Estimation of the temporal structure of the echo from the fish by using the broadband sonar signal of the dolphin.

Abstract

Dolphins can identify prey by using the broadband sonar signals. Behavioral experiments indicate that dolphins can detect and discriminate targets in highly cluttered and noisy environments (Au, (1993)]. The sonar system can be improved by clarifying the ability of the dolphin and this improved sonar system is useful for fish species identification. The echo from the fish contains components resulting from multiple reflections, for example, the swim-bladder and body surface of the fish. Therefore, it is necessary to determine the distance and intensity, corresponding to the reflectivity, of these reflectors. We analyzed the echoes from fishes in seas close to Japan by using the Hilbert transform. First, we analyzed echoes which were measured from three species of the anaesthetized fish in a water tank. It was clarified that both the numbers and intensity of reflectors and the duration of the echo were varied dependent on the species, the individual, and the tilt angle. Second, we analyzed the echo which was measured from the fish suspended by nylon monofilament line in the open sea. It was clarified that the numbers and intensity of reflectors were varied dependent on the species and movement of the fish.