Measurements of Lesser sandeel schools (Ammodytes marinus) using omnidirectional fisheries sonar
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The coverage of this master thesis is 58 pages, with 25 Figures and 9 Tables. The Simrad SH90 high frequency omidirectional sonar was used in a field experiment in 2011 to acoustically investigate the distribution of Lesser sandeel (Ammodytes marinus) during the North Sea sandeel survey by the Norwegian research vessel Johan Hjort". The sonar was applied together with conventional echo sounder survey, with trawl and dredge sample stations at various point. During the survey, sonar screenshots were recorded for 750 school from the total survey period, while two sandeel banks, the Inner Shoal East" and Hardangervidda" was investigated by using post-processing software LSSS for echo sounder data and PROFOS for sonar data. Sandeel school parameters were carefully analyzed from the screenshots by using the image editing software ImageJ". The results show that sandeel schools can well be detected with the Simrad SH90 omnidirectional sonar in good to fair weather conditions. The effective detection range for weak schools in shallow water is about 250 m. The sonar showed greater capability of detecting sandeel schools during a conventional acoustic survey, with 3.02 and 1.44 times the detections of the echo sounder for Inner Shoal East and Hardangervidda respectively. The school area, perimeter, shape and position relative to the vessel, show that sandeel schools are relative large with an average horizontal area of 1225 m2. However, the sonar system showed detection limitations with respect to the smaller schools situated near the seabed. A comparisons made between sandeel schools detected with both acoustic systems, show that the relative size of smaller sandeel schools are more often underestimated in areal size compared with the sonar, while the larger schools are overestimated. The sonar survey show examples of potential missing of schools on echo sounder transects, and have therfore the potential for being a good tool in future two stage adaptive surveys.