Combining multibeam-sonar and multifrequency-echosounder data: examples of the analysis and imaging of large euphausiid schools

The first high-resolution, quantitative, multibeam sonar (Simrad MS70) ever developed was mounted in a keel of RV “G. O. Sars” with port-orientated beams. Each ping samples a volume of 60° horizontally × 45° vertically with 500 beams, which is often enough to insonify a complete school of fish or zooplankton.

The large amount of resulting data is efficiently preprocessed with automatic, real-time detections of school candidates; these are accepted or rejected during post-processing. The system was used on the continental shelf near the Subantarctic island of South Georgia to study Antarctic krill (Euphausia superba), and some of the detected schools were immediately sampled with a six-frequency echosounder (Simrad EK60), then trawled with various nets to verify the target species and their size composition. For schools acoustically categorized as euphausiids, data from the two acoustic systems were used to estimate the school morphometrics and the krill size distributions. The principal objectives of this study were to explore the potential of combining data from a multibeam sonar, multifrequency echosounders, and nets, and to describe the efficient processing methods and software that facilitate the multi-instrument analyses. Three-dimensional morphometrics based on the MS70 data were consistent with corresponding two-dimensional morphometrics based on the echosounder data and could be used to improve the acoustic classifications of taxa or species. Additionally, automatic preprocessing and integration of data from different sources into the same user interface allowed efficient exploration and interpretation of all the acoustic data.

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