Acoustic data collected on pelagic fishing vessels throughout an annual cycle: Operational framework, interpretation of observations, and future perspectives

Acoustic data collection trials on pelagic freezer-trawlers were realised in 2012 during several fishing trips targeting blue whiting west of the British Isles in spring, North Sea herring in summer, and horse mackerel in the English Channel and Celtic Sea in autumn. Echosounders were calibrated and time- and position-stamped data logged along the path covered by the vessels. The acoustic detections recorded during different types of trawler activity within a fishing trip (‘searching’, ‘stationary’, and ‘fishing’) were compared between target species. The highest proportion of time spent for activity ‘fishing’ was observed in the blue whiting fishery (82%), while that value was lower in the horse mackerel and herring fishery (68% and 54%). In all fisheries the quantified mean fish densities recorded were significantly higher during ‘fishing’ than during ‘searching’. Changes in recorded fish density magnitudes over time before and after trawling also showed different patterns between fisheries. The quantified peculiarities exhibited by the specific fishing trip data is discussed in light of incorporating them in monitoring programs and analysis methods that can advance ecosystem understanding. Potential future approaches for analysis methods of opportunistically recorded acoustic fishing vessel data are discussed.