Noise in the Ocean

To a human observer, the sea appears to be a place of silence. Yet beneath the surface the ocean is filled with sound, and for the animals that live there hearing is often far more important than sight. Light fades quickly with depth, and even in clear water it is impossible to see very far, so many marine creatures rely on sound to find food, to avoid danger, to navigate and to communicate with one another. Sound has a great advantage in water: it travels much faster and much farther than it does in air, so a signal produced in one place can be detected a long way off. This makes the underwater world a rich acoustic environment, one that has been shaped over millions of years and that many animals depend upon completely.

The natural sounds of the ocean come from many sources. Waves, rain and the cracking of ice all add to the background, as do the animals themselves. Some fish grunt, click or drum; shrimps snap their claws in vast, crackling choruses; and the largest whales produce low, powerful calls that can travel across entire ocean basins. For these whales, sound is a lifeline. Because they may need to find one another across enormous distances, they use deep, far-carrying calls to stay in contact, and biologists believe some of their songs serve to attract mates or to coordinate movements over great stretches of sea.

Into this ancient soundscape, humans have introduced a great deal of new noise. The most widespread source is shipping. The propellers and engines of the world's cargo vessels generate a continuous low rumble that now fills much of the ocean, and as global trade has grown, so has the volume of this background hum. Other human activities add sharper, more intense bursts of sound. Surveys that search for oil and gas beneath the seabed often use powerful blasts of compressed air, repeated for hours or days at a time, to probe the rock below. Certain kinds of sonar, used for navigation and detection, send out loud pulses that can carry for many kilometres.

The concern is that this human-made noise interferes with the sounds animals rely on. When background noise rises, quieter signals can be drowned out, a problem scientists call masking. A whale trying to reach a distant mate, or a fish listening for the approach of a predator, may simply be unable to hear what it needs to hear over the rumble of passing ships. In effect, the useful range of an animal's calls shrinks as the ocean grows louder, so that signals which once travelled for many kilometres may now be lost within a much shorter distance.

Loud, sudden noises can have more direct effects. Studies have recorded animals fleeing an area when intense sound sources are switched on, abandoning feeding grounds or migration routes they would otherwise use. In some cases the disturbance appears to interrupt important activities such as feeding or breeding. Researchers have also linked certain very loud events to injury in marine mammals, although establishing a clear chain of cause and effect in the open ocean is extremely difficult, and much about the long-term consequences of noise remains uncertain.

Studying the problem is challenging precisely because the ocean is so vast and so hard to observe. Scientists use underwater microphones, moored in place for months at a time, to record the changing soundscape and to track both natural and human sources. By comparing recordings from busy shipping lanes with those from quieter regions, they can begin to measure how much the noise has increased and how animals respond to it. Such work has revealed that the din is not spread evenly; it is concentrated along trade routes and near industrial activity, leaving some areas far noisier than others.

Unlike many forms of pollution, noise has one hopeful feature: it does not linger. When a noisy ship passes or a survey ends, the added sound stops almost at once, and the natural soundscape can return. This means that measures to reduce underwater noise could bring benefits relatively quickly. Engineers are designing quieter propellers and hull shapes, and some authorities have encouraged ships to slow down or to avoid sensitive areas at important times of year. Whether such steps will keep pace with the steady growth of ocean traffic is not yet clear, but the recognition that sound itself can be a form of pollution marks an important change in how the health of the sea is understood.