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It's not just holidaymakers who need to keep covered up in the sun.

Scientists have discovered that a growing number of whales are suffering from sunburn - and that the hole in the ozone layer could be to blame.

The findings come from a study of 156 blue, fin and sperm whales swimming in the Gulf of California.

Researchers have long known that whales suffer blisters on their backs. However the reason why they have lesions has been a mystery - until now.

After analysing the sores on high resolution photographs and studying whale skin samples in the laboratory, a team of British and Mexican researchers now believe they are caused by acute sunburn.

Dr Laura Martinez–Levasseur, of the Zoological Society of London, said: 'Whales need to come to the surface to breathe air, to socialise and to feed their young, meaning that they are frequently exposed to the full force of the sun.'

Blue whales, who have fairer skin than most whales, suffered more than the darker fin whales, she reports in the journal Proceedings of the Royal Society B.

Sperm whales, which spend more time than other species at the surface, were also vulnerable.

The increase in sunburn appears to be worst for blue whales, the study found. In 2007, just six out of 48 blue whales had sunburn. By 2009, 15 out of 22 whales studied were suffering.

'The increase in skin damage seen in blue whales is a matter of concern, but at this stage it is not clear what is causing this increase,' Dr Martinez-Levasseur said.

'A likely candidate is rising ultraviolet radiation as a result of either ozone depletion, or a change in the level of cloud cover.'

Her co-author Professor Edel O'Toole, from Queen Mary, said: “As we would expect to see in humans, the whale species that spent more ‘time in the sun’ suffered greater sun damage.

'We predict that whales will experience more severe sun damage if ultraviolet radiation continues to increase.”

The researchers are unsure whether the lesions are painful - and whether sunburn increases the risk of skin cancer in whales.

One reason for the rise in sunburn could be the hole in the ozone layer - the protective layer that shields the Earth from the sun's harmful ultraviolet rays.

Since the 1970s, scientists have known that the protective layer is being depleted by manmade gases, in particularly the CFCs once used in aerosols.

Although CFCs have been banned, the ozone layer is still being depleted from gases released many years ago, and holes exist above both poles.

(Read by Lee Hannon. Lee Hannon is a journalist at the China Daily Web site.)

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(Agencies)