The effect of different sweep lengths on length composition of bottom sampling trawl catches

Abstract

Abundance estimates of North-east Arctic cod and haddock are given annually on the basis of combined bottom trawl and acoustic surveys. Both survey methods use data from bottom trawl catches in the calculations. The surveys are conducted in the Barents Sea and the Svalbard area with the same bottom trawl, but with different sweep wire length. The bottom trawl indices are calculated for both areas assuming an effective path width of 25 m. In this paper results from parallel trawling experiments with the standard sampling trawl equipped with different sweep lengths are analyzed. These show that the total catch increases with increasing sweep length. Small fish are relatively underestimated by the trawl with the longer sweeps, but no species selection was observed. It is concluded that combining survey results without compensating for the effect of using different sweep length will bias the estimates. The variability in the presented results demonstrates the complexity of combining data from trawl surveys conducted with different sweep lengths on the bottom trawl.