Effects of Noise on Acoustic Signal Production in Marine Mammals

Abstract

Marine mammals rely on sound for communication, orientation, and locating prey. Baleen whales use low-frequency sound, to frequencies below 10 Hz, to communicate over ranges of tens to hundreds of km. Toothed whales use clicks at center frequencies of 10−160 kHz to echolocate on targets at ranges of tens to a few hundreds of meters. Most marine mammals have sensitive enough hearing that they are limited by noise rather than the sensitivity of their auditory systems. Ocean noise is dominated by sounds of geological activity below about 20 Hz, by wind and waves above 200 Hz, but in the 20−200 Hz band, the dominant source of sound in the sea stems from a human source: the propulsion of ships. Other industrial and military activities also introduce very powerful, transient sounds into the oceans. As noise varies, the effective range for communication and echolocation would vary significantly if marine mammals did not have mechanisms to compensate for increased noise. Marine mammals have been shown to be able to compensate for noise by increasing the level of their own calls, by shifting their signal frequencies out of a noise band, by making their signals longer or more redundant, or by waiting to signal until noise is reduced. The mechanisms that involve modifying vocal output based upon auditory input have similarities with vocal production learning, and compensation for noise may have led to adaptations that close the neural loop between auditory input and vocal production. All of these mechanisms improve detection of signals in noise, but each is likely to incur costs, and it is not known whether they fully compensate for the effects of noise. At some levels of anthropogenic noise, animals leave an area near the source, reducing the amount of habitat available. As anthropogenic sound continues to increase in the ocean, the requirement for suitable conditions for communication means that effects of noise are one of the factors that must be monitored and regulated to maintain suitable environments for marine mammals.