Marine mammal auditory system noise impacts: evidence and incidence.

Sound is an inevitable element of every human activity in the oceans. Some, like exploration and military sonar exercises, produce impulse sounds that are intense but infrequent; others, like shipping, generate non-impulsive, less intense, but continuous noise. A recent National Research Council review (2003) found that the ocean's acoustic budget has increased by 3 dB, i.e., doubled, per decade in the last half century. In effect, in some ocean areas, and particularly along our fragile coasts, we are creating an environment akin to that of human workplaces. It is reasonable that we are concerned that any sound added to the marine environment may adversely impact a species within its "acoustic reach." Our concern for marine mammals is particularly acute because many species are endangered and hearing is arguably their primary sense. In fact, the important issues are multifactorial: what species will be exposed, for how long, to what frequencies, and at what levels, and then how do these parameters compare with an animal's hearing abilities. Only with all these factors in hand can we reliably determine the probability of adverse impacts affecting fitness or endangering populations. Before we can have a useful perspective, much less responsibly impose regulations and sanctions to prevent impacts from anthropogenic oceanic noise, it is first necessary to understand the susceptibility to noise damage in marine mammal stocks and the current status of their hearing. In the last decade, we have gained substantial ground in testing and documenting both normal and impaired hearing in some marine mammals, but we must still infer hearing characteristics for the majority of species. This paper summarizes our knowledge of sound impact mechanisms in land mammals, the current evidence for marine mammal hearing loss, and, finally, in the context of these data, the implications for when and how marine mammal ears may suffer noise damage.