Human-Generated Sound and Marine Mammals
Filed under: Discovery,News & Resources
(Click to enlarge image) Mark Johnson, design engineer of the Dtag, holds a tag in its housing. (Credit: Todd Pusser)
Loud anthropogenic noises can alter the behavior of whales and other marine mammals, sometimes with fatal consequences.
(From Physicstoday.org / by Peter L. Tyack) — Most species of large whales are endangered because for centuries whaling fleets have decimated their populations. In the late 1960s, marine-mammal biologists discovered that fishermen setting nets for tuna in the Pacific Ocean were killing more than 100 000 dolphins a year. The cause of marine-mammal conservation became so popular at the dawn of the environmental movement that one of the first environmental accomplishments of the US Congress was to enact the Marine Mammal Protection Act of 1972, which prohibits the killing or injuring of marine mammals.
Today, small remnant populations of whales, such as the North Atlantic right whale, are threatened by entanglement in fishing gear and collisions by ships. Indeed, marine biologists have estimated that hundreds of thousands of marine mammals are killed each year in fishing gear.1 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. A few decades ago, conservation efforts focused on reducing the intentional hunting of marine mammals.
Nowadays, when hunts for marine mammals are better controlled, the slow degradation of habitat from a combination of sources may have a bigger impact. 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. 2 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).
Behavior modification
Biologist Roger Payne and ocean engineer Douglas Webb were the first to raise the alarm about the effect of sound on marine mammals.3 In 1971 they considered the then recently discovered low-frequency calls associated with the reproduction of baleen whales. Payne and Webb noted that in the preindustrial ocean those calls could have been heard about 280 km away, but the low-frequency propulsion noise of modern commercial ships had so elevated ambient noise in the sea that the detection range for whale calls could be as low as 90 km. In addition to that decrease, whale populations had been greatly reduced by whaling. Thus the average separation between vocalizing males and females may have increased at the same time as their range of communication was reduced. If noise interferes with breeding behavior, it could inhibit the recovery of depleted populations.
Despite reason for concern, decades passed with little work on how shipping noise affects whale communication. One problem was methodological. How could one study whether shipping noise was the reason that a whale did not detect a call emanating from 200 km away? It has taken marine biologists decades to develop methods to study effects of sounds from sources only a few kilometers away.
We still do not know how often shipping noise prevents a whale from detecting important signals. Recent work, however, has shown that marine mammals can compensate for noise, at least to a point, by increasing the level of their own calls, shifting their signals out of the noise band, making their signals longer or more redundant, or waiting to signal until noise is reduced. Many of those compensation mechanisms involve increased energy expenditure or other costs to the signaler. For the signaler to modify its behavior and accept that cost suggests a problem with the reduced range of communication caused by shipping noise.
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. 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. 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. (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 air guns used for the seismic surveys were so intense that the whales rarely came within 20 km of a survey vessel.
(Click to enlarge image) Figure 1. Harbor porpoises avoid loud noises. In these plots, the white area is a part of the Clayoquot Sound in British Columbia, Canada; black lines indicate porpoise tracks; and the red bits indicate float lines from which an electric pinger was suspended. (a) In this control run, the pingers are off. (b) When the pingers are on, they emit short noises with a sound pressure level of 145 dB relative to 1 µPa. The gap in tracks shows that porpoises avoid coming within a few hundred meters of the pingers. (Source: Mar Ecologgy Progress Series Vol.211:255-260,2001)
A small industry has sprung from the observation that marine mammals may avoid loud sounds. 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 devices do not always deter seals, which may even interpret their loud sound as a dinner bell. The noises may be more likely to repel more sensitive animals such as the harbor porpoise. 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. Figure 1 (Source: Mar Ecologgy Progress Series Vol.211:255-260,2001), 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. 4 The porpoises avoided coming within a few hundred meters of the pinger; that is, they shied away from SPLs greater than about 100 dB. For fisheries where porpoises entangle in nets and die, pingers can be attached to the nets to reduce the number of animals killed. On the other hand, loud acoustic harassment devices may prevent porpoises, whales, and other cetaceans from using large swatches of their preferred habitat.
