Development of an acoustic-optical system to estimate target-strengths and tilt angles from fish aggregations

The J-QUEST instrument package, a quantitative 70 kHz split-beam echo sounder and a stereo TV camera, is being developed to observe mesopelagic fish. In situ acoustic and optical measurements of single fish within aggregations include swimming speed, target strength, length, and tilt angle. Acoustic data and two video signals are monitored in real-time via an optical-power composite cable and receivers on deck. The J-QUEST was deployed from the R/V Kaiyo-maru in the transient waters of the Kuroshio current from Kashima-nada to off the coast of Sanriku in the east of Japan. J-QUEST was tethered from the surface to a depth of 200 m and worked well. In Kashima-nada at a depth of 10 m, Pacific saury (Cololabis saira) were attracted to the underwater lights and formed a large aggregation around J-QUEST. We could monitor fish on both the TV monitors and on the echogram. Average target strength of Pacific saury was -37.9 dB and normalized TS (by squared fork length) was -66.8 dB