Length and species-dependent diurnal variation of catch rates in the Norwegian Barents Sea bottom-trawl surveys

Diurnal variation in capture efficiency may add to the variability in swept area estimates (or indices) of abundance from bottom-trawl surveys. In the present study the relationship between the day/night ratio of swept area e stimates and fish length was examined for five species observed in the Bare nts Sea bottom trawl survey in winter in the years 1985-1996. Generally, mo st species showed increased catch rates during daylight at all sizes as com pared with darkness. For cod the day/night ratio peaked at a length interva l 23-31 cm with a substantial reduction for larger fish. For haddock the ra tio was highest at the minimum size group, 12-15 cm, and decreased with inc reasing size. Some possible behavioural explanations for these findings are discussed. A change in groundgear during the time period analysed had a pr onounced effect on the day/night ratios for all species. In addition there seem to be a tendency for the day/night ratios of catch rates for both cod and haddock to increase with stock size. This is a matter to be aware of wh en survey results are interpreted and used in stock assessments. (C) 1999 I nternational Council for the Exploration of the Sea.