Study on the characteristics of the target strength of fish

The characteristics of the target strength of fish, which are essential for the assessment of fish stock using acoustic means, are studied. In Chapter 2, a simultaneous calibration method which is developed for precise target strength measurements is described. The most characteristic feature of this method is a small omni-directional hydrophone, which is put between a projector and target. Since the calibration is carried out simultaneously with the measurements, this method can save not only time but also labor, and calibration errors are very small. In Chapter 3, the measured target strengths of copper spheres are compared with the calculated values, and a good agreement is obtained. This indicates the reliability of the above measuring method. In addition, suitability of three kinds of material is examined for the calibration spheres. A group of tungsten carbide with a 6% cobalt binder is proposed for calibration spheres. They can be used as echo-sounders operating at any frequency within the 10-200 kHz range under environmental conditions likely to be encountered. In Chapter 4, the dorsal aspect target strengths of the eight species of tethered fish with swimbladder are measured, and they are shown as the patterns for the tilt angle of fish. The results are graphically presented