Research aids understanding of mercury risks to wildlife and people

Packed with omega 3 fatty acids, lean protein, B vitamins and other important nutrients, fish are considered a healthy food option for most people, but they can accumulate levels of mercury that are toxic. A College of Agriculture and Life Sciences graduate student’s research is helping pinpoint which fish in which areas of North Carolina pose the greatest risks – and some of her findings are surprising.

Dana Sackett, a Ph.D. student in the departments of Biology and Environmental and Molecular Toxicology, embarked on her study of mercury contamination in fish three years ago. Her collaborators are Drs. Derek Aday, an associate professor of biology; Jim Rice, a professor of biology; Greg Cope, an associate professor of toxicology; and David Buchwalter, an assistant professor of toxicology.

Fish contaminated with mercury are a concern because when people eat enough of these fish, they can develop nervous system problems. The problems are particularly risky for fetuses and young children.

The research began after the state’s Department of Health and Human Services issued an advisory cautioning people against eating largemouth bass harvested from waters statewide.

“We thought that a statewide advisory was overkill,” Sackett said, because even among the same species of fish in the same North Carolina river basin, there can be 10- to 100-fold differences in mercury levels.

A better understanding of that variation and of how mercury moves through ecosystems, they thought, could give public health officials — and the public — a better idea of where to expect the greatest risks.

The researchers’ first step was to analyze data that the N.C. Division of Water Quality had collected on mercury levels in fish from 1990 to 2006. Using easy-to-determine environmental factors such as water pH, the fish species and geographical region, they were able to explain a vast majority of the variation in mercury levels. They then used what they’d found to develop a model to predict which species in which lakes in which areas of the state would likely be safe to eat and which would not.

Public health officials and wildlife resource managers “don’t always have the means necessary to go out and collect fish from all lakes in the entire state,” she said. “We hope this model will help them make determinations about fish consumption advisories and wildlife management decisions.”

After developing the predictive model, Sackett and Aday began to look specifically at lakes near coal-fired power plants. These plants are the largest source of anthropogenic mercury air pollution, which rain can carry into lakes and rivers.

“What we wanted to do is say, ‘Well, next to these coal-fired power plants where you expect to see higher levels of mercury deposition, does that translate into higher levels in the fish?’” Sackett said.

What the scientists found was surprising: Fish in lakes at least 30 kilometers from a coal-fired power plant had mercury levels three times higher than fish of the same species in lakes within 10 kilometers.

The reason: selenium.

“Selenium is emitted from coal-fired power plants, too, and can prevent mercury from accumulating in fish tissue,” Sackett said. While high selenium levels help lower mercury levels in fish, high selenium levels can have negative effects of their own, causing health problems for fish and other wildlife.

Sackett’s next doctoral research project will look at how fish size and age relate to mercury levels.  Fish that are high on the food chain and those that are older are the ones most likely to have high levels of mercury. That’s because mercury levels add up in fish tissue over time, and they add up when one fish eats others that have mercury in them.

The problem, Sackett said, is that older fish and fish higher on the food chain tend to be bigger. And fishing regulations often encourage fishermen to keep for consumption bigger fish, returning those that are smaller to the wild so that fish populations are sustainable.

“What we don’t know is, Are we encouraging fishermen to always take home high-mercury fish?” she said.
“So what we want to see is, in lakes that are highly contaminated with mercury, are there any fish that are small enough that would still be considered safe to eat?” she said. “And in low-contaminated lakes — those we would consider safe lakes — are there any fish that are old enough and big enough that they would pass that limit of what would be safe to eat?”

Right now, Sackett is analyzing the data on mercury levels and fish size in six North Carolina lakes. Meanwhile, she’s also taking a look at how hurricanes affect mercury levels in fish in the southeastern United States.

— Dee Shore