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ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on December 18, 2007
ICES Journal of Marine Science: Journal du Conseil 2008 65(1):111-120; doi:10.1093/icesjms/fsm182
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Crown Copyright © 2007. Published by Oxford Journals on behalf of the International Council for the Exploration of the Sea. All rights reserved

Power of monitoring surveys to detect abundance trends in depleted populations: the effects of density-dependent habitat use, patchiness, and climate change

Julia L. Blanchard, David L. Maxwell and Simon Jennings

Centre for Environment, Fisheries and Aquaculture Science, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK

Correspondence to J. L. Blanchard tel: +44 1502 527701; fax: +44 1502 513865; email: julia.blanchard{at}cefas.co.uk

Blanchard, J. L., Maxwell, D. L., and Jennings, S. 2008. Power of monitoring surveys to detect abundance trends in depleted fish populations: the effects of density-dependent habitat use, patchiness, and climate change. – ICES Journal of marine Science, 65: 111–120.

Traditionally, trawl surveys were designed to collect fishery-independent data for assessing the population dynamics of commercially exploited species. However, trawl survey data are increasingly used to describe the abundance, distribution and ecology of a wide range of species in studies of climate change and fishing effects. These new uses of survey data suggest that improved understanding of the value and limitations of existing survey designs is required. We compared the power of different survey designs (where stations are fixed, fixed stratified, random, or random stratified) to detect known trends in the abundance of depleted populations. Modelled populations were characterized by different temperature preference, density-dependent habitat selection, and patterns of small-scale aggregation (patchiness). Temperature preferences and local patchiness had an influence on the power of different surveys to detect increases in abundance, and in some scenarios, survey-area indices would consistently underestimate or overestimate trends in overall abundance. As the distributions of many fish populations have shifted in response to climate change, exhibit distribution-abundance relationships, and have been depleted by fishing, we conclude that survey indices may provide partial or unreliable information on changes in the true abundance of the wider range of species now of interest. To disentangle the effects of fishing, climate, and biology on the abundance of fish populations, and to monitor the depletion and recovery of species threatened by fishing, there should be greater emphasis on coordinating the timing, areas of coverage, and methods of sampling of surveys of the Northeast Atlantic continental shelf.

Keywords: climate change, density-dependent habitat use, ecosystem approach to fisheries, fish population, monitoring, survey design, trawl survey

Received 11 November 2006; accepted 8 November 2007; advance access publication 18 December 2007.


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