Under the Microscope: Noise pollution of the oceans poses an increasingly serious threat to marine life, particularly to animals such as whales and dolphins that depend on hearing to navigate, locate food and to communicate with each other.
In particular, a new sonar technology called low frequency active sonar (LFA) developed by the US navy may cause major disturbance and damage.
The navy has long been interested in detecting underwater threats. Sonar is the detection system of choice, using transmitted and reflected underwater sound waves to locate submerged objects. The word sonar is an acronym for SOund, NAvigation and Ranging. Sonar technology, such as used in the Cold War, was passive: big microphones listening for the distinctive sounds of submarines.
Since then the US Navy, and others, have used mid-frequency active sonar designed to find new stealth submarines by sending out sub-surface sound waves and listening for returning echoes. Modern submarines make little noise. The 362-feet long USS Miami nuclear submarine weighs 395 tonnes and is powered by 35,000 horsepower engines. It radiates little more sound than the hum of a kitchen fridge.
The US Navy now also uses LFA which emits intense bursts of low frequency sounds that can travel extremely long distances to echo off solid objects. Navy ships examine the echoes to see whether the objects are rocks, fish schools or submarines.
Sound cannot travel in a vacuum. It is a periodic mechanical vibration of a medium by means of which sound energy is carried through the medium. The frequency (pitch) of sound is determined by the rate at which the initiator of the sound (e.g. tuning fork) vibrates.
The LFA sonar beam sweeps like a floodlight across tens to hundreds of miles of ocean to reveal objects in its path. This power calls for extremely loud sound. The LFA system uses an array of loudspeakers and each can generate 215 decibels of sound - equivalent to the sound of a twin-engine fighter jet at takeoff. One hundred miles from an LFA system sound levels can approach 160 decibels, far beyond the Navy's safety limit of human exposure. Many humpback whales stop singing when exposed to LFA originating hundreds of miles away.
Clear evidence that active sonar can cause serious harm to marine life emerged in March 2000 when whales from four different species beached themselves in the Bahamas after the US Navy used active sonar in the area. Seven of the 13 stranded beaked whales died. Investigation showed that the whales were bleeding internally. Since that time the local population of Cuvier's beaked whales has almost vanished. They have either left or died at sea.
Further investigations have uncovered many more mass whale beachings associated with military activities and active sonar - Greece (1996), Madeira (2000), Canary Islands (1985, 1988, 1989, 2002), NW coast US (2003), and more.
Undersea noise from naval exercises seems to give beaked whales the bends, a condition commonly associated with divers who rise too quickly from the deep to the ocean surface. When humans, whales or dolphins dive and are exposed to high pressure, nitrogen gas is expelled from their lungs and into the bloodstream where it dissolves and saturates local tissues. The longer/deeper the dive the more this happens.
If you quickly open a new bottle of sparkling water you get a dramatic rush of bubbles. The same thing will happen if an animal returns quickly to the surface from a deep dive. In order to avoid this, humans and whales return relatively slowly to the surface, exhaling as they rise. If they don't take this precaution, the explosion of bubbles can rupture tissue or block blood vessels. This is called decompression sickness, or "the bends".
Researchers autopsied beaked whales that stranded themselves in the Canary Islands in September 2002 and reported signs of gas-bubble lesions in blood vessels and tissues, consistent with decompression sickness in humans. The work was reported in Nature, October 9, 2003. Beaked whales are among the longest and deepest-diving cetaceans and may be particularly susceptible to the bends. The sonar probably either prompts a behavioural change in the whales causing them to surface too rapidly or it allows bubbles to form more easily in the nitrogen-saturated tissues.
Apart from whales and dolphins, naval sonar can also have bad effects on many other species. Marine mammals and many fish species rely on sound to locate each other over great distances, to follow migratory routes and to feed and mind their young. Impairment of their hearing by bombardment with artificial noise can impair their capacity to function and, in the longer term, to survive.
The US Navy has been under pressure from environmental groups to curtail its LFA activities. A federal court declared in August 2003 that the navy's plan to employ LFA in 75 per cent of the world's oceans was illegal. The navy agreed to limit the use to a fraction of the area originally proposed. The navy is also to negotiate seasonal exclusions and geographical limits in an effort to protect critical habitats and whale migrations. William Reville is associate professor of biochemistry and director of microscopy at University College Cork
• William Reville is associate professor of biochemistry and director of microscopy at University College Cork