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

Intercalibration of trawl surveys is needed whenever trawling technique. th e survey vessel, the season. or the localities of trawling are altered. Sta tistical modelling offers a generally cheaper method of estimating intercal ibration Factors than comparative trawling trials, Much variability of whol e-survey population abundance indices transformed to natural logarithms can he explained using only the year-class strengths and a coefficient of tota l mortality, Z. for the species. This modelling approach H as therefore use d to intercalibrate surveys forming part of the international Bottom Trawl Survey of the North Sea between 1977 and 1997 for four commercially importa nt galoid species. cod. haddock. whiting, and Norway pout. An age-related f actor was included to allow for apparently lower catchabilities of young fi sh. The models fitted satisfactorily. permitting intercalibration factors t o be estimated with standard errors. No indications of changes in Z were fo und 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 inform ation measure before testing factors in the model for significance or estim ating standard errors. A method for comparing the relative precisions of th e different surveys given the fitted model is also described.