Abstract
Acoustic waves are attenuated by fish schools as they propagate through the ocean. The attenuation by fish schools is not currently considered in fishery acoustics and sonar applications, especially at mid-frequency bands. In this study, fish school attenuation experiments were conducted with a number of individual fish in situ in a net cage at mid-frequency bands (3–7 kHz). The target fish species was the Japanese horse mackerel (Trachurus japonicus), which typically forms fish schools in the coastal ocean of northeastern Asia. The attenuated acoustic waves were measured for the cases of non-net, only net (0), 100, 200, 300, 400, and 500 individual horse mackerels in the net cage. Results showed that the acoustic signal attenuation increased with the number of horse mackerels. The mean and maximum attenuation coefficients were approximately 6.0–15.4 dB/m and 6.5–21.8 dB/m for all frequencies, respectively. The measured attenuation coefficients were compared with the ones from previous studies to propose new regression models with normalized extinction cross-sections of weight and length of fish. This study confirmed that the fish school attenuation could not be ignored and compensated at mid-frequencies in the ocean. These results would be useful for fishery acoustics, especially in the development of scientific echo-sounder, and naval applications of sonar operations and analysis.
Highlights
This study aims to measure the sound attenuation by dense fish schools of Japanese horse mackerel at a mid-frequency range (3–7 kHz) to understand the sound attenuation characteristics of fish schools
For all 100–300 pings acquired in experiments, the received level (RL) were averaged for comparative analysis
This study proposes the characteristics of sound attenuation by fish schools of Japanese horse mackerel at mid-frequencies measured in situ net cages
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Sound propagation is influenced by the complex interference of reflection, refraction, transmission, and attenuation in ocean media [1]. Sonar systems when operating in marine environments are affected by sound propagation through physical parameters including the sound speed and density of seawater, geological parameters such as bottom topography and sediment types, and sea surface, which can cause reflections and scattering [1]. Prior studies have extensively covered the propagation of acoustic waves by physical and geological parameters, but few studies have explored biological parameters affecting sound propagation, such as fish schools [2]
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