Acoustic Target Strength According to Different Growth Stages of Japanese Anchovy (Engraulis japonicus): A Comparison of Juvenile and Adult Fish

Japanese anchovy (Engraulis japonicus) is an economically important species found in the coastal waters of the northwestern Pacific Ocean. To conduct effective acoustic surveys on the spawning or fishing grounds of this species, the target strength (TS, dB re 1 m2) during its growth stages must be understood. However, knowledge regarding the TS of juvenile Japanese anchovies is limited. To address this knowledge gap, we measured the TS and proposed equation models for juvenile Japanese anchovies while describing the TS differences between young and adult anchovies. We evaluated 21 live anchovies using 38, 120, and 200 kHz transducers. TS models were fitted for the juvenile stage of anchovies, considering different frequencies. The mean TS values of young and adult anchovies were similar to those reported in previous TS studies, whereas the TS values of juvenile anchovies were significantly lower. Thus, applying the same TS equation based on the acoustic characteristics of anchovies at different growth stages, especially for juvenile anchovies, may not be appropriate. These findings underscore the importance of adopting a suitable TS equation specific to the growth stages of Japanese anchovies for accurate data analysis in acoustic surveys conducted on spawning and fishing grounds.