Traces of pesticides that were likely never used in Alaska and have -- in some cases -- long been outlawed are showing up in some Alaska fish, new research shows.

A study led by the National Park Service found "historic-use" contaminants in fish at three Alaska parks famous for their wilderness qualities and reputations of being pristine and protected: Lake Clark National Park, Wrangell-St. Elias National Park and Katmai National Park.

The study, which also examined contaminant levels in five high-altitude national parks in the western United States, also found PCB concentrations, with loads in Alaska fish exceeding those in the Lower 48 sites. The Lower 48 parks in the study were Yosemite, Great Sand Dunes, Rocky Mountain, Lassen Volcanic and Sequoia and Kings Canyon.

The study was published in the April issue of the Journal of the American Water Resources Association. It reported results from several varieties of trout and some samples of Arctic char. It follows up on earlier research published in 2008 that evaluated contaminants in fish from other national parks, including Denali and Gates of the Arctic national parks and Noatak National Preserve in Alaska.

Though the precise source was not identified, it is clear that the contaminants are dropping into the parks from the atmosphere, said co-author Colleen Flanagan Pritz, a Colorado-based ecologist with the National Park Service's Air Resources Division.

The contaminants are carried from distant points on atmospheric currents, she said, then brought down to earth with precipitation that occurs when atmospheric moisture condenses at cold sites -- high altitudes and high latitudes.

"That is beyond our control, at present, anyway," she said. "The airshed has no boundaries."

Other vectors, especially applicable to Alaska, could be anadromous fish like salmon that migrate between fresh and saltwater and encounter contaminants far out in the ocean, accumulate them in their bodies and carry them to spawning sites, spreading the pollutants when their bodies decompose after they die.

Contaminants found in the fish studied in Alaska's parks tended to be dominated by the old products, while currently-used pesticides were more likely to show up in the fish in the Lower 48 parks, the study said.

Some of the contaminants found in the Alaska fish, like chlordanes and HCB, are "persistent organic pollutants" on a list of compounds banned by an international treaty, the Stockholm Convention on Persistent Organic Pollutants. The treaty was signed by the United States in 2001, but the U.S. Senate has yet to ratify it.

Though contaminant levels in fish from Alaska parks were generally higher than those from more southern parks, that is likely because the tested Alaska fish tended to be bigger, not because conditions are worse in Alaska, Flanagan Pritz said. "These bigger fish accumulate more contaminants over time because they are older," she said.

Transport of faraway pesticides into the northern environment has long been recognized as a problem. For example, mirex, an insecticide used in past decades to combat fire ants in the southeastern U.S., has accumulated in fish and animals in the far north and affected the indigenous people who rely on wild foods, according to the U.S. Department of Health and Human Services. The pesticide has been found in the blood of people in Alaska, Canada, Greenland, Norway, Sweden Finland, Iceland and Russia, according to the department.

PCB contamination has also been considered a persistent problem around the north, with some of the contamination coming from regional sources such as abandoned military sites. PCBs -- compounds of polychlorinated biphenyl -- were used for decades in electrical, industrial and commercial products. They were banned in the United States in 1979 and, along with mirex, were among the first 12 compounds banned under the Stockholm Convention.

The amounts of persistent organic pollutants in Alaska fish have not risen to levels warranting consumption limits, according to the Alaska Department of Health and Social Services' Epidemiology Section.

Still, pesticides and their migration into Alaska's ecosystem will warrant continued watch, said Alaska State Veterinarian Bob Gerlach.

"In many parts of the world they, unfortunately, are still being used," he said. "They're very easy to make and they may go unregulated in many countries." In some parts of the world, he added, DDT -- long banned in the U.S. -- is still considered necessary to combat malaria, spread by mosquitoes that have proved resistant to other pesticides.

"It would be a long time period before we see a lot of these pesticides go away, below levels where we could detect them," he said.

Meanwhile, the U.S. Geological Survey and National Park Service issued an updated report on mercury in fish from 21 national parks in the western United States and Alaska. Mercury is also transmitted around the world from a variety of sources, including industrial air-pollutant emissions.

The study did find elevated mercury levels in some fish from Wrangell-St. Elias and Lake Clark national parks in Alaska and Zion and Capital Reef national parks in Utah.