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Marine Ecology Progress Series

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MEPS 441:225-239 (2011)  -  DOI:

Harbor seal foraging response to a seasonal resource pulse, spawning Pacific herring

Austen C. Thomas1,3,*, Monique M. Lance2, Steven J. Jeffries2, Benjamin G. Miner1, Alejandro Acevedo-Gutiérrez1

1Western Washington University, Department of Biology, Bellingham, Washington 98225-9160, USA
2Washington Department of Fish and Wildlife, Wildlife Science Program, Lakewood, Washington 98498, USA
3University of British Columbia, Marine Mammal Research Unit, Vancouver, British Columbia V6T 1Z4, Canada

ABSTRACT: Evidence suggests that Pacific harbor seals Phoca vitulina are likely to alter their foraging behavior in response to seasonal prey pulses. We hypothesized that spawning herring Clupea pallasii aggregations are seasonally important prey for harbor seals, predicting that (1) harbor seal consumption of adult herring would peak during the spawning season, (2) harbor seals would seasonally change their foraging areas to take advantage of spawning herring aggregations, and (3) seal diving behavior would reflect the vertical distribution of herring during the spawning season. The predictions were tested using an analysis of harbor seal prey remains, GPS telemetry, and satellite-linked time/depth recorder data. Contrary to predictions, herring in harbor seal diet was comprised of 74% juveniles and 26% adults in the spawn season, versus 37% juveniles and 63% adults in the post-spawn season. Harbor seal use of documented herring areas was highest during the season when herring did not spawn, and seal diving behavior did not reflect the vertical distribution of herring. The lack of response by harbor seals to spawning herring pulses is likely explained by the low energy density of adult herring during the spawn season, and the availability of profitable alternative prey such as juvenile herring. This study highlights the influence of relative prey profitability on the foraging behavior of harbor seals, and may help to explain why predators do not always respond as predicted to resource pulses.

KEY WORDS: Phoca vitulina · Resource pulse · Utilization distribution · Diet · Diving · Marine predators

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Cite this article as: Thomas AC, Lance MM, Jeffries SJ, Miner BG, Acevedo-Gutiérrez A (2011) Harbor seal foraging response to a seasonal resource pulse, spawning Pacific herring. Mar Ecol Prog Ser 441:225-239.

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