The analysis of echotrace obtained by a split-beam echosounder to observe the tilt-angle dependence of fish target strength in situ

Abstract

A method of echotrace analysis (ETA), proposed by Furusawa and Miyanohana in 1988 utilizes successive echodata from an individual fish to obtain the target-strength (TS) pattern as a function of fish tilt angle and behaviour. The method can be applied to data from an echosounder with any beam configuration, single-, dual-, and split-beam. However, the method has not been applied rigorously to split-beam sounder data, which provide most comprehensive and accurate information. The aim of this work is to establish an accurate and practical ETA method for split-beam echosounder data. Although the basic theory had already been developed, some theoretical and practical enhancement was needed; smoothing of an echotrack by linear and quadratic regressions, correction for transducer motion, and comparison of an observed TS pattern with a prolate-spheroid theoretical model. The analysis starts from careful selection of a single echotrace on an echogram and ends with a composite display of results including a three-dimensional fish track and a TS pattern as a function of the tilt angle of the fish. The method was applied to typical echotrace examples, and utility and practicality were confirmed. A means of increasing the number of applicable echotraces is discussed. © 2009 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved.