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.
Publisher
ICESSeries
ICES CM documents2004/ R:23