An attempt at estimating the effective fishing height of the bottom trawl using acoustic survey recordings

Abstract

Abstract For species like cod and haddock, commonly distributed from bottom to far above bottom, swept area (bottom trawl) indices and acoustic indices of abundance cover different fractions of the stock. It has been argued that a combination of the two density estimates, into one estimate of absolute abundance, will improve the reliability of the survey results. The effective fishing height of the trawl may not be equal to the headline height of the trawl. In response to vessel noise, fish in the pelagic zone may swim towards the bottom. The bottom trawl will thus catch fish higher in the water column than the height of the trawl opening. Vertical herding is highly dependent on the size of the fish and their vertical distribution pattern. In this paper, bottom-trawl-catches and acoustic recordings of northeast Arctic cod, from annual surveys conducted in the Barents Sea seasonally over the past few years, have been compared. Differences in the relationship between the two methods are discussed regarding fish length, time of day, season, year, vertical distribution, and depth. Only “clean” stations, with regard to species and length distribution, were used in the analysis. In order to combine the swept area and acoustic estimates the effective catch height of the bottom trawl must be known. Therefore, the catchability of the trawl as a function of height above bottom has been estimated for three different length groups by fitting a logistic model to the data and by examining patterns in the correlation between trawl catches and acoustic densities. The results were equivocal.