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Depletion of deep marine food patches forces divers to give up early.
MedLine Citation:
PMID:  22881702     Owner:  NLM     Status:  Publisher    
Abstract/OtherAbstract:
Many optimal foraging models for diving animals examine strategies that maximize time spent in the foraging zone, assuming that prey acquisition increases linearly with search time. Other models have considered the effect of patch quality and predict a net energetic benefit if dives where no prey is encountered early in the dive are abandoned. For deep divers, however, the energetic benefit of giving up is reduced owing to the elevated energy costs associated with descending to physiologically hostile depths, so patch residence time should be invariant. Others consider an asymptotic gain function where the decision to leave a patch is driven by patch-depletion effects - the marginal value theorem. As predator behaviour is increasingly being used as an index of marine resource density and distribution, it is important to understand the nature of this gain function. We investigated the dive behaviour of the world's deepest-diving seal, the southern elephant seal Mirounga leonina, in response to patch quality. Testing these models has largely been limited to controlled experiments on captive animals. By integrating in situ measurements of the seal's relative lipid content obtained from drift rate data (a measure of foraging success) with area-restricted search behaviour identified from first-passage time analysis, we identified regions of high- and low-quality patches. Dive durations and bottom times were not invariant and did not increase in regions of high quality; rather, both were longer when patches were of relatively low quality. This is consistent with the predictions of the marginal value theorem and provides support for a nonlinear relationship between search time and prey acquisition. We also found higher descent and ascent rates in high-quality patches suggesting that seals minimized travel time to the foraging patch when quality was high; however, this was not achieved by increasing speed or dive angle. Relative body lipid content was an important predictor of dive behaviour. Seals did not schedule their diving to maximize time spent in the foraging zone in higher-quality patches, challenging the widely held view that maximizing time in the foraging zone translates to greater foraging success.
Authors:
Michele Thums; Corey J A Bradshaw; Michael D Sumner; Judy M Horsburgh; Mark A Hindell; Graeme Hays
Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2012-8-7
Journal Detail:
Title:  The Journal of animal ecology     Volume:  -     ISSN:  1365-2656     ISO Abbreviation:  J Anim Ecol     Publication Date:  2012 Aug 
Date Detail:
Created Date:  2012-8-13     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  0376574     Medline TA:  J Anim Ecol     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
Copyright Information:
© 2012 The Authors. Journal of Animal Ecology © 2012 British Ecological Society.
Affiliation:
Marine Predator Unit, Institute of Marine and Antarctic Studies, University of Tasmania, Private Bag 129, Hobart, Tas., 7001, Australia; School of Environmental Systems Engineering and The UWA Oceans Institute, The University of Western Australia, M470, 35 Stirling Highway, Crawley WA, 6009, Australia; Australian Institute of Marine Science, The UWA Oceans Institute, MO96, 35 Stirling Highway, Crawley WA, 6009, Australia.
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