Summary of Issues

Summary of Issues Related to Mercury Contamination of Fish

Why are we studying mercury?

In 1992, data from fish tissue sampling on the Ouachita River resulted in issuance of a fish consumption advisory by the Louisiana Department of Health and Hospitals (LDHH) and the Louisiana Department of Environmental Quality (LDEQ). The advisory consists of two recommendations: First, pregnant or nursing women and children under seven years of age should not consume any bass, and should limit consumption of other species to two meals per month. Second, non-pregnant women, men, and children over seven years of age should limit consumption of bass from the Ouachita to two meals per month. There is no limit on the consumption of other species by this second population. In addition to the LDHH/LDEQ advisory, the U.S. Fish and Wildlife Service (USFWS) strongly recommends that no fish or raccoons taken on either Upper Ouachita or D'Arbonne National Wildlife Refuges be consumed. The USFWS used different fish samples, analytical techniques and risk assessment methodologies.

Since that time, extensive work has been done to determine the scope of mercury contamination in Louisiana. This has involved two separate studies, first on 12 North Louisiana lakes, and second a statewide study originally designed for a three-year span which will be continued as funding permits. Results of these two studies as well as additional information on the effects of mercury on humans and wildlife are presented here.

What are the health risks due to mercury?

The LDHH and LDEQ coordinate in the assessment of data for health risks and jointly issue advisories if warranted. The Louisiana Department of Wildlife and Fisheries and the Louisiana Department of Agriculture and Forestry are also apprised of the situation and allowed to comment. LDHH and LDEQ use a limited meals approach in establishing health advisories. The two lead agencies will consider issuing a health advisory limiting fish consumption for pregnant or breast feeding women and children under seven for locations and species where the average concentration of mercury exceeds 0.5 parts per million (ppm) in fish and shellfish. At average concentrations exceeding 1.0 ppm, the agencies will recommend limited meals or no consumption for pregnant or breast feeding women and children under seven and limited consumption for the general population. In addition, LDHH considers other types of information when making advisory decisions. These considerations include, but are not limited to, information on sensitive subpopulations and local fish consumption practices that can affect exposure, the number of samples within a species, and the size and number of fish collected. The Food and Drug Administration (FDA) uses a maximum allowable level of 1.0 ppm, which was established to protect consumers at mercury concentrations 10 times lower than the lowest levels associated with the initial adverse effects of mercury.

Adverse effects on the nervous system, particular in developing fetuses and young children among whom effects can be permanent, is of primary concern with mercury contamination. Paresthesia, which consists of numbness and tingling of lips, fingers and toes, is generally the first symptom of mercury poisoning. Continued exposure results in stumbling, slurred speech, constricted visual fields, and impaired hearing. In extreme cases tremors and jerks can occur, followed by coma and death.

It is important to note that both the mercury concentrations found thus far in Louisiana and the alert levels used by FDA and LDHH/LDEQ are well below those found to cause mercury poisoning due to consumption of contaminated fish. Two notable mercury poisoning cases occurred in the 1960's in which over 100 persons were killed or became ill as a result of eating contaminated fish almost daily over an extended period of time (Foulke, 1994). These episodes took place in Minimata and Nigata, Japan. In each case, industrial discharges of mercury were routed to bays where fish were obtained for local consumption. Average mercury concentrations in fish tissue from both areas ranged from 9-24 ppm, with some fish having concentrations of up to 40 ppm. By comparison, the highest average concentration found to date in Louisiana was 0.984 ppm for king mackerel collected south-southwest of Grand Isle. The highest single sample concentration of 4.04 ppm was found in a largemouth bass from the Ouachita River. Therefore, the lowest average concentration found in Japan was over nine times higher than the highest reported average for Louisiana. For individual samples, the highest concentration in Japan was over 10 times higher than the highest individual sample reported in Louisiana. This illustrates both the highly protective nature of FDA and LDHH/LDEQ alert levels and the significantly lower mercury concentrations found in Louisiana and the United States. There have been no cases similar to the Japanese episode reported in the United States.

What about the fish and animals?

Besides posing a human health risk, elevated levels of mercury in fish can also have ecologically significant effects, such as affecting reproduction in fish and wildlife. Although fish can generally excrete inorganic mercury, methylmercury crosses biological barriers more readily and is not readily excreted nor sequestered in a form less harmful to the fish. Because of the ease with which methylmercury crosses biological barriers, embryonic fish are at much higher risk of mortality than adults even at mercury concentrations 1% to 10% lower than concentrations associated with adult mortality in fish. Due to biomagnification, predatory fish at or near the top of aquatic food chains and larger, older fish tend to have the highest concentration of mercury and, therefore, pose the greatest risk to human consumption.

