A humpback whale (Megaptera novaeangliae), one of the many cetaceans relying on sound across vast distances.
Credit: Louis Herman/NOAA
Descend 150 metres or so into the ocean and objects that are a mere arm's length away take on fuzzy outlines. Go deeper still and your hands appear like shadow puppets as the light begins to fade. At 1,000 metres you enter endless night, where eyes are of limited use and hearing is everything. Here, noise moves five times faster and infinitely farther than on land. The oceans are the true realm of sound.
Most sea creatures, from whales and dolphins to fish, sharks, squid, shrimps and possibly even anemones and jellyfish respond to sound and many can produce it. They use it to hunt and to hide, find mates and food, form and guide shoals, navigate 'blind', send messages and transmit warnings, establish territories, warn off competitors, stun prey, deceive predators, 'illuminate' their surrounds acoustically, avoid obstacles and sense changes in water and conditions.
Marine animals click bones and grind teeth; use drum-tight bladders and special sonic organs to chirp, grunt, sing and boom; belch gases and liquids; and vibrate special organs or their entire bodies. They gather to form great choirs. Sounds emitted by sea creatures span the range from 0.1 Hertz to 300 kiloHertz, says Michael Stocker, consultant acoustician and naturalist. Far from the 'silent deep', the oceans are a raucous babel.
Into this age-long tumult, in the blink of an evolutionary eye, has entered a new thunder: the throb of mighty engines and the thrash of propellers as 46,220 large vessels displacing 600 million tonnes of water plough the world's shipping lanes. The hammer of diesels and the scream of outboards reverberate through the waters as 4 million fishing boats and more than 10 million ferries and pleasure craft surge to and fro. Then there's the thump and ping of military and fishing sonars; the deafening crash of seismic ships seeking oil; blare of acoustic harassment devices; grinding of drills and dredges; and low-frequency growl of scientific experiments designed to detect global warming.
Within barely a century, the long-silent oceans have been filled with a cacophony of human origin. Scientists say that background noise in the ocean has increased roughly 15 decibels in the past 50 years. It may not sound like much in overall terms; but it is enough, according to many marine biologists, to mask the normal sounds of ocean life going about its business.
At its most intense, some even say noise causes whales to become disoriented and die, dolphins to develop 'the bends', fish to go deaf, flee their breeding grounds or fail to form shoals - enough to disrupt the basic biology of two thirds of the planet.
"Undersea noise pollution is like the death of a thousand cuts," says Sylvia Earle, former astronaut and chief scientist of the U.S. National Oceanic and Atmospheric Administration. "Each sound in itself may not be a matter of critical concern, but taken all together, the noise from shipping, seismic surveys, and military activity is creating a totally different environment than existed even 50 years ago. That high level of noise is bound to have a hard, sweeping impact on life in the sea."
Sarah Dolman of the British-based Whale and Dolphin Conservation Society agrees. "Today the seas are a far noisier place than they were a few decades ago." The society's recently published report, Oceans of Noise, states: "There is increasing concern amongst scientists and conservationists that noise pollution poses a significant and, at worst, lethal threat to whales and dolphins and other marine wildlife. A deaf whale can be expected to be a dead whale."
