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.