Analysis of Echo-Sounder Records Acoustic Information of Fish Size

When attempt is made on the direct estimation of the number of commercial fishes, using wide-beam echo-sounder, coupled with a pulse counter, it is necessary to consider statistically the echo strengths from individual fishes. The echo strengths from a given target passing through the sound beams, varies with its directional angle from the acoustic axis; fishes of identical size will give all possible echo levels below the maximum on the axis. The apparent loss through fish reflection, which is determined by the maximum of a series of echoes from a fish, included two components, the actual reflection loss which varied with the third power of fish size, and the directivity loss on the angular distribution of fish. Actual reflection loss were only observed when a fish existed on the acoustic axis. In this paper, a method is described on the correction for angular distribution of fish and also for the distribution frequency of observed reflection loss of fishes which are the acoustic information of fish size. It describes also the determination of echo sampling volume and average number of pulses received from a fish.