Mercury Level in Lake Istokpoga

by Beacham Furse and Ted Lange, Division of Freshwater Fisheries, FFWCC

In response to concerns by individuals that the 2001 Aquatic Habitat Enhancement Drawdown, as well as other habitat enhancement activities conducted by the Florida Fish and Wildlife Conservation Commission, may increase mercury levels in fish in Lake Istokpoga, Division of Freshwater Fisheries biologists collected largemouth bass from Istokpoga in January 2002 to determine mercury content. Results of this study have shown that mercury levels in Istokpoga largemouth bass have decreased since samples were last collected in 1989. Bass collected in 1989 had an average mercury level of 0.60 parts per million (ppm). Average mercury level for bass in 2002 was 0.44 ppm with bass less than 15 inches (bass harvestable under the 15" - 24" protective slot limit) having an average concentration of 0.39 ppm. The most likely explanation for this decrease is that, in general, mercury emissions to the atmosphere have decreased over the last decade and therefore mercury loading to our lakes has reduced. Also, as Lake Istokpoga has become more eutrophic, less mercury is expected to reach largemouth bass at the top of the food chain. That being said, because bass greater than 14 inches are used as the standard for determining consumption advisories and the average mercury level for Istokpoga bass over 14 inches is 0.51 ppm (A 0.50 ppm level triggers a consumption advisory), the Florida Fish and Wildlife Conservation Commission continues to recommend limited consumption of largemouth bass from Lake Istokpoga (see definition of "Limited Consumption" in the following Q/A section). Below are some frequently asked questions concerning mercury.

What is mercury?

Mercury belongs to a group of elements collectively known as "heavy metals". It exists in many forms such as elemental mercury (the mercury found in scientific thermometers), inorganic mercury (used in manufacturing), and organic mercury (form usually found in contaminated foods). In high enough doses, all three forms present serious threats to human health.

Where does mercury come from?

Although mercury is naturally found in rock, soil, air, water and plant and animal tissue, it was formerly used by industry in many different applications. It was used in the manufacturing of chlorine and lye. It was also used in pulp and paper mills to control slime growth and by agriculture to prevent fungus growth on seeds. Presently, the most common man-made source of mercury in Florida is from the burning of coal, medical waste, and household trash containing mercury cell batteries. Inorganic forms of mercury are transported over great distances in the atmosphere and deposited onto the earth's surface through dry fall and rain.

How does mercury get into fish?

Once mercury enters the aquatic environment, it can be transformed through the actions of bacteria to its most toxic form, methyl-mercury. This process, called methylation, occurs predominantly in sediments and algae. Methyl-mercury is absorbed by small plants, crustaceans, and insects and is transferred up through the food chain as these prey items are consumed by larger predators in a process called bioaccumulation. Fish obtain virtually all methyl-mercury through their diet. Once a fish takes in methyl-mercury, much of it is deposited in its muscle tissue. Because fish eliminate mercury from their bodies very slowly, mercury can gradually accumulate in high concentrations. Generally, the larger and older the fish or animal gets, the greater the concentration of mercury in their body.

What are the health effects of mercury?

By eating fish with high levels of mercury, you are absorbing that mercury into your body. All animals will eventually eliminate mercury from their

bodies naturally, but only at a slow rate. If you take in more mercury than your body can get rid of, mercury will accumulate in your body. No danger of mercury poisoning exists when fish are not eaten from affected areas. Therefore, fish caught and released do not present any danger from mercury poisoning.

Toxic effects of mercury differ from person to person and are primarily based on the amount of mercury consumed. Although mercury levels in Florida fish are not high enough to cause acute illness, frequent consumption of fish containing low levels of mercury may result in serious health risks. Hand tremors, speech impediments, and lack of coordination are some nerve and brain symptoms of mercury poisoning. Sensory disturbances include tunnel vision, blindness, and deafness. Birth defects such as deafness, blindness, and cerebral palsy may result from fetal exposure to mercury. Unborn and very young children are most vulnerable to mercury due to the sensitivity of their developing nervous systems.

What mercury levels are of concern?

When fish are found to have mercury concentrations greater than 0.5 parts per million (ppm), the Florida Department of Health issues a health advisory. Water bodies in which mercury levels are between 0.5 and 1.5 ppm receive a "Limited Consumption" advisory. If fish have concentrations exceeding 1.5 ppm, the Florida Department of Health recommends that fish from those systems should not be consumed under any circumstances ("No Consumption" advisory). Lake Istokpoga, as well as all other lakes in Highlands County, are considered "Limited Consumption" lakes.

What does "Limited Consumption" mean?

Certain fish species (largemouth bass, gar, and bowfin) caught in water bodies with "Limited Consumption" advisories should not be eaten more than once per week by adults and not more than once per month by pregnant women, nursing mothers, women who intend to have children, and children under 15 years old. This recommendation is based on a portion size of 8 ounces of fish per person.

Can anything be done to make these fish safe to eat?

No special cooking or cleaning methods exist to reduce mercury levels in fish. Because mercury is not stored in fatty tissue, mercury cannot be removed by discarding fish fat.

Where can I get more information about mercury?

You may contact your County Public Health Department or the Florida Department of Health Bureau of Environmental Epidemiology ([850] 488-3385). You may also contact Beacham Furse ([863] 462-5190) or Ted Lange (Chemistry Section; [352] 742-6438) with the Florida Fish and Wildlife Conservation Commission. Web sites with good information include,  and