Application of echo-trace analysis to estimation of behaviour and target strength of fish.

Abstract

The echo trace of a single fish is theoretically analyzed. The shape and echo level at each point on the trace are expressed as functions of the target strength, orientation, velocity of the fish, the directivity pattern of transducer, etc. From the theoretical results, we derive estimation methods of the fish target strength, swimming velocity and orientation. These methods are applicable not only to a single-beam echo sounder but also to a dual-beam and a split-beam echo sounders. When the swimming speed of a fish is faster enough than a ship speed or when the split-beam sounder is used, even the tilt angle dependence of the target strength can be observed. We developed a quantitative echo-sounder suitable for the echo-trace analysis. These make it possible to read echo levels and ranges at each point on echo traces. The accuracy and effectiveness of the methods were confirmed by analyzing the echoes of calibrated steel spheres. Field observations on fish were performed and the results were examined.