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."

Others disagree, arguing the human addition to natural ocean noise is vestigial, being localised to certain busy spots, and falling away rapidly only metres from the source as the water blankets it. While it is possible to deafen fish in a tank, it is very difficult to prove that changes in the health or behaviour of wild whales, seals or fish are due to sound alone. Indeed, the fine details of their behaviour patterns are extraordinarily difficult to monitor and interpret, so who really knows? In short, they claim, ocean noise is just another overheated issue generated by environmentalists and fed by a media frenzy based on too much emotion and too little data.

We know sound can travel both fast and far in the ocean - but for many reasons it does not travel evenly. "In the sea, noise drops off very quickly with distance from the source, then it drops off much more slowly," said Rob McCauley, a marine biologist at Perth's Curtin University of Technology who has studied ocean bioacoustics for the past 15 years.

"It's a logarithmic decay. High-frequency sound attenuates very rapidly because the water literally absorbs it. But low-frequency sound in the range 10 to 200 Hertz can travel a long, long way," he said.

To make the picture more complicated, shallow waters absorb sound both at the sea-bottom and at the surface - so they are, potentially, less 'noisy' than deep waters.

The oceans also contain a structure that allows low-frequency noise to propagate along a 'sound channel', about 1,000 metres down, through which it can travel for thousands of kilometres, all over the globe. It is this sound channel, says McCauley, that has given rise to a misconception that all noise in the ocean travels all round the world.

"That's not true," said McCauley. "There is certainly a lot of manmade noise in the busier parts of the Atlantic, but it rapidly decays to background levels and we are not seeing its effects in waters around Australia. There's a lot of noise here but it is mostly biological, made by whales and so on."

However, he adds, there are still local problem areas for noise - including busy ports and Australia's northwestern coast where ship-borne air-guns blast away in the seismic quest for oil and gas.

And some species are more susceptible than others: toothed whales and dolphins, for example, are sensitive to high-frequency sound, whereas baleen whales operate at lower frequencies. Seismic survey ships "act like a giant plough", McCauley says, scaring away sea life with each blast. "But they tend to come back afterwards. Permanent displacement of sea life due to noise has been claimed, but there's not much evidence for it."

However McCauley has firsthand proof that seismic blasts do affect snapper. In a behavioural experiment in Western Australia's Cockburn Sound, he exposed a cage of pink snapper to shots from a seismic air-gun over a distance of 500 metres and less. At each shot, the snapper bunched together, circled wildly and dived for cover at the bottom of the cage. But when McCauley repeated the experiment 58 days later, he was surprised to see the fish didn't react at all.

"When we came to look at their ears, the sensory part was a mess," he said. "We concluded they were stone deaf, and that any fish in the wild that suffered such an injury would be dead, eaten by sharks or whatever."

Michael Smith, a biologist at the University of Maryland, has likewise demonstrated noise-induced stress and deafness in goldfish, a species that tends to rely on its hearing. And Sonja Amoser, a research associate at the University of Vienna, has shown that catfish, which communicate vocally and rely on sound even more than goldfish, suffered longer hearing loss.

There is mounting evidence that noise can cause sea life to change behaviour. Canadian scientists have shown that killer whales are deterred by 'acoustic harassment' - loud alarms set off to keep them away from salmon farms. Californian researchers found that low-frequency sound can change the diving behaviour of elephant seals. And a U.S. Navy study shows that humpback whales change their song when obliged to compete with military low-frequency sonar.

But research into floating hotels on the Great Barrier Reef suggests some sounds may actually attract sea larvae - and hence the larger fish and animals that feed on them, says McCauley. Amusingly, fish also seem better able to regulate their noise emissions than people: his early research revealed that the deafening choruses of certain fish on the Barrier Reef were timed or located in such a way as not to interfere with the singing of other sea species. Ocean noise pollution is a rising issue in environmental and conservation circles. The seminal report, Sounding the Depths: Supertankers, Sonar and the Rise of Undersea Noise, was published in New York in 1999 by the Natural Resources Defence Council.

However before this came a number of fiery spats, including a rather unedifying one between physicists and biologists over a global experiment in transmitting acoustic waves through the deep ocean as a way of measuring global warming, known as ATOC (Acoustic Thermometry of Ocean Climate). The physicists argued their low-frequency sound waves were harmless; the marine biologists contended they would interfere with one of the main communication channels used by whales, in effect scrambling their long-distance conversations.

