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Mercury is released into the environment from natural deposits in rocks, volcanoes, and soils. It is also released into the environment when power plants burn coal, incinerators burn mercury containing wastes, and during the production of other industrial chemicals. Airborne mercury can travel in the atmosphere for years. It eventually attaches itself to dust and water particles and enters Florida waters with rain and runoff.

Mercury is found, usually at extremely low concentrations, in virtually all Florida waters. Naturally occurring bacteria that process sulfate in aquatic systems take up mercury and convert it into methyl-mercury through a metabolic process. These bacteria may be consumed by the next higher level in the food chain and pass on their burden of methyl-mercury. Because animals accumulate methyl-mercury faster than they eliminate it, animals consume higher concentrations of mercury at each successive level of the food chain. This process, called biomagnification, results in high concentrations of mercury in long-lived predatory fish at the top of the food chain.

Nearly all fishes and shellfish contain at least trace amounts of mercury. In Florida's freshwaters, small, short-lived species such as sunfish (bream), brown bullhead, and black crappie (specs) generally have low concentrations of mercury. Largemouth bass and other long-lived predatory fish have higher concentrations of mercury; however, smaller largemouth bass have less mercury than larger individuals. In marine waters, shorter lived species such as striped mullet, Florida pompano, sheepshead, common dolphin, gray snapper, gulf flounder, and southern flounder generally have much lower concentrations while king mackerel, swordfish, and sharks tend to have the highest concentrations. Ultimately, mercury concentrations in fishes depend on diet and lifespan: those that consume other fish and live longest have the highest mercury concentrations.

Since mercury accumulates in the muscle tissue of fishes, which is the part you eat, trimming excess fat and skinning do not reduce the amount of mercury you consume. The only way to reduce mercury consumption is to eat fish from less contaminated water bodies and to select species that are lower in mercury. By choosing a variety of fish low in mercury from different water bodies and avoiding eating only one type, anglers can enjoy catching and eating fish from Florida waters.

Florida's rich freshwater resources include over 12, 000 miles of rivers and streams, 3 million acres of lakes and ponds, and 11,000 miles of canals. Not all water bodies can be tested for mercury in fish. Therefore, the fish-sampling program is frequently adjusted in order to identify species and locations to be avoided and to identify both species and water bodies where higher levels of consumption can be recommended.
Since testing all waters is too expensive, Florida's fish collection and testing program focuses on water bodies with known or suspected pollution, those thought to be susceptible to pollution, those with important fisheries (popular fishing spots), and those being tested for changes in trends. Testing is a continuous process, so consumption recommendations may change over time.

If elevated concentrations of chemicals such as mercury or dioxin are found in a local fish population, the Florida Department of Health may issue health advice to the public in the form of a fish consumption advisory. Fish consumption advisories may include recommendations to limit or avoid eating certain fish species caught from specific water bodies or, in some cases, from specific water body types (e.g., all lakes). An advisory may be issued for the general population, for specific groups such as recreational and subsistence fishers, or for sensitive subpopulations such as pregnant women, nursing mothers, and children. Consumption advisories are not regulations; they are voluntary recommendations issued to inform people.

FWC has continued to test new water bodies and new species of fish as well as monitor trends in mercury concentrations at many sites in Florida waters. It is to be expected that if we look harder we will find more places where fish advisories are warranted.