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DETROIT - It's not supposed to be like this.

Though advisories about toxic mercury in fish have continued in Michigan and the surrounding Great Lakes, with recommendations to limit consumption of certain species to a few times per month, the amount of mercury found in fish tissues has dropped steadily over the decades since the 1970s. That corresponded with the reduction of pollution coming from Midwestern smokestacks as regulations tightened, pollution prevention technology improved, and coal-fired factories and power plants went offline.

But over the past several years, that started changing. Scientists are finding mercury levels rising in large Great Lakes fish such as walleye and lake trout. Curiously, it's occurring with fish in some locations but not others. Researchers are still trying to figure out why.

The mercury levels are not surpassing U.S. Environmental Protection Agency thresholds. But researchers want to determine if what they are seeing is a temporary trend or a trajectory that's only going to worsen.

The answer has large ramifications for Michigan's vital sports fishing industry. Anglers spent $2.4billion in trip-related expenses and equipment in 2011, according to the Michigan Department of Natural Resources.

Mercury is toxic to humans and animals — and unlike many other toxins, mercury remains in the environment for very long periods of time, moving up the food chain and compounding inside animals that ingest it.

The EPA has found that mercury in water has the potential to cause kidney damage from short-term exposures at levels above the maximum contaminant level of just 0.002parts per million. Mercury can inhibit brain development in fetuses and children and harm immune systems and adult heart function.

"Out of 19 data sets, we see eight where we can see a significant trend" of mercury levels rising in certain fish, said Joseph Bohr, aquatic biologist for the Michigan Department of Environmental Quality's Water Resources Division, which does its own fish monitoring.

On Lake Michigan, walleye and lake trout from Grand Traverse Bay show increases, Bohr said. On Lake Huron, walleye from Saginaw Bay and lake trout from Thunder Bay have rising levels. Mercury levels in Lake Erie walleye have also risen. The average rate of increase is about 2percent per year in the fish, he said.

But at least for now, the mercury spikes are in isolated locations.

"We have 11 other data sets where we're not showing any significant increase," Bohr said.

Scientists only have hypotheses for why this is occurring. The trend of warming Great Lakes could be a factor, said Shane De Solla, an ecotoxicologist with Environment Canada and co-author of the recent study.

Many types of mercury in the environment tend to pass through fish when ingested. But a type known as methylmercury tends to be absorbed into fish tissues. As small fish eat contaminated insects, medium-size fish eat the smaller fish and large game fish eat the medium fish, those mercury concentrations get magnified exponentially, a process known as bioaccumulation.

"The lakes are slightly warmer, and that increases the production of methylmercury," De Solla said.

The region's more frequent and intense storms could also be a factor, said Agnes Richards, a research scientist with Environment Canada.

"That results in a lot of flooding and the re-suspension of sediments," she said. "What was buried before can become exposed, and that can increase the conversion of mercury to methylmercury."

Invasive species in the Great Lakes likely also play a role. "It's really significantly changed the food web," Bohr said.

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