Ocean ambient noise from anthropogenic and natural sources in the context of marine mammal acoustics

Abstract

The effect of anthropogenic ambient noise on great whales is difficult to assess because of the limited knowledge of the way in which these whales used sound. Some idea of the impact can be obtained by comparing anthropogenic noise with the natural ambient noise, i.e., the background noise that whales have always experienced. Most measurements of ambient noise, however, have been made in the northern hemisphere where there is significant contamination from anthropogenic noise. Around Australia, there are many areas where anthropogenic noise is low or negligible because of the relative isolation of the continent, and this has allowed us to make reliable measurements of the natural ambient noise throughout the frequency range of relevance to marine mammals, and in areas frequented by vocalizing whales. The results show that there is more than 30-dB temporal variation in natural ambient noise, with 20 dB being common. Much of the variation is due to the contribution of biological sources, especially fish and whale choruses, and sea surface noise (wind-dependent noise). Natural ambient noise levels are often comparable to those of traffic noise (noise from distant shipping), and at times reach the high-traffic noise levels of the northern hemisphere.