Scenting another rising environmental issue, green groups have been quick to take up the cause of ocean noise. At its meeting in Bangkok, Thailand in November 2004, the World Conservation Union adopted a resolution that declared noise was a form of ocean pollution. That same month, a U.N. Environment Program meeting of 16 countries formed to protect whales and dolphins in the Mediterranean, Black Sea and Atlantic adopted a resolution acknowledging manmade ocean noise as "a dangerous pollutant which can disturb, injure and even kill whales and other marine species." Groups have sprung up, like the North American Ocean Noise Coalition and the European Coalition for Silent Oceans (ECSO), which are all are calling for sea sanctuaries where all human activity is banned.

Whether sound can cause whale or fish deaths at sea remains debatable. The Whale and Dolphin Conservation Society report says "evidence of the lethal nature of noise for marine mammals comes from recent strandings associated with the use of military sonar, such as the mass stranding of beaked whales in the Bahamas in March 2000".

The ECSO adds "a series of mass mortalities of cetaceans [was] associated with the use of mid-frequency active sonar in coastal environments." It said the best-documented cases were in the Bahamas in the Caribbean and Madeira off the southwestern coast of Portugal in 2000, and the Canary Islands off the coast of North Africa in 2002. Other incidents have occurred in Greece in 1996, the U.S. Virgin Islands in 1998 and 1999, the Canary Islands again - in 1985, 1986 and 1989 - and, most recently, the northwest coast of the United States in 2003. "However, the magnitude of the problem is not known as evidence indicates whales may die at sea where carcasses sink," the report stated. "The proliferation of active sonar poses a threat to already depleted fish stocks throughout the world's oceans."

The Royal Australian Navy, for one, isn't taking chances. When it discovered the Perth Canyon - the nearest deepwater exercise area to the submarine base on Garden Island in Western Australia - was also a favoured haunt of pygmy blue whales, the navy asked a team of local biologists to investigate. Their research found that whale visits to the canyon were seasonal, coinciding with a flush of krill from February to May every year. The navy tactfully decided to yield right of way to the whales, moving its exercises.

French biologist Michel André, who chairs the European Cetacean Society, considers sound can kill sea life in various ways. André investigated the rising carnage in the Canaries as fast ferries crossing between the main islands 25 times a day collided with sperm whales - killing them and, sometimes, even people. His study of the dead whale's ears led him to conclude the whole population had been partially deafened by noise pollution.

"The more noise we put in the ocean, the more we affect the way the animals that live there communicate and orientate," André said. "And this means that we are compromising their survival. We're compromising the way they look for their food. We're compromising the way they breed.

"Nowadays there is no region anywhere that's unaffected by noise pollution. Due to its low-frequency components, a sound source may propagate for thousands of miles. The control of these sources constitutes a scientific challenge and involves an important responsibility for society and governments."

Amid the rumble of claim and counter-claim one fact stands out: there is no law - international or otherwise - that controls noise pollution at sea. So far, it has mainly been the strength of public and scientific opinion that has induced oil explorers, ship owners and navies to investigate the possible side effects of their noisy activities on marine life.

Some governments are erring on the side of caution. Australia's Department of the Environment and Heritage classifies acoustic disturbance as an 'intermediate threat' to whale populations: "Any human activities which produce loud and persistent sounds under water are likely to interfere seriously with the acoustic perception and communication of any cetaceans in the vicinity, and have the potential to induce significant levels of stress ... incessant or repeated acoustic disturbance could cause abandonment of important habitats such as narrow migration paths and calving and nursery sites".

Meanwhile, the World Conservation Union has urged governments to apply the precautionary principle to marine noise pollution, to investigate mass strandings that may be associated with intense manmade noise, and to encourage the development of less noisy alternative technologies. They also say powerful noise sources should be controlled until their effects are better understood, should be avoided in sensitive areas, and should be subject to restriction under marine and coastal management guidelines.

"Regulating sound sources can be difficult, but one has to start somewhere," says Sylvia Earle. "Every breath we take is dependent on the ocean. Unless we really understand how that vast system works and take better care of it, it isn't just the ocean that's in jeopardy; it's our whole future that's at stake."

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