The basic concept is to measure the faintest sound an animal can hear, then expose the animal to a noise stimulus and retest hearing. On the other hand, loud acoustic harassment devices may prevent porpoises, whales, and other cetaceans from using large swatches of their preferred habitat. For fisheries where porpoises entangle in nets and die, pingers can be attached to the nets to reduce the number of animals killed. The porpoises avoided coming within a few hundred meters of the pinger that is, they shied away from SPLs greater than about 100 dB. Figure 1, for example, shows tracks of harbor porpoises in British Columbia, Canada, observed in control conditions and when they were exposed to short, 145-dB sounds. Many studies of porpoises in the wild and in the lab show that the animals avoid low levels of sound exposure at much greater distances than do seals.

The noises may be more likely to repel more sensitive animals such as the harbor porpoise. The devices do not always deter seals, which may even interpret their loud sound as a dinner bell. Acoustic harassment devices are typically electronic sound sources designed to deter seals from catching farmed fish or to keep marine mammals away from such dangers as nets. The air guns used for the seismic surveys were so intense that the whales rarely came within 20 km of a survey vessel.Ī small industry has sprung from the observation that marine mammals may avoid loud sounds. (Henceforth, I’ll drop explicit mention of the 1 µPa reference pressure see the box on page 41 for a review of definitions and notations of sound levels.) Aerial observations of bowhead whales migrating past a seismic survey vessel showed that those whales also avoided exposures greater than about 120 dB. The whales, which were exposed to experimental playback of continuous industrial sounds such as those from ships or drill rigs, avoided sound pressure levels (SPLs) of 120 dB relative to 1 µPa. Some early experiments, motivated by concerns about the impact of offshore oil-industry activities, tracked migrating gray whales as they passed a sound source moored in the migration corridor off California. Since Payne and Webb’s paper was published, researchers have increasingly developed and applied methods to study whether exposure to sound disrupts the normal behavior of marine mammals. Noise in particular is at issue in legal actions that have been brought against the US Navy for sonar exercises that may have caused whales to strand and die (see Physics Today, February 2008, page 23). Leatherwood, eds., Academic Press, Orlando, FL (1984), p. Lafferty, in The Gray Whale, “Eschrichtius robustus,” M. For example, biologists have documented cases in which the effects of coastal development-including noise, pollution, and dredging-have caused marine mammals to abandon critical breeding habitat. Nowadays, when hunts for marine mammals are better controlled, the slow degradation of habitat from a combination of sources may have a bigger impact. A few decades ago, conservation efforts focused on reducing the intentional hunting of marine mammals. Inadvertent effects of human activities can pose a serious risk to coastal populations, as evidenced by the recent extinction of the Chinese river dolphin due to fishing, pollution, and overdevelopment of the Yangtze River. Indeed, marine biologists have estimated that hundreds of thousands of marine mammals are killed each year in fishing gear. Today, small remnant populations of whales, such as the North Atlantic right whale, are threatened by entanglement in fishing gear and collisions by ships.

Underwater sound reference detachment