Acoustic data collected during and between bottom trawl stations:: consistency and common trends

Acoustic data are often collected during bottom trawl surveys. Their use can potentially improve the precision and accuracy of fish abundance estimates if acoustic data collected between trawl stations are consistent with those collected during trawling operations. This question is addressed here through the analysis of 20 bottom trawl surveys (three survey areas and five different survey series) with coincident acoustic measurements during and between trawl stations. Firstly, on-station and underway acoustic data were compared using statistics computed globally over each survey (average vertical profiles, global indices of collocations, and spatial structures) for various combinations of depth layers. Secondly, we focussed on underway acoustic data recorded in the vicinity of stations, distinguishing between data recorded before and after the tows. On-station and underway acoustic data were highly consistent, and no systematic perturbation of the acoustic sign due to the presence of the gear a few hundred metres behind the vessel was observed.