ターゲットストレングス推定に必要なミナミマグロ(Thunnus maccoyii)幼魚の魚肉内音速測定

The ratio between the speed of sound in animals and that of the surrounding water (the sound-speed contrast), particularly for fish lacking any gas-filled organs or for zooplankton, is an important parameter in a theoretical scattering model for estimating target strength (TS). Since the swimbladder of a juvenile southern bluefin tuna (SBT) is not developed or gas-filled, the accurate and precise measurement of the sound-speed inside the body is required for TS estimation by the scattering model. The sound-speed of juvenile SBT tissue was measured using a FURUNO medical bone density-measuring instrument, the CM-100, during acoustic surveys conducted in Western Australia. This instrument measures the ultrasound travel time in tissues (and converts sound-speed to bone density). Since it has been recognized that sound-speed in tissues changes as time progresses and that it varies between fresh and defrosted tissues, the measurement should be taken immediately after the fish is caught. The sound-speed of juvenile SBT tissue ranged from 1610 to 1655 m/s and showed a tendency to depend on body temperature. On the assumption that the muscle temperature of a living SBT was 5 degrees higher than the surrounding water, the sound-speed contrast ranged from 1.06 to 1.09.