When there are fish in the sea, ultrasonic pulses transmitted into the sea from the echosounder will hit the fish and return to the vessel. To determine the abundance of fish using the acoustic method, the number of returned ultrasonic pulses should be counted. However, in the case of fish school, the ultrasonic pulses are often reflected from the entire school. In such a case, the number of fish in the school is estimated by assuming that if the school consists of three fish, ultrasonic waves with the scattering strength of three fish will be returned. In this case, it is necessary to know how much ultrasonic waves per fish are reflected.
This index is called target strength. In this experiment, we examine the target strength. We measure the target strength in the dorsal aspect as if we were using an echosounder and the target strength in the side aspect as if we were using a sonar.