1Department of Marine Science and Technology, Tokyo University of Fisheries, Minato, Tokyo 108-8477,2Furuno Electric Co., Ltd. Nishinomiya, Hyogo, 662-8580, Japan
Since a quantitative echo sounder uses a sharp and vertical beam and fish near the sea surface avoid approaching the vessel, accurate abundance measurement of surface fish schools can not be achieved by the quantitative echo sounder. A quantitative horizontal scanning sonar is expected to solve this problem. As a first step, this study investigates a quantitative commercial fisheries scanning sonar to measure the volume backscattering strength (SV) of surface fish schools. A sonar system for quantitative use is constructed and the receiver output signals are A/D converted and fed to a personal computer. The transmitting and receiving coefficients of the sonar are calibrated successfully by using a standard sphere. Using this system, experimental observations of surface fish schools are conducted in Tokyo Bay. The raw digital output signals are recorded and analyzed to track the echoes of surface fish schools and to calculate SV of the surface fish schools. Introducing data from a gyrocompass and a GPS, the distribution of SV is exhibited synoptically at absolute position in the surveyed area. Through this study, accurate information regarding surface fish schools by the scanning sonar is found to be effective, and the possibility of making a quantitative sonar is confirmed.