Whales living near the Galapagos Islands appear to have been exposed to higher levels of pollutants than those in other areas of the Pacific, say an international team of researchers.
The results are surprising given the Galapagos Islands - a UNESCO marine reserve - are considered pristine.
The research, published today in Environmental Health Perspectives, was part of a five-year voyage of the world's tropical and subtropical oceans which began in 1999.
Persistent organic pollutants (POPs), such as polycyclic aromatic hydrocarbons (PAHs) and the insecticide DDT, cannot be broken down. Instead they travel in air and water currents around the world, eventually finding their way into the food chain.
Marine mammals such as sperm whales are good indicators of ocean health because their long life-spans and high position in the food chain means their tissues accumulate these chemicals. Given females and juveniles tend to stay within a 1000-kilometre range, they can also provide an indicator of the presence of pollutants in different areas.
The researchers, led by Dr Celine Godard-Codding of Texas Tech University, took skin samples from 234 whales at five locations across the Pacific Ocean: the Gulf of California, the Galapagos Islands, waters between the Galapagos Islands and Kiribati (Pacific Crossing), Kiribati and Papua New Guinea.
They found that levels of an enzyme called CYP1A1, which is induced by POPs, were highest in whales found near the Galapagos Islands and lowest in those near Kiribati and the Pacific Crossing, furthest away from the continents.
This isn't the first time that such chemicals have been found in sperm whales, but it is the first to compare potential exposure in different areas, says Godard-Codding, an assistant professor in the University's Department of Environmental Toxicology.
Mixed messages from results
Age, gender, position in the food chain or whale migration could not account for the difference in CYP1A1, she says, adding the team was surprised by the findings.
"If we'd have had to guess we would have said that we might find higher levels in whales that had been close to industrialised countries, so perhaps whales from the Gulf of California. But everybody thinks of the Galapagos as a pristine area," says Godard-Codding.
But CYP1A1 levels did not appear to correlate with the levels of pollutants found in the whale tissue. Godard-Codding says this is probably because the samples that can be taken humanely were too small to adequately test for all the chemicals that induce CYP1A1.
She says it's also possible that natural sources of the pollutants, such as PAHs from volcanic eruptions, could account for some of the CYP1A1 present.
"We cannot know for sure that the whales from the Galapagos actually have higher levels of pollutants in their tissue. It's not that that's not the case, we just don't have the data to prove it."
Godard-Codding says the research has helped to identify the Galapagos as an area of interest and that further research should be done to look at the marine food chain there to see if it is more contaminated than other areas.
"You have to think about the ocean as the final sink for contaminants," she says. Given enough time, whatever pollutants are in the environment - from car exhausts, industry, agricultural practices - all of those contaminants end up in the ocean. No area of the world is pristine."