The exact source of the mercury cannot be definitively pinpointed, but generally, coal plants and coal-fired boilers are the single largest source of man-made mercury pollution in the U.S. Mercury is released into the atmosphere by coal-fired plants and then settles back to earth, contaminating soil and water.
Mercury increases in concentration with each step up the food chain, a process called “bioaccumulation” that is particularly acute in fish species. Mercury is absorbed by smaller microorganisms, which are in turn eaten by small fish, which are in turn eaten by larger fish. While smaller fish may have only trace amounts of mercury, a big fish that dines regularly on those smaller fish can end up with much higher levels of the toxin.
Large predator fish such as walleye and bass can have mercury levels one million times higher that of the surrounding water, according to the National Wildlife Federation.
Nantahala and Chatuge join two other lakes with fish consumption advisories already in place in WNC: Lake Fontana and Santeetlah Lake.
But sometimes, the absence of a particular lake — or particular fish species — from the advisory list comes down to whether that lake or fish species has been sampled.
Some lakes, such as Lake Glenville in Jackson County, simply haven’t been sampled recently. Mercury may be present, but biologists simply haven’t gotten around to sampling it yet. Some species may not have been sampled either.
But likewise, some species and lakes in the region have been sampled and have mercury limits below the consumption advisory threshold.
Hiwassee Lake in Cherokee County was tested, for example, but was not high enough to trigger a consumption advisory, according to David Yow, warm water fisheries research coordinator for the mountain region with the N.C. Wildlife Resources Commission.
“In general, mercury is present in most fish, but there is a full range of mercury levels,” Yow said. “Those with higher mercury, you have to make a point of having fewer meals of those fish.”
Even though two lakes could be just 30 miles away from each other as the crow flies, one could have unsafe levels of mercury in fish and another might not. Or, within the same lake, walleye could have mercury levels that dictate limited consumption while perch doesn’t.
“The way that fish are bioaccumulating mercury, each water body is going to be different,” said Dr. Ken Rudo, a epidemiologist with the N.C. Division of Public Health.
Rudo is one of two epidemiologists with the state that make the final call on when to issue an advisory.
Analyzing fish for toxins and deciding whether there could be health risks from eating them is a three-step process involving three state agencies. Fish are caught by biologists with the N.C. Wildlife Commission, usually in conjunction with other routine monitoring.
Tissue samples of the various fish species are then sent to the Division of Environment and Natural Resources for analysis. Those numbers are in turn sent to epidemiologists at the Division of Public Health to determine what, if any, advisory is needed.
The news isn’t great for fishermen who regularly trawl the lakes for their supper.
“From talking to folks on the other two reservoirs (Fontana and Santeetlah) where the advisories have been there for a while, some have reduced their intake,” Yow said.
But, he doesn’t think it will have a major impact on the popularity of lake fishing in the mountains or be a deterrent for the sport.
Some fishermen practice catch-and-release anyway, or just like the excuse to get outside on the water. For those who do make meals of their catch, they simply must cut back to once a week or avoid it altogether in the case of children and nursing or pregnant women.
Mercury can adversely affect nerve cells in the brain and spinal cord, especially in unborn babies and young children. Prenatal mercury exposure can affect the way children think, learn and problem solve later in life. Adverse health effects also can occur in adults at much higher doses.
Every year, 300,000 infants are born at risk for developmental defects because of their mother’s exposure to toxic mercury pollution, according to the Sierra Club.
Mercury pollution isn’t good for wildlife either. Mercury levels in fish can cause difficulty in schooling and decreased spawning success, according to the National Wildlife Federation.
Roughly half of U.S. lakes and reservoirs have mercury amounts exceeding safe levels and nearly all fish and shellfish contain traces of mercury, according to the Environmental Protection Agency.
Almost two-thirds of the nation’s coal-fired plants lack the needed modern pollution controls to keep toxic air pollution, like mercury, out of our air and water. That is slated to change in coming years, however.
In December 2011, the Environmental Protection Agency finalized new air pollution standards that will result in the first-ever national limits on the amount of mercury spewing from the nation’s coal-fired power plants.
The new pollution limits on power plants will cut mercury emissions by 91 percent, while also cutting acid gas, arsenic, lead and nickel emissions. Environmental, conservation and wildlife groups had been fighting for the power-plant mercury emission standards for more than two decades.
“At long last, these prudent and overdue limits on unchecked mercury and toxic air pollution will ensure our fish will be safe to eat, and our children can breathe easier,” Larry Schweiger, the president and CEO of the National Wildlife Federation, said in a statement.
Advisories on mountain lakes
Due to contamination, women who are pregnant, nursing or are of childbearing age and children under 15 should not eat:
• Walleye and largemouth bass from Fontana Lake.
• Walleye and largemouth bass from Santeelah Lake.
• Smallmouth bass, walleye, yellow perch and largemouth bass on Nantahala Lake.
• White bass and large mouth bass from Lake Chatuge.
Other adults should limit their consumption to six ounces of the fish species listed on these lakes per week.
The mercury contamination does not present a health risk for people engaging in other recreational activities such as wading, swimming, boating or handling fish.