Mercury Testing
The illustration shows how mercury is bioaccumulated, starting with bacteria at the bottom of the food chain and up to top level, including apex predators like largemouth bass.
Since 1983, the FWC has actively investigated the occurrence of mercury in Florida's freshwater and marine environments.
The data collected by the FWC over the last 25 years contributed to the recent publication, “Basic Guidelines for Eating Freshwater Fish”. This section of the DOH Health Advisory Brochure gives general advice on the safest species to eat and applies to all state waters. Women of childbearing age and young children are encouraged to eat up to one meal a week of bluegill sunfish, redear sunfish, or brown bullhead catfish. The general population can eat up to two meals per week of these same species, with the addition of redbreast sunfish. Three of these sunfish species are the most popular panfish throughout the state and brown bullhead is one of the most commonly harvested catfish species. As an agency that promotes recreational fishing, the FWC is happy to report that these species are very safe for consumption no matter where they are caught.
Many sources of natural and manmade mercury exist in the environment. Natural sources include volcanoes, mercury deposits in rock and oceans through a process similar to evaporation called volatilization. Human-produced mercury comes from waste incineration, coal-fired power plants, mining and smelting, and industrial processes including chlorine-alkali processing, cement production and pulp and paper production. In the 1950s, scientists recognized that manmade mercury pollution was contaminating food sources. Early studies indicated that fish and wildlife gathered significant amounts of mercury from their environment and certain aquatic bacteria converted mercury into a more toxic form called methyl mercury. When the bacteria that produce methyl mercury are eaten by the next organism in the food chain, they pass on the mercury, and as it moves up the food chain it increases in concentration. This process is called bioaccumulation. Top level predators contain the highest concentration of mercury because they consume high numbers of contaminated prey.
Such is the case in Florida’s freshwater ecosystems, where the highest concentrations of mercury occur in top-level predatory fish like largemouth bass, bowfin, chain pickerel and gar. Because largemouth bass are a popular food fish among anglers in Florida and they can bioaccumulate high concentrations of mercury, they have been the major focus of monitoring for mercury contamination in fresh water lakes and rivers. Fish species with typically high levels of mercury in marine ecosystems are also upper-level predators (i.e. tuna, mackerel, shark, snook, etc.).
In 1983, the Florida Fish and Wildlife Conservation Commission (FWC), the Florida Department of Environmental Protection (DEP) and the Florida Department of Health (DOH) started actively investigating the occurrence of mercury in Florida's freshwater and marine environments. This multi-agency effort has focused on surveying important marine and freshwater bodies and fish species throughout the state.
- FWC biologists collect target species.
- DEP scientists analyze fish tissue for mercury.
- DOH officials conduct risk assessments and issue fish consumption advisories.
Advisories for mercury in Florida waters have been issued since 1989. Florida's freshwater and marine fish are generally considered safe to eat; however, it is a good idea to review the current fish consumption advisories for specific advice on fish species or individual bodies of water. The advisories are not intended to discourage anglers from eating fish but should be used to choose fish lower in mercury while limiting consumption of some species of fish from certain waters.
Data collection for these health advisories begins each year by compiling a list of 50 to 60 lakes and rivers. Other FWC personnel or the public may also suggest new water bodies to be added and the sampling list may change throughout the year, as some areas are unsuitable for sampling due to lack of fish or access. Researchers at FWC created a seven-year rotating re-sampling schedule for previously sampled lakes and rivers. The schedule is designed to keep the data and associated consumption levels as current as possible. Seven years is also a rough estimate of the average lifespan of most fish; so it is assumed that the sampling will target fish from the next generation.
Biologists attempt to collect at least eight harvestable individuals of each targeted fish species in each water body. Fish are collected by electrofishing then put on ice and transported to the laboratory. Then, researchers determine total length, weight and gender and a sample of muscle tissue is taken. The tissue samples are then labeled, frozen and shipped to the DEP Chemistry Laboratory in Tallahassee for analysis. Once mercury results are returned to FWC, this data can be submitted to the Department of Health for creating consumption guides. When creating a consumption guide, the average mercury level for a species in a single water body is taken into account. Depending on that average, this is an example of an individual consumption guide:
Black crappie from Lake Monroe: Recommends one meal per week for women of childbearing age and young children/Two meals per week for all others.
The most recent data was submitted in July of 2013 from lakes and rivers sampled between February 2012 and February 2013. Data from this period contained a total of 1,267 individual fish from 27 locations and a total of 19 species, including: blue tilapia, black crappie, bluegill, butterfly peacock bass, channel catfish, common snook, chain pickerel, gray snapper, largemouth bass, Mayan cichlid, Midas cichlid, redbreast sunfish, redear sunfish, spotted bullhead, spotted sunfish, spotted tilapia, warmouth sunfish, white catfish, and yellow bullhead.
For more information from the Department of Health and other organizations monitoring Florida’s water quality and contaminants, check out the links below.