Interdecadal trends in ocean ambient sound at seven sites in the northern Pacific Ocean basin

A long-term observation program begun in approximately 1994 has amassed time-series of ambient sound short-time spectra from omni-directional hydrophones deployed on the ocean floor at seven locations in the northern Pacific Ocean. Each time-series consists of spectra estimated every 5 min over a useful band of 10–500 Hz, thereby encompassing the vocalizations of baleen whales, the anthropogenic contribution of ship traffic noise, and wind/wave noise due to sea surface processes. Simple linear trend lines show that traffic noise in the northern and northwestern reaches of the Pacific has increased by as much as 3 to 4 dB during this program. In the eastern North Pacific, however, the ambient sound shows a decrease of about 3 dB. The number of ships in the world merchant fleet increased by approximately 25% over this period. This change is insufficient to explain the increases in noise levels along northern and northwestern regions, and provides no explanation for the decreases observed in the north-east. The traffic noise field is evidently dependent on more complex temporal and geographical patterns of shipping traffic. [Work supported by ONR.]