Journal Article

Intercalibration of North Sea International Bottom Trawl Surveys by fitting year-class curves

A. J. R. Cotter

in ICES Journal of Marine Science

Published on behalf of ICES/CIEM

Volume 58, issue 3, pages 622-632
Published in print January 2001 | ISSN: 1054-3139
Published online January 2001 | e-ISSN: 1095-9289 | DOI: http://dx.doi.org/10.1006/jmsc.2001.1068
Intercalibration of North Sea International Bottom Trawl Surveys by fitting year-class curves

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Intercalibration of trawl surveys is needed whenever trawling technique, the survey vessel, the season, or the localities of trawling are altered. Statistical modelling offers a generally cheaper method of estimating intercalibration factors than comparative trawling trials. Much variability of whole-survey population abundance indices transformed to natural logarithms can be explained using only the year-class strengths and a coefficient of total mortality, Z, for the species. This modelling approach was therefore used to intercalibrate surveys forming part of the International Bottom Trawl Survey of the North Sea between 1977 and 1997 for four commercially important galoid species, cod, haddock, whiting, and Norway pout. An age-related factor was included to allow for apparently lower catchabilities of young fish. The models fitted satisfactorily, permitting intercalibration factors to be estimated with standard errors. No indications of changes in Z were found over the period or over different year classes for any of the species. Residual errors were positively correlated among-ages-within-years for each survey. Residual degrees of freedom were therefore reduced using an information measure before testing factors in the model for significance or estimating standard errors. A method for comparing the relative precisions of the different surveys given the fitted model is also described.

Keywords: trawl surveys; abundance indices; intercalibration; year-class curves; North Sea; cod (Gadus morhua); haddock (Melanogrammus aeglefinus); whiting (Merlangius merlangus); Norway pout (Trisopterus esmarki)

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Subjects: Environmental Science ; Marine and Estuarine Biology

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