Horizontal target strength of the Japanese anchovy Engraulis japonicus for sonar surveys

The horizontal target strength (TS) of Japanese anchovies (Engraulis japonicus) remains poorly understood and inaccurately estimated for acoustic fish resource surveys using quantitative scanning sonars. In this study, the TS was investigated in a fishing port using a tethering method and estimated using a prolate spheroidal model. Measurements were conducted at a frequency of 28 kHz, with ultrasound emitted horizontally at 0° and at angles of 10° and 20° above the fish, while the fish were rotated in the horizontal plane. Consequently, the measured and estimated TS values demonstrated good agreement. At depression angles (tilt angle in sonar surveys) of 10° and 20°, the yaw angle characteristics of the TS showed no significant changes; however, the directivities of the TS became sharper as the depression angle increased. In addition, the average normalized maximum TS values obtained by measurements were −61.9, −62.3, and −60.8 dB at depression angles of 0°, 10°, and 20°, respectively. A simulation showed that the pitch angle distribution of fish and the orientation of individual fish within a school had a minimal impact on lateral TS, whereas the yaw angle had a significant effect. Results indicate that a shallow depression angle of 20° or less is optimal for resource surveys using sonar, and it is essential to develop a method for accurately determining the yaw angle distribution within a fish school.