Underwater cacophony caused by commercial and military ships has become so intense that it is killing whales, scientists at the World Conservation Congress here say.

Sounds ranging from the hum of yacht motors to sonar blasts strong enough to destroy a whale's inner ear are wreaking havoc on the ability of these cetaceans to migrate, feed and breed, they said on Thursday as a historic case began to be heard by the US Supreme Court.

"The noises generated by ships create what I call acoustic smog," said Michel Andre, director of the Laboratory of Applied Bio-Acoustics in Barcelona.

Just as air pollution reduces one's field of vision, "noise pollution in the sea reduces the zone in which whales can feed and hampers their ability to communicate," he told AFP in an interview.

"There is no place in the world's oceans that is untouched."

Many shipping lanes follow the coastal routes that whales have traced for millions of years as they roam the planet's seas.

The result is a crescendo of beachings, strandings and collisions as whales and other sea mammals disoriented or physically damaged by noise lose their bearings.

Recent research on a population of some 300 sperm whales living around the Canary Islands provide an unique window onto the problem.

Sperm whales normally migrate, but the squid upon which they feast are so plentiful in these waters that this group has made the region their permanent home, Andre said.

Maritime traffic, however, is taking a terrible toll -- since researchers began monitoring the area, six to 10 whales have been killed each year by collisions with ships.

"If we don't do something, in a few years there won't be any sperm whales left here," Andre said.

The consequences would extend beyond the loss of a whale population, he warned. "Each whale eats about one tonne of squid per day. All that uneaten squid would completely disrupt the food chain," he said.

Some forms of noise pollution are so powerful that "a whale can be killed outright by the shock," said Carl Gustav Landin, head of marine programmes for the International Union for the Conservation of Nature (IUCN).

Sonars used by the military and the oil industry can exceed 230 decibels in volume, and can be deadly within a one- or two-kilometer (-0.6 or 1.2-mile) radius, Andre said.

Eighty-five decibels -- the unit used to measure sound pressure -- can cause permanent damage to the human ear.

Research published in the United States last week shows that climate change is amplifying the problem.

The acidification of oceans caused by rising sea temperatures reduces sound absorption in the water by up to 40 percent, meaning that noise travels much further.

"Ambient noise levels in the ocean ... are set to increase significantly," the study, published in the Geophysical Research Letters, concluded.

A website created by Andre's team demonstrates the scope -- and volume -- of the problem (www.sonsdemar.eu).

The site features an interactive map showing the heavy maritime traffic around the Iberian peninsula, as well as the auditory footprint in red of each vessel in realtime.

By moving an icone representing a whale, one can hear the extent to which the sound the animal produces is masked by noise pollution.

Working in partnership with other researchers around the world, Andre will soon extend the map to cover the world.

The plight of the whales has come before the US Supreme Court.

On Thursday, some of the judges indicated they favoured slapping down a lower court ruling that curbs the use of powerful sonar in US Navy training exercises.

Even if the sonar harms the giant sea mammals, national security would likely take priority, some of the justices suggested.

Claudia McMurray, assistant secretary of state for oceans, environment and science and head of the US delegation at the congress in Barcelona, acknowledged it was hard to reconcile security and environmental interests.

"It is a delicate balance for us," she told AFP.

But Andre insisted solutions are available. "Technology exists that would allow military to continue their activities without putting the future of whales in peril," he said. "It is a shame this is not happening."