By Helen Briggs BBC News Online science reporter

They are the loudest love songs in the ocean, sung by the largest animals on Earth.

But scientists fear that the serenades of whales will disappear from the deep, drowned out by the noise of ships.

New research confirms that for the fin whale, and probably the blue, it is the males only which sing these long-distance ballads.

By towing microphones through the waters off the Mexican coast, biologists in the United States have been able to eavesdrop on the animals' distinctive low-frequency sounds.

The calls travel over thousands of kilometres to attract females.

Breeding grounds

It is potentially bad news for the denizens of the deep. Ambient noise levels in the sea are growing, due to increased commercial shipping, military sonar and seismic surveys.

The oceans of the world are becoming noisier and noisier

Vassili Papastavrou, International Fund for Animal Welfare

Conservationists warn that the manmade din could eventually reduce the distance over which the love songs can be heard, threatening the future of the animals.

Leading bioacoustics expert, Christopher Clark, says the sounds are very distinctive and can be detected throughout the world's oceans.

By tracking the whales and listening in on their chatter, his team has been able to confirm that they come from male fin whales - not females.

It supports the idea that the distinctive songs are a breeding display, designed to tell the females when it is time to mate.

Unlike other whales, such as the closely-related humpback, fin whales do not gather at specific breeding grounds.

Writing in the journal Nature, Dr Clark, of Cornell University in New York, US, said: "Our results help to focus growing concern over the effects of human-produced sound on Balaenoptera species (fin and blue whales).

He says an increase in ambient sound in the oceans could impede their recovery from past exploitation.

Noise pollution

Vassili Papastavrou, a whale biologist with the International Fund for Animal Welfare, says acoustic pollution is a largely unrecognised but serious problem.

Fin whale

Second largest animal, after blue whale Once abundant, it has been hard hit by hunting Found in North Atlantic, North Pacific and Southern Hemisphere

"The oceans of the world are becoming noisier and noisier," he told BBC News Online.

"Because whales live in a world that is dominated by sounds, they need to be able to distinguish natural sounds from manmade sounds.

"These natural sounds may be drowned out by some of the noise of ships' engines and military sonar.

"Male whales' ability to be heard by females is easily disrupted by the other sounds in the oceans."