For most marine mammal species, the data are inconsistent. For instance, some observers report that beluga whales are quite tolerant to noise from larger ships traveling near their summer feeding grounds in eastern Canada. But belugas scatter from smaller boat traffic. And another study showed a strong avoidance reaction to noise from ships and icebreakers when the whales were in shallow water or near an ice shelf.
Whales in the mysticete suborder, which includes gray, humpback, blue, fin, right, and bowhead whales, show consistent disturbance reactions to noise. They often flee from ships and boats, particularly from fast-moving vessels. When they hear high frequency sonar they also try to escape. And most gray and bowhead whales swim away from ships conducting sonic oil explorations.
Some researchers find the existing data unsatisfying. In most cases the studies don't include control groups or large sample sizes. Bill Watkins, a senior researcher at Woods Hole Oceanographic Institution in Massachusetts, says it will be difficult to understand the true impacts of ocean noise, "until we can design experiments that really ask the questions from the animal's perspective."
When marine animals first hear a loud, unusual sound, they usually swim away from it. But eventually, an animal could become accustomed to the noise. This seems to happen with fin whales, says Watkins. "Fin whales at sea will scatter very quickly from ship noises; and yet there are fin whales living in Boston harbor," he says. The whales in the heavily-trafficked harbor may have learned to ignore ship noises, in a phenomenon known as habituation.
Watkins recently began experiments that should help him hear the sounds a sperm whale hears, and to see how the animal responds to different sounds. Watkins and his colleagues strap a hydrophone listening device and satellite/radio tracking devices onto sperm whales. When they play different noises underwater, the hydrophone records the sounds as they reach the whale. The satellite and radio tags show how the whales' swimming patterns change when the sound is turned on. Watkins says his group hasn't collected enough data to draw conclusions.
The ATOC marine mammal project includes a similar tracking experiment, on elephant seals. Burney Le Boeuf, the long-time elephant seal expert at the UC-Santa Cruz, attaches recording and monitoring equipment to the two-ton behemoths while they are beached at Ano Nuevo, just north of Santa Cruz. Then he transports them by boat to an area near the ATOC source and releases them. Back in the lab, the researchers can monitor where and how fast the elephant seals swim, and how long they stay under during dives.
In addition to ATOC sounds, the hydrophone picks up the seals' breathing rate and heartbeat, which helps show them whether the loud noise causes a stress reaction. Preliminary results suggest that the deep-diving seals are not avoiding the loud ATOC source.
But some scientists have suggested that after centuries of evolution, certain noises are coded in an animal's brain to represent danger. Dave Kastak, a graduate student in Ron Schusterman's group, noticed that whenever he knocked on the side of the tank, Burnyce the elephant seal would "haul out" of the water and onto a nearby platform. The researchers designed an experiment to compare Burnyce's knock response with her response to an equally loud sound of varying pitch. They found that when she heard the varying-pitch sound, Burnyce only turned her head slightly; but when she heard knocking she hauled out. After repeated knockings, Burnyce became so disturbed that she refused to go back in the water, even when tempted with fish. To the seal, the knocking may sound like a danger signal, such as a killer whale call, Schusterman says; and perhaps the seal was trying to escape a perceived threat.
If specific sounds could scare an animal away from a food source, says Schusterman, then certain kinds of noise pollution could create a huge problem. If humans generate a steady, danger-coded noise near a food source, an animal might starve itself to avoid it. Another complicating factor: a sound that affects one species could be completely harmless to another.
Perhaps certain sounds can attract animals as well. The rumbling of a ship's engine might be what attracts dolphins to swim alongside it. But an attractive noise could be harmful too, Schusterman says. You may enjoy yourself at a rock concert, but if you hear enough loud music, you can experience a temporary hearing loss. Hearing researchers agree that it's likely that marine mammals also experience temporary hearing loss.
Temporary hearing loss could be quite harmful to marine mammals. A dolphin could become unable to use its echolocation system to hunt for food. A deafened elephant seal would have trouble hearing its pups' calls. Whales could become separated from their pods or have trouble hearing mating calls.
Because animals sometimes don't swim away from man-made noise sources, people too often assume the noise is not a problem, says Schusterman. "No response doesn't mean something isn't happening." If a large animal simply lifts and turns its head or gently changes course in response to a noise, that does not mean the animal isn't under stress, he says. Experiments like Watkins' and Le Boeuf's tracking studies should provide more of these details.