Modelling Diurnal Variation in Bottom Trawl Catches and Potential Application in Surveys
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Diurnal variability in bottom trawl survey catches may influence the quality of the survey results when not taken properly into account. This is especia]!.y the case if diurnal effects vary substantiaIly from year to year. In this study, catches of cod from the winter and autumn surveys in the Barents Sea during the years 1985-1999 were explored using both parametric and nonparametric statistical methods. Diurnal variation in winter has a smooth threshold form with a high catch period at daytime lasting approximately 8 hours. Small cod seems to be triggered more by the clock than by the time of sunrise. In the autumn the diurnal variation is much less distinct. Both in winter and autumn the effect tends to decrease with fish size and increase with depth. Diurnal effects on small cod vary substantially from year to year, whereas larger fish show a higher degree of stability. We suggest a bootstrap based method for adjusting the catches for diurnal variation. This can be applied to the catches prior to the calculation of indices, and also includes measures of the uncertainty. Analyses are done by size groups and the effects of size dependent vertical migration dynamics and catching efficiency are discussed and considered in relation to the performance of the simultaneous acoustic survey.
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