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Marine Ecology Progress Series

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MEPS 388:243-261 (2009)  -  DOI:

Steller sea lion foraging response to seasonal changes in prey availability

Michael F. Sigler1,*, Dominic J. Tollit2,*, Johanna J. Vollenweider1, John F. Thedinga1, David J. Csepp1, Jamie N. Womble1, Mandy A. Wong2, Michael J. Rehberg3, Andrew W. Trites2

1National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Alaska Fisheries Science Center,
17109 Point Lena Loop Road, Juneau, Alaska 99801-8626, USA
2Marine Mammal Research Unit, Fisheries Centre, University of British Columbia, 2202 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada
3Alaska Department of Fish and Game, Statewide Marine Mammal Program, 525 West 67th Avenue, Anchorage, Alaska 99518-1599, USA
*Corresponding authors. Email: or *

ABSTRACT: 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 non-breeding 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.

KEY WORDS: Prey availability · Foraging · Steller sea lion · Prey selection · Diet · Walleye Pollock · Herring

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Cite this article as: Sigler MF, Tollit DJ, Vollenweider JJ, Thedinga JF and others (2009) Steller sea lion foraging response to seasonal changes in prey availability. Mar Ecol Prog Ser 388:243-261.

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