A submersible multifrequency acoustic TS-probe was used for measuring the target strength of Antarctic krill in situ at short range. The experimental methods, procedure for data retrieval and analysis with some results for one selected station are presented. A Simrad EK60 split beam system operating at 38, 120 and 200 kHz is installed in the probe, connected to three pressure resistant transducers with 7o half power beam opening angles. The orientation of the probe was monitored with sensors for pressure, compass, pitch and roll. A stereo camera system was also mounted directly on the transducer platform with the purpose of measuring the orientation of the organisms. All system communication between the ship and the probe was through an optical Ethernet link. Firstly, krill presence was documented through recordings with hull-mounted ship echosounders and verified through net samples. The probe was subsequently lowered to suitable depth from the vessel in fixed position and data were acquired at short pulse duration, high ping rate and corresponding short maximum detection range, often limited to 25 – 50 meters below the transducers. The data analysis could then be performed by selecting single target tracks from the echograms. From synchronized detections at the three frequencies within single target tracks, individually based TS frequency response could be determined with fair accuracy. The results from the target strength measurements can further be compared with theoretical model predictions and further utilized in the biomass evaluation.
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