In June, the Alabama Department of Public Health (ADPH) released its annual report of statewide fish consumption advisories, which indicated this year more than a half-dozen freshwater locations in Mobile County showed significant levels of mercury in larger species of fish.
Whether caused by pollution or nature, mercury is the most common toxic chemical found in fish throughout the country. However, blackwater rivers — like several running through the forested swamps and wetlands of the Mobile-Tensaw River Delta — can carry more mercury than water flowing through less vegetated areas.
Dr. John Guarisco, one of the environmental toxicologists who authored the 2015 report, explained that mercury collects on foliage, and as leaves fall, it’s easily swept into the water supply.
“What’s sad about the whole thing is, swamps are notorious for this,” Guarisco said. “They have a lot of tree cover so they pick up a lot mercury, and they’re usually under at least partial anaerobic conditions.”
According to Guarisco, once mercury vapor is in the atmosphere, it can deposit anywhere in the world, which means mercury found in local fisheries could have origins in a coal-fired power plant in another hemisphere or from the erosion of natural sediments in the area.
Once it’s in the water, a natural process converts mercury into methylmercury, which can readily get into a population of fish through their gills. Over time, mercury builds up in the tissue of a fish.
“Mercury is an element, which means it can’t ever be created or destroyed,” Guarisco said. “Once it’s in the water, it’s picked up by plankton or smaller freshwater animals. Then it’s eaten by the little fish, the little fish are eaten by bigger fish and then predator fish, like the bass family, eat those fish and slowly [mercury] builds up to a big concentration.”
Mercury collects in the proteins of a fish’s muscles or tissue and passes into humans through consumption. Guarisco said even water bodies with strict advisories are safe for recreational swimming, boating and watersports.
Additionally, it’s larger species of fish that tend have more mercury exposure over time. With regard to seafood, the ADPH advises that children under the age of 14 and women who are pregnant or nursing not consume king mackerel, shark, swordfish or tilefish because of their size.
One of the most common culprits among freshwater fish is the bass.
Chickasaw Creek and Cold Creek Swamp are the only bodies with across-the-board “do not eat” advisories on all fish species. The Mobile River also has a “do not eat” advisory associated with multiple species of bass, which was the only species of fish tested at those locations.
According to the report, there were no advisories for any fish at Heron Bay, Dog River, Mobile Bay, Mississippi Sound, Porterville Bay and Three Mile Creek. The full report can be reviewed on the ADPH website, and more excerpts are available at lagniappemobile.com.
Each year, fish are tested by the Alabama Department of Environmental Management (ADEM). It’s then up to ADPH to inform the public through annual reports and public outreach.
However, because mercury exposure is a long-term danger rather than an immediate threat, there typically aren’t any signs indicating that a particular body of water is under an advisory — even in the case of publicly maintained water bodies like the Big Creek Reservoir.
“There is no fish in the state of Alabama that if you eat it, it’s going to kill you,” Guarisco said. “We take the long view, chronic toxicity over time. We’re looking at a lifetime exposure.”
According to the U.S. Environmental Protection Agency, mercury exposure can increase the risk of heart disease in adults and can accumulate to cause damage to the nervous, digestive and immune systems, lungs and kidneys.
According to Guarisco, the best way for an individual to control his or her mercury intake is to stay informed about area advisories, eat smaller fish and to correctly prepare fish. Broiling, baking, poaching or boiling fish will allow the fatty juices drip away, which can reduce the level of mercury as well as perfluoroalkyl sulfonate — another toxic contaminant less common in the area.
“When the recommendations are made, they’re made with the understanding that the vast majority of people are also consuming fish as part of a balanced diet,” Guarisco said. “If you’re eating fish for two of your three meals a day, quite possibly you could have a problem. But, nobody does that. Most people are going to at least have a cheeseburger somewhere along the line.”
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