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Decomposing and explaining the variability of bottom trawl survey data from the Barents Sea

Survey data have become an integral part of the annual evaluation of most commercially important fish stocks. However, many physical and biological factors may bias the estimates or reduce their precision. The use of systematic signals in the survey time series is the most cost‐efficient way to improve the estimates. In this study, we analysed records from 1938 trawl stations from the annual Norwegian winter surveys in the Barents Sea from 1986 to 2000. We quantified the explanatory power of variables such as depth, location, light, water temperature and fish density for (i) diurnal variation and (ii) haul‐to‐haul variation in length‐stratified bottom trawl catches of cod and haddock. The largest diurnal effects were found for small fish, with depth being the most important cofactor. On the log‐scale, the diurnal amplitude of small cod and haddock increased linearly with depth. It also increased with the density of large cod, which was the most important biological factor. The density of fish of the same species and similar length was the most important explanatory variable for haul‐to‐haul variation. Water temperature was the most important physical factor for all length groups of both cod and haddock. The explanatory variables explained more for haddock than for cod for both haul‐to‐haul and diurnal variation.