Day-night and depth effects on catch rates during trawl surveys in the North Sea

Data from the Scottish participation in the International Young Fish Survey for the period 1976-1993 were analysed to examine the effect of light level and depth on catch rates. The species selected for this study were common dab. herring, haddock and whiting. Differences between day and night were observed for the juvenile common dab and herring and for the adult common dab and haddock. Differences between shallow and deep water were observed for the juveniles of all the species and for the adults of common dab, haddock and whiting. The mean lengths and the composition of the common dab and whiting catches were affected by both light and depth, indicating behaviour differences: between the juveniles and adults of these species. Whilst light seems to have no effect on the mean length and the catch composition of herring. differences were observed between shallow and deep waters. In the case of haddock, neither the light level nor the depth had any noticeable effect on the mean length and catch composition of the catches. We conclude that the diel behaviour and the geographic distribution are important factors in determining the quantity and composition of trawl catches, but their effects are species dependent. When trawl surveys are not tract replicates in terms of fishing times and areas, the estimation of catch indices should allow for the possible bias introduced by these factors. There is a need for models of the capture process that take account of such effects.