Mid-frequency sound attenuation by dense fish schools

The propagation of sound waves through sea water is affected by fish schools, which act as obstacles scattering and attenuating incident sound waves, resulting in a loss of energy as the sound waves propagate through the water column. It is very difficult to measure the acoustic characteristics of fish schools because their sizes and positions vary significantly in natural conditions. The attenuation experiments were carried out in the artificial sea-cage at a coastal floating fish-farm of marine science station, which is located in the South Sea of Korea. The target fish species was the Japanese horse mackerel (Trachurus japonicus), which typically forms fish schools from summer to autumn in the coastal ocean. We carried out sound attenuation experiments for mid-frequency with a large number of fishes in a net cage. As a result, we observed that the signal attenuation varied with the number of individual fishes. The received level decreased as the number of individual fishes increased. This was mainly due to signal attenuation by the fishes at all frequencies. We applied statistical methods to analyze the signal attenuation by various free-swimming fish schools. Our results showed that the presence of the fish school affected the acoustic propagation by attenuating the sound waves.