Study on the Underwater Sound Environment in Kagoshima Bay

The underwater sound environment in shallow water area was complicated by time and depth under the influences of the biological and artificial noises from land and sea, e.g., trains, cars, feeding, cruising boats, etc. The purpose of this study is to clarify the underwater noise background and the underwater sound environment weighted by auditory sensation of fish in Kagoshima Bay. The emitted noise by drifting and cruising vessels has the largest influence on the underwater sound environment in shallow water area. There were recorded the vertical underwater noise by drifting vessel and approaching noise by cruising vessel. The transmission rays of underwater noise by drifting vessel have two directions, i.e., direct and reflected rays ; interference occurs while receiving the two direction sound waves with a hydrophone simultaneously. As an interference theory of underwater sound in shallow water was obtained as an equation with the range of transmission rays from drifting and cruising vessels to hydrophone for the reflection number. A comparison between theoretical and actual interference was made with a measurement of the ambient noise and traffic value in the heavy traffic area of Kagoshima Port. It was confirmed that the main noise was emitted by vessels, and the characteristic noise was temporal and sharply increased. In the biological noise, the underwater sounds measured in net cages at a culture ground were analyzed by the zerocrossing method to obtain the period change, power, and continuation time of swimming sounds by fish. The audiogram of a bastard halibut Paralichthys olivaceus was measured by a classical respiratory conditioning technique. The hearing response of bastard halibut was also measured by sweeping sound, and clarified the upper limit frequency of hearing. The lowest threshold of bastard halibut was at about 100 Hz and the threshold increased rapidly at 200-400 Hz. The ambient noises are mainly broadband and appears under 3 kHz of fine auditory sensation in fish. The hearing ability differs among fishes, thereby the underwater sound environment also differs. Arrangement was made on the frequency-weighted characteristics for auditory sensation of fish, fish weighting using these audiograms, in comparison with the underwater sound environment and underwater noise level of each fish. The fish weighting of bastard halibut is a type of low frequency with narrow band ; the underwater noise level is strongly affected by emphasis on the interference level of low frequency. The fish weighting of a Japanese horse mackerel Trachurus japonicus is a type of high frequency with wide band ; the underwater sound environment is influenced by the interference with wide band, but a small effect on variation of the underwater noise level