Transducer positioning system with sights of multiple optical beams for target strength measurements of marine organisms

Information on target strength (TS) is indispensable to the acoustic estimation of the existing biomass of the important small oceanic organisms. These kinds of target are lower in echo level, and more complex in shape than fish. We constructed a TS measurement system considering these characteristics. A live target was suspended and fixed in seawater, and a transducer rotated by a step angle around the target horizontally. In particular it has been loaded horizontally and vertically with multiple optical beam devices to observe the sample aspects. TS measurements of Euphausia pacifica were performed in 2006 using a ultrasonic frequency of 200 kHz. A little inclination was observed in the horizontal line type beam. In vertical beams no inclination was observed. The error of TS values due to this inclination was permissible on a practical basis in comparison with the beam width of the ultrasonic transducer.