Characteristics of Sound Attenuation by Individual and Multiple Fishes

Fish biomass and stock assessment are estimated from acoustic volume backscattering strengths (Sv) obtained from various hydroacoustic equipment. Although sound attenuation due to fish schools and water influences the Sv value, only attenuation from water is considered during the acoustic data process. For these reasons, it is necessary to understand the characteristics of sound attenuation by fish. Unfortunately, little is known about sound attenuation from fish. In the present study, the attenuation from one to four fish specimens was precisely measured during ex situ experiments in a water tank. The scientific echo sounder of a split-beam 200 kHz transducer and a miniature hydrophone were used for the attenuation measurements. Results show that the maximum attenuation coefficient (alpha) was approximately 25 dB/cm when the 4 multiple fishes had high fish heights. The relationship between the attenuation coefficient (alpha(200kHz)) and the total fish height (H-total) was approximately alpha(200kHz) = 0.54 * H-total +/- 0.06 (r(2) = 0.72). This work describes the sound attenuation characteristics to provide basic information for the compensation of Sv from fish schools or layers.

Automated summary

Fish biomass and stock assessment require consideration of sound attenuation caused by both water and fish. However, limited knowledge is available on fish-induced acoustic attenuation. This study measured the acoustic attenuation from one to four fish specimens in a water tank and found that the maximum attenuation coefficient (alpha) was approximately 25 dB/cm when multiple high fish heights were present. The analysis showed a linear relationship between the attenuation coefficient (alpha(200kHz)) and the total fish height (H-total). This research provides basic information on the sound attenuation characteristics of fish, which can be used for compensating Sv from fish schools or layers.