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 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 last 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.
Scientists only have hypotheses regarding 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 on 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, and medium-sized 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 in recent years could also be a factor, says 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.”
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