It is all but invisible and must be measured with sensitive scientific equipment. Still, scientists assert there is more than enough atmospheric mercury falling upon East Texas to explain elevated mercury levels found in gamefish in some East Texas lakes.

Now marine species, including the Gulf of Mexico's king mackerel, have been added to the growing list of fish consumption advisories issued to protect the public health in Texas, Florida and other Gulf Coast states.

Raw data from a Mercury Deposition Network (MDN) test site at Gregg County Airport near Longview indicates a similar amount of airborne mercury is falling on East Texas, as it is at other sites in the U.S. and Canada-where mercury contamination in fish has long been recognized, including places for which there are no known point-sources of contamination.

Data collected February 1996-97 at the Longview site, established by the Texas Natural Resource Conservation Commission (TNRCC) in cooperation with the National Atmospheric Deposition Program (NADP), indicates that Texas is getting more than enough mercury (8 to 11 micrograms per-meter-squared per year) from atmospheric deposition to explain levels of methylmercury found in the edible flesh of freshwater fish in East Texas lakes and in king mackerel in the Gulf of Mexico. Atmospheric deposition is believed responsible for elevated mercury levels found in king mackerel off the Texas Gulf Coast that resulted in the June 1997 issuance of a fish consumption advisory by the Texas Department of Health.

The TDH advises no consumption of king mackerel longer than 43 inches, and limits consumption of king mackerel with lengths of 37 to 43 inches to one 8-ounce serving per week for adults and one 8-ounce serving per month for children and for women of childbearing age. The advisory does not restrict consumption of king mackerel under 37 inches long, nor does it apply to any other mackerel species, including those sold commercially in food stores.

The king mackerel lengths roughly correspond to expected mercury levels of 1.5 parts per million (ppm) or more in the edible flesh of king mackerel over 43 inches; 0.5-1.5 ppm in mid-length king mackerel; and less than 0.5 ppm in king mackerel less than 37 inches, according to Kirk Wiles, an avid offshore angler and assistant director of the TDH Seafood Safety Division.

Wiles urges anglers to take advisories very seriously, but added, "This does not mean anyone who occasionally eats more king mackerel than recommended definitely will suffer adverse health effects. We're concerned primarily with children, females of childbearing age, and anyone who regularly consumes the larger king mackerel."

While the Texas advisory applies to king mackerel only, the federal Food and Drug Administration (FDA) on Feb. 16, 1994, issued a chemical hazard alert for bonito, halibut, Spanish mackerel, king mackerel, shark, marlin, and blue tuna based on a federal action level of 1 ppm mercury in the edible flesh of food fish bound for market. A debate is ongoing in federal circles as to whether the action level should be lowered to 0.5 ppm or somewhat higher, but less than 1 ppm.

Current advisories in Florida cover selected species in certain areas including: gafftopsail catfish, jack crevalle, spotted seatrout (speckled trout), ladyfish (skipjacks or "horse mackerel"), Spanish mackerel and-in places like Tampa Bay, Florida Bay, Charlotte Harbor and Indian River Lagoon-sharks. Their king mackerel advisory is for the west coast of Florida. Their advisories cover freshwater gamefish in a state where largemouth bass mercury levels have been measured as high as 4.4 ppm.

Methylmercury, more toxic than elemental mercury or most other forms (or species) of mercury, is the form of mercury most harmful to fish, wildlife and humans. Humans are most often exposed to mercury through consumption of fish. However, natives of the Faroe Islands, Arctic regions and Northern Canada have been exposed through consumption of seals and whales.

According to a Draft (June 1996) Mercury Study Report to Congress being prepared by the U.S. Environmental Protection Agency (EPA), scientists believe mercury in sediments and water bioaccumulates in small plants and animals in lakes, rivers and oceans before biomagnifying up the food chain when small plants and animals are consumed by larger predators, including man. Experts said biomagnification in animals may result in mercury levels in animals up to 1 million times higher than in the water column they live in or drink from.