Inter-calibration of participating vessels in the ICES coordinated surveys of North Sea herring

Summary: Three pairwise acoustic inter-calibrations of the performance of EK500 scientific echosounders were carried out in July 1997. The first inter-calibration was carried out at about 57 40'N by OE by GO Sars and Walther Herwig Ill on the morning of 1 July. Due to severe weather Tridens was unable to reach this location in time, and could not participate as planned. The inter-calibration was carried out throughout the entire survey day, during which no fishing took place. The second calibration carried out between Walther Herwig Ill and Dana after the completion of the first inter-calibration. During 2 July Walther Herwig Ill sailed eastward and contacted and Dana. The inter-calibration was carried out in the morning of 3 July at a position 57.45'N and 06 00'E about 30 Nm south west of the Norwegian coast. The third inter-calibration was carried out between GO Sars and Scotia on 16 July at about 60 45'N 30'W. The results of the calibrations have been examined, analysis includes careful matching of threshold values and an examination of the impact of weather dependent losses. The relationships between sequences of points in space are shown and as an approximate maximum likelihood regression are presented giving confidence intervals for the three calibrations. The results show excellent agreement between the four vessels involved