Among terrestrial wildlife, top predators of the aquatic food chain tend to have the highest concentrations of mercury. Such species include raccoons, mink, otters, cougar and fish-eating birds such as eagles, ospreys and great blue herons. Effects on terrestrial wildlife are similar to those of aquatic organisms and range from adverse effects on growth and reproduction to behavioral effects and mortality.

Where are we looking and what have we found?

The North Louisiana 12 Lakes Study was conducted as a follow-up to the initial Ouachita River study. A total of 15 largemouth bass from each lake were tested for mercury in fillets. The 12 lakes covered by the study are listed below along with the average concentration for each lake (Table 1.).

Table 1.

Average concentration of mercury (ppm) in largemouth bass fillet samples from 12 North Louisiana lakes sampled in 1993.


Mercury in ppm


Mercury in ppm

D'Arbonne Lake


Lake Bartholomew


Cheniere Brake


Lake Claiborne


Cross Lake


Caney Lake


Upper Toledo Bend


Caddo Lake


Wallace Lake


Lake Bruin


Lake Bistineau


Lake Providence


No advisories were issued following the North Louisiana 12 Lakes Study; however, five of the lakes were scheduled for resampling as part of a statewide study conducted in conjunction with the United States Geological Survey (USGS). These included D'Arbonne Lake, Cheniere Brake, Cross Lake, Toledo Bend and Caddo Lake. Although mercury concentrations in fish tissue from Caddo Lake were found to be below the 0.5 ppm trigger level for resampling, additional sampling was recommended because Texas had issued an advisory for their side of the lake. Resampling of those lakes with average concentrations greater than 0.5 ppm did not confirm the presence of high levels of mercury, and no advisories were issued.

Following the 12 lakes study, LDEQ and the USGS began a statewide mercury contaminants study in early 1994 to sample approximately 33 new sites per year over a three year period. At the completion of the first three years, LDEQ elected to continue the study on its own in an effort to more fully characterize the extent of health risks due to mercury. This study will continue as funding permits. Targeted species for the statewide study included largemouth bass, channel catfish, blue catfish, crappie and bowfin. Substitutes included sunfish, freshwater drum, gar, striped bass, white bass, and buffalo. Composites consisting of at least three fish of the same species are collected for each sample, with generally five to 12 samples at each water body or sample site. This initial study was discontinued in approximately 2009 due to changes in budget priorities. However, the sampling program was restarted in 2014 with fish samples collected in LDEQ's Acadiana region followed by sampling in the Southwest region. It is hoped that sampling can continue throughout the state as current and future funding is made available. Data from the mercury in fish tissue sampling program can be found at the following link.

Data query tool for the mercury in fish sampling program

Despite the fact that over 100 water bodies have been tested during the statewide study only a small percentage have been selected for fish consumption advisories. The presence of an advisory on any of these water bodies does not necessarily indicate that no one should eat any of the fish taken from that water body. Rather, it means that some precautions should be taken regarding the type of fish consumed or the segment of the population you are in (for example pregnant or not; adult or child under 7). Details on each of these advisories can be found on LDEQ's fish consumption and swimming advisory web page, or by calling the LDHH hotline at 1-888-293-7020 or the LDEQ at (225) 219-3189.

Current listing of fish consumption advisories

Other water bodies will be considered for advisories following additional testing and data analysis. All of the water bodies with average fish mercury concentrations of 0.5 ppm or above are either under an advisory or are being further evaluated for a possible advisory.

In addition to fish consumption advisory development and dissemination, LDHH, in cooperation with LDEQ, issued a brochure describing how to choose, clean and cook recreationally caught fish and shellfish. The brochure, entitled Eating Recreationally Caught Seafood in Louisiana, describes ways to reduce the potential for consuming hazardous compounds that may be present.

Where is the mercury coming from?

Possible sources of mercury in Louisiana and the nation are many and varied. Ambient concentrations of mercury have increased significantly since the beginning of the industrial revolution. One study on Minnesota and Wisconsin lakes found that annual mercury deposition increased from 3.7 µg/m2 prior to 1850 to 12.5 µg/m2 in modern times. In addition, they found that mercury deposition rates were similar among widely scattered lakes indicating regional or global sources. These lakes were relatively undisturbed and had no known direct sources of mercury. Other reports have confirmed these results, indicating that anthropogenic sources are increasing the level of mercury contamination in the environment.

