海洋音響学会誌

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Acoustic techniques are commonly used to study the distributions and abundances of fish and zooplankton, because these techniques can survey large areas relatively quickly. Recently, acoustic-scattering techniques such as echosounders have been applied to jellyfish monitoring. Several studies have examined the acoustic-scattering characteristics of Nemopilema nomurai, but the influence of changes in the swimming (swimming angle) of this jellyfish on target strength (TS) has yet to be determined. This information has important implications for estimations of the abundance of jellyfish. In this study, the TS of live jellyfish in a seawater tank was measured to clarify the relationship between TS and swimming angle. Both measured and modeled scattering patterns depended on the swimming angle, revealing that the TS was largest for the head-aspect (incident on the bell of the jellyfish: 0 degree) swimming angle, and 10 dB lower for the intermediate-aspect swimming angle. The reduced target strength (RTS) in the side-aspect angle (range, 20–160º) varied between −60 and −50 dB; there was good agreement between the estimated and measured TS values.

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