1Graduate School of Fisheries Sciences, Hokkaido University, Hokkaido 041-8611, Japan, 2College of Fisheries and Ocean Science, Yosu National University, Yosu 550-749, Korea
The purpose of this study was to estimate the acoustic target strength (TS) of live Nomura's jellyfish Nemopilema nomurai found in the Yellow Sea and the Sea of Japan. The experiments were conducted at Yosu National University, Korea. Seven jellyfish were collected from coastal waters in southern Korea by SCUBA diving and from set-nets. Individual live jellyfish were suspended at the center of a seawater tank (5 m×5 m×5 m) tethered by thin fishing line, and their behavior was monitored by underwater cameras. The TS of each jellyfish was measured at 38 kHz (split-beam system) and 200 kHz (dual-beam system).
The mean TS values ranged from −79.0 to −47.5 dB at 38 kHz and from −72.2 to −46.0 dB at 200 kHz, and tended to be higher at 200 kHz than at 38 kHz by 1.5 to 10.6 dB. The TS values of similar sized jellyfish were approximately 20 dB lower for those collected by SCUBA diving than those collected in the set-nets. The TS values also varied depending on the form change of the jellyfish.