A vessel specific bias in echo sounder recordings during trawling

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We have analysed acoustic data recorded by RV 'Johan Hjort' and (old) RV 'G.O.Sars' during the bottom trawl survey in the Barents Sea from 1997 through 2002. The observed acoustic density of demersal fish during trawling was compared to that just before and just after trawling. For RV 'G.O.Sars', the echo abundance was on average about 12 % lower before and after trawling than during trawling, and the difference was highly significant and consistent between years. For RV 'Johan Hjort' the average difference was about 1%, which was not significant (Figure 1).