Diel variation in the catchability of gadoids and its influence on the reliability of abundance indices

Abstract

Present management of the north-east Arctic cod and haddock stocks is based on scientific assessments of stock abundance. The assessment procedures apply acoustic and bottom-trawl survey indices of abundance for tuning of the VPA (virtual population analysis). The reliability of the survey indices is strongly dependent on the precision and accuracy of the fish sampling conducted with trawls. Variation in bottom trawl catches and vertical distribution of cod and haddock from a small area were studied with the aim of finding environmental factors of explanatory importance for the observed variation. Data were obtained over a period of three days from two vessels trawling in parallel, one of which simultaneously collected acoustic information on vertical distribution. Variation in catch size and length composition was analysed and related to the observed vertical distribution. The results show that tidal currents affect the vertical distribution of cod and haddock and, hence, the availability to the bottom trawl. On the other hand, light intensity apparently affects the catch efficiency of the sampling trawl. Consequently, the abundance estimates from acoustic and bottom-trawl surveys are affected by the interaction of diel rhythms in tidal current and light intensity. This illustrates the importance of monitoring the environmental factors that influence fish behaviour in order to increase the reliability of the abundance estimates. © 1996 International Council for the Exploration of the Sea.