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ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on June 23, 2009
ICES Journal of Marine Science: Journal du Conseil 2009 66(10):2272-2277; doi:10.1093/icesjms/fsp179
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© 2009 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Biased stock assessment when using multiple, hardly overlapping, tuning series if fishing trends vary spatially

Sarah B. M. Kraak1, Niels Daan2 and Martin A. Pastoors3

1 Marine Institute, Rinville, Oranmore, Co. Galway, Ireland
2 Waaldijk 39, 6621 KH Dreumel, The Netherlands
3 Wageningen IMARES, PO Box 68, 1970 AB IJmuiden, The Netherlands

Correspondence to S. B. M. Kraak: tel: +353 91 387392; fax: +353 91 387201; e-mail: sarah.kraak{at}marine.ie

Kraak, S. B. M., Daan, N., and Pastoors, M. A. 2009. Biased stock assessment when using multiple, hardly overlapping, tuning series if fishing trends vary spatially. – ICES Journal of Marine Science, 66: 2272–2277.

Fishing-effort distributions are subject to change, for autonomous reasons and in response to management regulations. Ignoring such changes in a stock-assessment procedure may lead to a biased perception. We simulated a stock distributed over two regions with inter-regional migration and different trends in exploitation and tested the performance of extended survivors analysis (XSA) and a statistical catch-at-age model in terms of bias, when spatially restricted tuning series were applied. If we used a single tuning index that covered only the more heavily fished region, estimates of fishing mortality and spawning-stock biomass were seriously biased. If two tuning series each exclusively covering one region were used (without overlap but together covering the whole area), estimates were also biased. Surprisingly, a moderate degree of overlap of spatial coverage of the two tuning indices was sufficient to reduce bias of the XSA assessment substantially. However, performance was best when one tuning series covered the entire stock area.

Keywords: North Sea plaice, simulation, spatial effort variation, statistical catch-at-age, stock-assessment performance, tuning indices, XSA

Received 2 December 2008; accepted 27 May 2009; advance access publication 23 June 2009.


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