Detection of free-swimming fish passing through a narrow passage using MHz-band ultrasonic reflected and transmitted waves

Abstract

In this study, we evaluated the validity of detecting free-swimming fish using echoes from the fish, transmitted waves, and backwall echoes propagated through the fish, using a pair of transducers with MHz-band ultrasound in a small, shallow water tank. The increases in the amplitudes and the numbers of peaks in the echoes, the decreases in amplitudes and time-of-flight in the backwall echoes and the transmitted waves were observed for each event in which the fish either stayed or passed between two transducers. We found that the point at which the fish were irradiated with ultrasound had a major effect on the properties of the measured ultrasound. We demonstrated that free-swimming fish passing or staying between two transducers could be detected via the MHz-band ultrasonic echoes from the fish and from the backwall and from the waves transmitted through the fish.