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Identifying patterns in the diet of mackerel icefish (Champsocephalus gunnari) at South Georgia using bootstrapped confidence intervals of a dietary index

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Abstract

Ontogenetic, inter-annual and regional variations in diet were investigated for mackerel icefish, Champsocephalus gunnari, in three successive summer seasons around South Georgia. Stomach contents from 2239 C. gunnari (130–560 mm total length) were examined. A bootstrapping technique was used to calculate confidence intervals for an index of relative importance of prey categories (% IRIDC). Diet varied significantly between years and age classes but there was little regional difference in diet. In general, diet was dominated by krill, Euphausia superba and by the amphipod Themisto gaudichaudii. Smaller (younger) fish tended to prey on a higher proportion of T. gaudichaudii and small euphausiids such as Thysanoessa sp. and took smaller quantities of E. superba. In a season of poor krill availability (summer of 2003–2004) the proportion of krill in the diet, stomach fullness and fish condition (indicated by length–weight relationships) were significantly lower than in the other summer seasons. A large reduction (>80%) in the estimated annual (2005) biomass of the C. gunnari stock directly followed the season of poor krill availability. This decline was largely because of mortality of 2+ and 3+ fish, which were more krill dependent than 1+ fish. Younger fish appear to have survived, leading to an increase in the estimated population biomass in 2006.

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Acknowledgments

We wish to thank Len Featherston (Master) and the crew and scientists on Dorada during the surveys in January 2004, 2005 and 2006. Thanks also to Sarah Clarke, Will Reid and Jamie Watts who assisted with some of the stomach contents analysis, and thanks to Pete Rothery for statistical advice. The study contributed to the BAS Discovery 2010 programme.

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  1. Charlotte E. Main

    Present address: Fisheries Research Services, Marine Laboratory, Aberdeen, AB11 9DB, Scotland, UK

Authors and Affiliations

  1. British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge, CB3 0ET, UK

    Charlotte E. Main, Martin A. Collins, Richard Mitchell & Mark Belchier

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  1. Charlotte E. Main
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Correspondence to Charlotte E. Main.

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Main, C.E., Collins, M.A., Mitchell, R. et al. Identifying patterns in the diet of mackerel icefish (Champsocephalus gunnari) at South Georgia using bootstrapped confidence intervals of a dietary index. Polar Biol 32, 569–581 (2009). https://doi.org/10.1007/s00300-008-0552-7

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