The amount of toxic mercury in Minnesota walleye and northern pike has been going up since the mid 1990s, the Minnesota Pollution Control Agency reported Tuesday.
The unexpected increase in mercury was found in an analysis of 25 years of fish from 825 Minnesota lakes by the PCA and published last week in the journal Environmental Science & Technology.
The increase surprised scientists because mercury levels in fish had been slowly but steadily declining in recent decades.
"It's surprising. We didn't expect to see it going up at all,'' said Bruce Monson, PCA scientists who discovered the trend.
While mercury levels in fish declined 37 percent from 1982 to 1992, they spiked back up 15 percent from 1996 to 2006, Monson said. Other studies that looked at the entire period didn't pick up on the recent increase.
Scientists say the mercury spike may come from a decade of global economic growth that saw a massive increase in coal-generated electricity in places like China and India. Emissions from those powerplants, including mercury, can circle the globe and fall into Minnesota lakes.
Monson said global climate change also may be increasing mercury levels as water levels show greater fluctuation through increased drought and flood cycles. That fluctuation can pull more methyl mercury of the environment.
Similar increases in mercury in fish were found in Lake Ontario from 1999-2003, PCA officials said, indicating the problem of increasing mercury is not limited to Minnesota.
State officials say they won't change fish consumption advisories for people who eat fish because the guidelines already are broad enough to be protective even at the higher mercury levels.
The news comes just a day after the Obama administration reversed years of U.S. policy and called for an international treaty on mercury, calling it the world's gravest chemical problem. Some 6,000 tons of mercury enter the environment each year, much of it settling in oceans and lakes, contaminating big fish like tuna, walleye and pike.
"The group of states that have been active in mercury emission reductions has been calling for an international agreement since 2003, so this is great news,'' said Ned Brooks, the PCA's mercury coordinator.
Mercury is a natural substance and is emitted into the atmosphere when many things are burned, especially coal at electric power plants. It also can come from burning products and garbage that contain mercury and naturally from volcanoes and evaporation from oceans.
Minnesota taconite plants also are a major source of mercury when ore pellets are hardened in giant furnaces.
Mercury falls back to the ground and in some cases becomes toxic, called methyl mercury, when it falls into waterways. That mercury then moves up the food chain from small organisms to fish and then animals that eat fish, including loons, otters and humans.
Exposure to high levels of mercury over a long period can cause severe neurological disorders, even death. Mercury also can cause major developmental problems in fetuses and small children.
The PCA and federal authorities have issued advisories for people to limit the number and size of fish meals they eat, especially women and children. Smaller fish usually have less mercury.
Minnesota has taken major steps to remove mercury form products and from incinerators. And last year the state became a national leader on the issue when it reached a landmark agreement with most major industries that emit mercury to lower their pollution to levels that would keep Minnesota fish safe to eat.
Minnesota industries agreeing to cut mercury over the next 20 years include powerplants, taconite plants, dentists and even funeral homes which emit mercury when humans with dental work are cremated.
But because more than half of the mercury that falls in Minnesota lakes is from outside the region, including as far away as China, national and even international laws are needed to curb the problem, state officials said.