Can subjective evaluation of echograms improve correlation between bottom trawl and acoustic densities?

Abstract

Direct comparison of acoustic densities and trawl catches involves fundamental difficulties related to species and size selectivity of the trawl and efficiency of the acoustic instrumentation. In some surveys, therefore, accumulated experience on the visual appearance of various species on the echograms combined with species and size information from the trawl is used to scrutinize the echogram and split the observed total acoustic density to species or groups of species. In this paper, combined bottom trawl – acoustic survey data collected by Scotland and Norway from the North and the Barents Seas are used to study correlations between bottom trawl catches and the acoustic density measures logged from vessel mounted transducers during the trawl operations. We want to evaluate if classifying echo traces by species or groups of species prior to the analysis can improve the correlations between trawl catches and acoustic densities. Although the classification process contains elements of subjectivity, it is based on characteristics of the echo traces, temporal changes in the echograms, geographic location and time of day as well as species and size compositions of the trawl catches. Initial results indicate that scrutinized information improves the correlation between trawl and acoustic density information and that harmonized rules for the scrutinizing procedures should be developed.