1Hokkaido National Fisheries Research Institute, Kushiro, Hokkaido 085-0802 ,2Graduate School of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan
The attenuation of acoustic energy due to the effects of sailing were measured on the RV Kaiyo-maru No. 3 by integrating the area backscattering strength (Sa) of the sea bottom. The attenuation of the bottom Sa appeared with the increase of the ship speed. In particular, the attenuation was apparent under the head sea condition of sailing; maximum attenuation of 1.3dB was attained at 38kHz, and 0.7dB at 120kHz. On the other hand, no clear attenuation occurred under the conditions of a following sea. Under the head sea condition of sailing, the attenuation by sailing occurred at a slower sailing speed than the appearance of a missing echo or apparent attenuation shown on the echogram. That is, the attenuation by sailing exerted an influence upon the acoustic data without the operator being aware of it. When planning an acoustic survey by the RV Kaiyo-maru No. 3, or even in a survey field, setting the sailing direction in a following sea will prevent underestimation of the acoustic biomass index caused by the acoustic attenuation due to the effects of sailing.