The properties of echoes and transmitted waves from a fish body, in megahertz bands, were investigated to construct a fish detection method in a narrow path by a simple measurement structure using a pair of transducers. Experiments were performed in a water tank using dead Japanese jack mackerel (Trachurus japonicus). From the echoes, the outlines of fish could be observed. High amplitudes of echoes from the swim bladder and fins were observed, while relatively low amplitudes of echoes were observed from other body parts, e.g., the head, body surface, and vertebral columns. From the transmitted waves, decreased amplitudes and times-of-flight in waves transmitted from the fish body were observed. The fish could be detected using ultrasound, in megahertz bands, and the combined use of the echo and transmitted waves could increase the information obtained about fish compared with the use of echoes only.

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