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Abstract

This paper discusses the three-dimensional target strength (3DTS) of fish for a horizontal scanning sonar. Experiments were conducted in a tank using a defrosted Japanese mackerel. Measurements were made by changing the horizontal incident angle of a 50 kHz acoustic beam and the pitch angle of a tethered fish. Following the experiment, morphological characteristics, including the swimbladder size and shape, were measured using soft X-ray and digitized. The theoretical 3DTS was estimated using the prolate spheroid model and compared with the measured 3DTS. Results showed that the TS strongly depended on the fish orientation. The maximum and minimum TS were recorded at the broadside and the head/tail aspect of the fish, respectively. The horizontally averaged TS, which is necessary to estimate fish abundance by horizontal scanning sonar, gradually increased as the pitch angle increased, showing a minimum at 0° and maximum at 90°. The measured horizontally averaged TS was 3-5 dB lower than the theoretically estimated TS. Possible theoretical, biological, and technical reasons are discussed.

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