London, England (CNN) -- Whales in Mexico's Gulf of California are showing worsening signs of sunburn according to new report published Wednesday.
Photos and skin samples gathered by scientists from the UK's Zoological Society of London (ZSL) and Mexico's Interdisciplinary Marine Science Center revealed blisters and changes in skin pigmentation in blue whales, fin whales and sperm whales.
The most badly-affected species was the blue whale -- which has paler skin -- and whales which spend most time on the surface exposing their skin to the sun.
"Whales need to come to the surface to breathe air, to socialize and to feed their young, meaning that they are frequently exposed to the full force of the sun," lead author Laura Martinez-Levasseur said in a statement.
Martinez-Levasseur says the rises in skin damage in the blue whale were "a matter of concern," but it isn't clear yet why they were happening.
"A likely candidate is rising UVR (ultraviolet radiation) as a result of either ozone depletion of a change in the level of cloud cover," Martinez-Levasseur said.
Ultra-violet levels in the Gulf of California generally remain high or very high on the UV index (the international standard measurement of the strength of the ultraviolet radiation) throughout the year.
Edel O'Toole, professor of molecular dermatology at Queen Mary, University of London and co-author said the changes in the whales' skin were "dramatic" and "significant."
"In the cells of the epidermis there were blisters which we could observe under the microscope, as well as the ones you can see on the skin. We also observed sunburn like you would see in humans," O'Toole told CNN.
The damage appears to be getting worse, but there is no evidence yet that whales are developing more skin cancers, O'Toole says.
Now they have established that exposure to strong sun is damaging to whales' skin, scientists will now look at the knock-on effects and monitor if the whales are able to respond to increasing radiation, and enhance their natural sun protection mechanisms.
The research, which was conducted between 2007 and 2009, appears online in the journal Proceedings of the Royal Society B.