Fish finder, which use ultrasonic waves to locate fish in the sea, are widely used in commercial and recreational fisheries. Among fish finders, those that have the ability to quantitatively evaluate the characteristics of fish schools are called a quantitative echo sounder, and are used to estimate the amount of marine resources. The measurement results of a quantitative echo sounder are output as an image called an "echogram" (Figure 10), which shows the depth and strength of the echoes returned. By using sound waves of multiple frequencies together and analyzing the differences in each echo, an attempt is made to identify the species of fish and their sizes.
