A multipurpose quantitative echo sounding system is developed. The sounder operates at two frequencies of 25 kHz and 100 kHz. The echoes by the two frequencies are effectively combined to give estimation techniques such as the quasi-dual-beam method, frequency difference observation of target strength and volume backscattering strength, and school contour observation. Further, the echo trace analysis and two-step echo integration method are applicable to give behaviour-associated target strength values and accurate abundance estimates, respectively. The designing is performed under the principle of the minimum error and high practicality. The paper recorder and color display can show absolute echo levels enabling direct reading of the target strength and volume backscattering strength in school. The echo integrator simultaneously integrates echoes in medium and large scale integration meshes. The integrator outputs are fed to personal computer to store data and draw abundance map online. We show several field applications using this system.
