scats), dive behavior (satellite telemetry) and prey availability and caloric density (nearshore, pelagic and demersal fish surveys). We found that Steller sea lions shifted diet composition in response to changes in prey availability of pollock Theragra chalcogramma, hake Merluccius productus, herring Clupea pallasi and salmon Oncorhynchus spp. They selected intermediate-sized fish and avoided small (<10 cm) and large (> 60 cm) fish, and moved between areas as prey became available seasonally. The number of sea lions present depended on the amount of prey available; a standing biomass of 500 to 1700 t of prey in a nonbreeding area such as Frederick Sound, depending on species composition, can attract and sustain about 500 sea lions. Pollock was more frequent in sea lion diet in inside waters of SE Alaska -including Frederick Sound, Stephens Passage and Lynn Canal -than anywhere else in Alaska and contributed ~1⁄ 3 of the dietary energy in Frederick Sound. This finding implies that a diet with substantial year-round contributions from less nutritious, but abundant prey such as pollock can form part of a healthy diet as long as more nutritious prey such as herring, salmon or eulachon Thaleichthys pacificus also are consumed. Our study supports the conclusion that the Steller sea lion is an opportunistic marine predator with a flexible foraging strategy that selects abundant, accessible prey and shifts among seasonally available species.
Quantifying the nutritional quality of forage fish is integral for understanding upper trophic levels as forage fish are the dominant prey for top predator fish, marine mammals, and sea birds. Many existing reports documenting body composition of forage species are not comparable due to confounding effects. This study systematically assessed the variability in proximate composition and energy content of 16 forage species in southeastern Alaska (57.2626 N/133.7394 W) between 2001 and 2004. Variation in energy and lipid contents was related to habitat, epipelagic planktivores varying most, mesopelagics intermediate, and demersal species relatively invariable. Season was the greatest source of variation as a result of short growing seasons at high latitude and energy allocation strategies for reproduction and growth. Among species that varied seasonally, energy and lipid increased over summer and declined during winter. Annual differences in body composition occurred during periods of peak energy content. Sampling recommendations and guidance for bioenergetics models are provided.Electronic supplementary materialThe online version of this article (doi:10.1007/s00227-010-1569-3) contains supplementary material, which is available to authorized users.
The availability of seasonally abundant energy-rich prey can be a significant factor for the survival and reproductive success of predator populations. Large numbers of Steller sea lions (Eumetopias jubatus) were attracted to a prespawning aggregation of eulachon (Thaleichthys pacificus) in Berners Bay in southeast Alaska during AprilMay in 2002 and 2003. Sea lion abundance increased as eulachon gathered in Berners Bay, peaked as eulachon abundance peaked, and decreased as the eulachon moved up-river. As sea lion abundance increased in Berners Bay, sea lion abundance decreased at Benjamin Island, a sea lion haulout located 22 km away. The eulachon provided an abundant, energy-rich, predictable prey source for the Steller sea lions: (i) eulachon energy density was 9.70 ± 0.24 kJ·g1, much higher than that of any forage species reported in the North Pacific Ocean except northern lampfish (Stenobrachius leucopsarus); (ii) a large surplus of prey was available per sea lion while the eulachon aggregation was present; and (iii) the spawning run usually begins between late April and early May. The eulachon pulse may be critical to Steller sea lions during a period of high energetic demands.
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