Diurnal variations in bottom trawl catch rates of cod and haddock and their influence on abundance indices

Abstract

Analyses of sampling data from Norwegian bottom trawl surveys for cod and haddock in the Barents Sea and the Svalbard area show that the trawl catches do not provide a representative sample of small fish in the area. In this paper, catch rates of cod and haddock from six different experiments carried out within a limited area of the same survey strata were examined according to time of day. Catch rates of cod and haddock were higher by day. Diurnal differences were also observed in species composition and length-frequency distribution. It is concluded that the trawl catch rates cannot be relied upon to provide a truly representative estimate of the entire length range of the species in question. Effects of the observed diurnal differences on bottom trawl index estimates and on the conversion of acoustic abundance to absolute fish density are discussed.