Acoustic characteristics of Japanese anchovy during sonar surveys

In quantitative scanning sonar surveys of Japanese anchovy Engraulis japonicus, accurate characterization of target strength (TS) is critical yet remains insufficiently resolved. Here, we combine in-port tethered measurements and a prolate spheroidal acoustic model to examine the horizontal TS behavior of Japanese anchovy. Experiments were performed at 28 kHz, with the transducer emitting pulses horizontally (0° depression) and at shallow downward angles (10° and 20°), while individual fish were rotated in the horizontal plane to sample yaw dependent backscattering. Measured TS patterns closely matched model predictions across all depression angles, confirming the model’s validity. Although yaw angle sensitivity persisted regardless of beam tilt, the main lobe of the TS directivity sharpened as the depression angle increased. Simulations further revealed that variations in pitch or intra school orientation had negligible effect on lateral TS, whereas yaw fluctuations significantly influenced lateral backscatter strength. These findings suggest that sonar surveys conducted with depression angles up to 20° maximize measurement consistency, and underscore the importance of developing techniques to characterize yaw angle distributions within anchovy aggregations.