As a result of the proliferation of mercury in the environment, many of the fish people consume, including ocean caught species such as tuna, swordfish and shark purchased at local stores, are contaminated with low levels of mercury. Much of this is due to the fact that mercury is present in coal used at electrical power plants and is used in many products such as thermometers, fluorescent and mercury vapor lights, and electrical switches which may eventually be incinerated or placed in landfills. Mercury in these materials is released to the atmosphere as a gas by coal burning, trash incineration or direct volatilization. In a process similar to acid rain, the mercury is later deposited on the earth's surface through atmospheric deposition.

Other sources of mercury emissions to the atmosphere include chloralkali plants, which use mercury cathodes to generate chlorine and alkali from brine using electricity; hazardous waste incinerators; and pulp and paper mills. In addition to these regional and global sources of mercury, potential localized sources of mercury, both natural and anthropogenic, are known to occur. These include, but are not limited to, mercury leaching or vaporization from bedrock; active and inactive mercury mines; metal processors; natural gas meters containing mercury; and past use of mercury as a fungicide or pesticide. A complete list of mercury uses and sources is beyond the scope of this summary.

Paper mills, waste incinerators, and chloralkali plants that are major sources under LDEQ's Air Toxics rule are required to report mercury emissions under the Toxics Emissions Data Inventory (TEDI). Results from the past five years indicate that, with the exception of chloralkali plants, the other facilities generally report mercury emissions of less than 100 lbs. per year. One chloralkali plant is part of a major source facility and emits approximately one ton per year. The second chloralkali plant is not a major source facility and does not report mercury emissions; however, other reports indicate emissions of less than one ton per year. Electrical power plants are currently exempt from LDEQ's Air Toxics rule but not other Air Quality regulations, and are not required to report mercury or any other emissions as part of the TEDI. In addition, power plants generally do not have any type of pollution abatement systems for mercury. EPA is currently in the process of preparing a report on mercury emissions from power plants and other sources. Results of this study will be provided in future summaries. LDEQ's Air Analysis Section does not currently monitor ambient air for mercury. However, if funding can be obtained mercury monitoring may be incorporated in order to support LDEQ's research on mercury in fish.

So what now?

Fortunately, due to the nature of mercury's bioaccumulation contamination levels of fish in Louisiana are generally below those considered harmful to humans. This is evidenced by the fact that despite the widespread testing done in Louisiana, and the application of even more protective fish consumption advisories than some other states, we currently have a limited number of advisories for mercury. In addition, these advisories usually permit at least limited consumption of most fish by most people. This fact is often overlooked when a new fish consumption advisory is first issued by LDHH/LDEQ and then publicized by the media. LDHH and LDEQ will consider issuing additional fish consumption advisories due to mercury as the statewide mercury survey continues.

To summarize, it is unfortunate that mercury has become, over many years, all too common in Louisiana's and the world's environment; this includes our air, soil, sediments and water. However, even those water bodies currently under advisory in Louisiana are still capable of supporting sport fishing and fish consumption, provided some precautions are taken.

For more information regarding mercury and other fish consumption advisories contact the LDHH hotline at 1-888-293-7020, or call the LDEQ at (225) 219-3189.


Anthropogenic - environmental contaminants released by human activities.

Bioaccumulation - the rate and means by which a substance is incorporated and concentrated in an organism.

Biomagnification - process by which contaminants are accumulated to higher concentrations in predatory species due to consumption of many smaller organisms with lower concentrations of the contaminant. Bioaccumulated contaminants are frequently biomagnified as one moves up the food chain to higher predators, including humans.

Inorganic mercury - the commonly known elemental form of mercury, Hg 0, found as a silvery liquid or vapor, or the mercuric ion, Hg(II). Liquid inorganic mercury is not absorbed during digestion, but the vapor is absorbed by the lungs. Neither form is readily bioaccumulated by fish, humans or other organisms.

Methylmercury - mercury that has been converted by bacteria or other processes into an organic (containing carbon) compound, CH3Hg-. Methylmercury is the only form of mercury that can be readily bioaccumulated by fish, humans and other organisms; therefore, essentially all mercury found in fish is methylmercury.

TEDI - Toxics Emissions Data Inventory - a data base containing actual toxic air pollutants emissions as listed in LAC 33:III.Chapter 51. Major sources of toxic air pollutants are required to report these emissions annually.


Page last updated December 6, 2016

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