The concentration of mercury in walleye and northern pike has shown an unexpected long-term rise in lakes throughout Minnesota, and scientists believe the problem has global causes and consequences.
A Minnesota Pollution Control Agency study released Tuesday after being published last week in the journal Environmental Science & Technology showed an unexpected rise in mercury concentrations in fish collected from 845 Minnesota lakes. The finding, from an analysis of records kept over 25 years, is a concern because methylmercury, the form of mercury that contaminates fish, is toxic to humans and wildlife.
The new finding, however, won't affect current fish-consumption guidelines from the Minnesota Department of Natural Resources. Those guidelines are based on current data about mercury levels in fish, while the MPCA study showing the rise in those levels is a historic look at the data.
Under the Minnesota Deptartment of Health guidelines, men and women not planning to get pregnant can safely eat one meal a week of Minnesota-caught walleye and northern pike. Pregnant women, those who may become pregnant and children younger than 15 should eat only one meal a month of walleye shorter than 20 inches and northern pike shorter than 30 inches from most Minnesota lakes. It's recommended they don't eat fish larger than that.
A change in direction
At one time, methylmercury concentrations in fish were decreasing in Minnesota. Those levels in northern pike and walleye fell 37 percent from 1982 to 1992, after the state began limiting the use of mercury, said Sam Brungardt, MPCA spokesman.
But then mercury levels in fish began to fluctuate. From 1996 to 2006, mercury concentrations in northerns and walleye rose by 15 percent, he said.
MPCA scientist Bruce Monson, who conducted the analysis, said the source of the mercury probably isn't local because the trend is statewide. Scientists also have found an increase in mercury concentrations in coho and chinook salmon from Lake Ontario from 1999 through 2003.
The cause is probably either increased global mercury emissions by sources outside the United States, such as China or India, or factors associated with climate change, or both, Monson said. Global mercury emissions increased between 1990 and 1995, largely because of an increase in electricity produced by coal-fired power plants in Asia.
Mercury deposited from the air is converted to methylmercury by bacteria that live in the sediments of wetlands and lakes. It then accumulates in the aquatic food chain, with predator fish such as northerns having the highest concentrations.
Compounding that problems is climate change, which creates major variations in rainfall, sometimes drying up wetlands and causing more mercury to be converted to methylmercury.
Seeking a solution
Monson said he and others are working with scientists from the other Great Lakes states and provinces in an effort to look at trends throughout the region.
"Increased mercury pollution of fish underscores the need for nations worldwide to reduce mercury emissions to the greatest extent possible and address the problems brought on by climate change," said MPCA Assistant Commissioner David Thornton.
Earlier this week, the United States endorsed negotiations for a new global treaty to control mercury pollution. The Bush administration had opposed legally binding measures to control mercury.
With mercury concentrations rising in Minnesota fish, residents here should advocate for a worldwide solution, MPCA's Brungardt said.
Mary Lynn Smith • 612-673-4788