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ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on January 5, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(2):234-247; doi:10.1093/icesjms/fsl025
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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

Estimating stock parameters from trawl cpue-at-age series using year-class curves

A. J. R. Cotter1,, B. Mesnil2 and G. J. Piet3

1 Cefas Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK
2 Département EMH, IFREMER, BP 21105, F44311 Nantes Cedex 03, France
3 Institute for Marine Resources and Ecosystem Studies (Wageningen IMARES), Haringkade 1, IJmuiden, Netherlands 1970 AB

Correspondence to A. J. R. Cotter: tel: +44 1502 524323; fax: +44 1502 513865; e-mail: john.cotter{at}cefas.co.uk

Cotter, A. J. R., Mesnil, B., and Piet, G. J. 2007. Estimating stock parameters from trawl cpue-at-age series using year-class curves. – ICES Journal of Marine Science, 64: 234–247.

A year-class curve is a plot of log cpue (catch per unit effort) over age for a single year class of a species (in contrast to the better known catch curve, fitted to multiple year classes at one time). When linear, the intercept and slope estimate the log cpue at age 0 and the average rate of total mortality, Z, respectively. Here, we suggest methodological refinements within a linear least squares framework. Candidate models may include a selectivity term, fleet-specific parameters, and polynomials in year to allow for gradual variations of Z. An iterative weighting method allows for differing precisions among the different fleets, and a forward (one-step ahead) validation procedure tests predicted cpue against observed values. Choice of the best approximating model(s) is made by ranking the biological credibility of each candidate model, then by comparing graphic plots, precision of prediction, and the Akaike Information Criterion. Two example analyses are (i) a comparison of estimated and true results for five stock simulations carried out by the US National Research Council, and (ii) modelling three beam trawl surveys for plaice (Pleuronectes platessa) in the North Sea. Results were consistent with known, age-related, offshore migrations by plaice. Year-class curves are commended as a widely applicable, statistically based, visual, and robust method.

Keywords: analysis of relative residual variance, cpue, fish stock assessment, forward validation, iteratively weighted least squares, North Sea, plaice, year-class curve

Received 5 May 2006; accepted 26 October 2006; advance access publication 5 January 2007.


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