Length and species dependent diurnal variation in 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) from bottom trawl surveys. In the present study the relationship between the day/night ratio of swept area estimates and fish length was examined for 5 species observed in the Barents Sea bottom trawl survey in winter in the years 1989-1996. Generally, most species showed increased catch rates during day light at all sizes as compared with darkness. For cod and haddock a substantial reduction in the day/night ratio with fish length was observed. Some possible behavioural explanations for these findings are discussed for cod. In addition is there a tendency that increased stock size increases the difference in catch rates for both cod and haddock. The implications of the findings for the conduct of the surveys and the reliability of swept area estimates/indices are considered