In spite of what Jacques Cousteau has said, the oceans are a long way from being a "world of silence". The wind, tectonic plate movement, and whale song compose a complex web of sound. But for fifty years now, human activities have completely transformed this sound web. Biologists have coined the term "noise pollution" and are worried about what effects it may have on marine mammals. Why ? One reason is that the oceans have become very noisy, and the level of noise is on the increase. Another reason is that marine mammals depend on sound in order to find their bearings as well as to feed, reproduce, and socialise. First there was hunting, then habitat loss and chemical contamination. Does noise also represent a serious threat to whales ?

Shipping, the mining and petroleum industries, military activities, acoustic thermometry, and fisheries all contribute to the relentless increase in sound levels in the oceans.

With globalisation, the merchant fleet has doubled in size and quadrupled in tonnage. All of these ships (tankers, tug-boats, freighters, ice-breakers, etc.) fill every nook and cranny of the seas with a constant rumbling in the frequency band around 500 Hz.

Drilling activities are also important sources of low frequency noise. For example, the search for petrol requires the use of airgun arrays of , towed by a boat, which causes tens of thousands of explosions. In fact, from the initial search to the production and right up until the destruction of the oil installations, every single stage in the exploitation of the ocean depths adds to noise levels.

The American army and NATO also contribute to turning up the volume in the oceans. In order to detect submarines that have become very stealthy they have developed low frequency active sonar systems (LFA). These systems not only listen , they also produce powerful sonar beams (230 decibels at the source) which are propelled in a radius of hundreds of kilometres.

Low frequency sound travels a great distance and this characteristic has made it an interesting research tool. As the speed of sound is directly related to temperature, it is possible to evaluate the average temperature of the water by measuring the time sound takes to cover a known distance. Within the programme baptised ATOC (Acoustic Thermometry of Ocean Climate), American researchers placed two transmitters in the Pacific (one in California and the other in the Hawaiian Islands) and a dozen receivers in order to study climatic changes. For ten years, these transmitters will produce sounds of 195 decibels at regular intervals. The same group of researchers intend to place such transmitters in all of the oceans.

Finally, fisheries have also added to sound pollution by trying to prevent the accidental entanglement of marine mammals in fishing gear. Fishermen install sound "scarecrows" to keep whales, seals, and sea lions at a distance. The effect of this noise is relatively local in scale in comparison to the sources discussed above. Nevertheless, these sound beacons aim to create an effect on marine mammals and could have serious impacts on the use of critical habitats by coastal species such as the harbour porpoise.

The effects of noise pollution on whales depend, among other things, on the distance the whales are from the source of the noise. If the sound is very powerful and close to the animals it could well bring about permanent ear damage, internal injuries, and even death. Even sounds less powerful can induce temporary deafness, as was shown by studies carried out on seals, dolphins and belugas in captivity. Knowing just how dependent whales are on sounds, there is much cause for concern.

As well as physiological effects, sounds that derive from human activities can have an effect on cetaceans behaviour. Studies have shown that relatively powerful sounds can cause whales to deviate from their original trajectory. Chronic exposure could even force marine mammal populations to abandon habitats. When exposed to low frequency sounds, certain species of cetaceans stop vocalising for a few hours or even a few days. Moreover, even thousands of kilometres from sources of noise whales could suffer from the increase in background noise which could hide certain important sounds. This effect could be the clincher between detecting prey or not, escaping a predator or not, finding its group members or not. The impact of this form of pollution is all the more disturbing as the frequency bands used by whales are the exact ones experiencing the highest rise in the seas.

Unfortunately, there exists very little data to evaluate the real problems caused by noise pollution. Published studies mainly treat the short-term effects and raise many questions. What do these reactions really signify for the biology of the animals ? When there are no apparent reactions, are the animals actually out of danger ? And what about the rest of the ecosystem ? Could whales suffer from noise pollution via its impact on food sources ? Biologists have not yet found answers for these complex questions.

Long term monitoring of the impact of noise pollution on life in the seas in general, and on marine mammals in particular, is essential. International co-operation should also be fostered to help find solutions to questions raised and to create practical ways of applying standards aimed at reducing noise pollution. These are huge challenges when we consider the vast range of activities that bring about noise pollution in the oceans. In fact, this type of pollution is but one aspect of a larger problem, one of the concerns of the United Nations’ environmental programme: our growing use of the seas.

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"Whales and Sonar" of Natural Resources Defense Council (NRDC)

CNN Report: Making Waves.