exploration for locating new oil and gas deposits run hundreds of thousands of miles of survey lines in the Gulf of Mexico alone each year. Commercial sonar systems are on all but the smallest pleasure craft and permit safer boating and shipping and more productive fishing. Military sonar systems are important for national defense. Ocean noise from human and natural sources can also originate in the air, as in sonic booms, lightning, and wind (NRC, 2003b).

The intentional and unintentional introduction of sound in the oceans associated with activities beneficial to humans has known deleterious effects on individual marine mammals. Mass strandings of beaked whales, defined as strandings involving two or more animals other than female-calf pairs (Geraci and Lounsbury, 1993), in some cases have clearly been associated with the use of midrange tactical military sonar (D’Amico, 1998; Evans and England, 2001; Jepson et al., 2003). Beluga whales (Delphinapterus leucas) have strong and prolonged behavioral responses to icebreakers 50 km away under some circumstances (LGL and Greeneridge, 1986; Cosens and Dueck, 1988; Finley et al., 1990). Gray whales (Eschrichtius robustus) and killer whales (Orcinus orca) have shown multiyear abandonment of critical habitats in response to anthropogenic noise (Bryant et al., 1984; Morton and Symonds, 2002). Although there are many documented, clearly discernible responses of marine mammals to anthropogenic sound, responses are typically subtle, consisting of shorter surfacings, shorter dives, fewer blows per surfacing, longer intervals between blows, ceasing or increasing vocalizations, shortening or lengthening vocalizations, and changing frequency or intensity of vocalizations. Some of those changes become statistically significant for a given exposure, such as increases in descent rate and increases or decreases in ascent rate of northern elephant seals (Mirounga angustriostris) in response to Acoustic Thermometry of the Ocean Climate (ATOC) signals (Costa et al., 2003). But it remains unknown when and how these changes translate into biologically significant effects—effects that have repercussions for the animal beyond the time of disturbance, effects on the animal’s ability to engage in essential activities, and effects that have potential consequences at the population level.

The basic goal of marine mammal conservation is to prevent human activities from harming marine mammal populations. The threat from commercial whaling was obvious, but it is more difficult to estimate the population consequences of activities that have less immediately dramatic outcomes, such as those with indirect or small but persistent effects. The life histories and habitat of marine mammals compound the difficulties.

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