Mercury on rise in Great Lakes
After long decline, toxin increasing in Great Lakes fish such as walleye, trout
DETROIT — It’s not supposed to be like this. Although advisories about toxic mercury in fish have continued in the 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 past several years, the decline has reversed. Scientists are finding mercury levels rising in large Great Lakes fish such as walleye and lake trout. Curiously, it’s occurring in fish in some locations but not others. Researchers are trying to figure out why.
The mercury levels are not surpassing U.S. Environmental Protection Agency thresholds, but researchers want to determine whether what they are seeing is temporary or a trend that will worsen.
The answer has large ramifications for the Great Lakes’ sportsfishing industry. Anglers spend billions of dollars in equipment and triprelated expenses.
Mercury is a heavy, silvery metal, unusual in that it’s liquid at room temperature. It’s naturally occurring, but is rare to find uncombined with other elements.
Mercury is extremely toxic to humans and animals — and unlike many other toxins, it remains in the environment for a long 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 0.002 parts per million. Mercury can inhibit brain
development in fetuses and children, and it can harm immune systems and adult heart function.
Environment and Climate Change Canada, an agency similar to the U.S. Environmental Protection Agency, looks at data collected from multiple fish species and monitoring of herring gull eggs across the Great Lakes.
“We’ve been monitoring since the 1970s, and the (mercury contamination) trends overall have been declining — as have been the emissions of mercury into the atmosphere and deposition into the lakes,” said Agnes Richards, a research scientist with Environment Canada. “We decided to look at recent trends, from 2000 to 2015. What we found is, at some specific sites, trends have reversed.”
The researchers published a finding of their studies in late December.
The issue has been noticed on the U.S. side of the Great Lakes as well.
“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 monitors fish.
Mercury levels in Lake Erie walleye have risen, Bohr said.
On Lake Michigan, walleye and lake trout from Grand Traverse Bay show increases, and on Lake Huron, walleye from Saginaw Bay and lake trout from Thunder Bay have rising levels, he said.
The average rate of increase in the fish is about 2 percent per year, Bohr 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 have only hypotheses regarding why this is occurring. The trend of the Great Lakes warming 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-size fish eat the smaller fish, and large game fish eat the medium-size 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 also could be a factor, Richards said.
“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.”
And invasive species in the Great Lakes probably also play a role. “It’s really significantly changed the food web,” Bohr said.
The timelines for the explosion of invasive zebra- and quaggamussel populations in the Great Lakes, and of the round goby, a small fish, rather neatly correlate with the reversal of declining mercury levels in sport fish, he noted. “You can’t just ignore that.”
Further evidence of invasive species’ disruption of fish diets as a possible culprit for the mercury mystery comes from carp. In Grand Traverse, Saginaw and Thunder bays, carp — unlike walleye and lake trout — aren’t showing an increase in mercury levels, Bohr said. From the St. Clair River through Lake St. Clair and the Detroit River carp are even showing decreases in mercury, he said.
That’s significant because bottom-feeding carp are eating different meals than the large sport fish are.
“They’re low on the food chain,” Bohr said of carp. “They’re just mucking around on the bottom eating insects, basically.”
De Solla said he doesn’t “see anything catastrophic in the next little while.” But if the mercury numbers continue to increase in Great Lakes fish, “it could become a problem again.”
That highlights the importance of continued monitoring, Richards said, “to see if this is a slight oscillation or a growing trend.”
That could be jeopardized on the U.S. side, as President Donald Trump’s 2017 budget proposal calls for the elimination of virtually all Great Lakes restoration funds.
Bohr said most of the Michigan Department of Environmental Quality’s work to evaluate toxins in fish is done through state funding. But, “indirectly, any federal budget cuts are likely to have an impact,” he said.