Mercury a problem for fish-eaters

Seafood is more than just a love along Bayou Lafourche and throughout South Louisiana. For many, it is a means of survival, in terms of economic gain and nutritional sustenance.

For that reason, as scientists continue to study the levels and effects of toxins such as mercury in local waters and aquatic animals, residents of the numerous water-bound towns of the state and other regular seafood eaters should pay attention.

Mercury levels in humans came under increased scrutiny beginning in the 1950s when residents of the small fishing town of Minamata, Japan, started displaying inexplicable irregularities – numbness, vision problems, slurred speech and uncontrollable shouting and bodily twitches – all signs of neurological damage.

The outbreak was later linked to mercury compounds dumped into area waters by a local factory and subsequently ingested by locals through their frequent consumption of fish.

Mercury is a naturally occurring element that can be found throughout the environment. It is pumped into the atmosphere through the burning of fossil fuels, and is carried back to earth by rain, sometimes far away from where the initial emission took place, the Louisiana Department of Environmental Quality reports.

Mercury accumulates in sediments in waters where bacteria converts the element to its methyl form – that most readily absorbed by fish.

Scientists have identified sediments in local waterways that contain mercury deposits at least 50 years old, said Barry Kohl, a Tulane-based geologist who is working with the state to combat the mercury problem.

And it is likely that deposits date back even further.

If mercury-laden sediments are left undisturbed for a period of time, new sediment deposits can cover the contaminated ones. One foot of clean sediment can prevent further contamination of fish, Kohl said. That is, until dredging is performed on that area or new sources of mercury are added to the environment.

The concentration of mercury in the tissue of fish and other aquatic animals generally increases as small bottom feeders are eaten by larger and older predators, such as large mouth bass and King mackerel.

Flathead catfish and bowfin are also among Louisiana fish that merit mercury warnings, Kohl said. Many local fish meet the 1 part per million mercury threshold used by the U.S. Food and Drug Administration to restrict interstate sales, he said.

Accordingly, while nearly all fish contain trace amounts of mercury, larger fish typically are higher in mercury levels and pose a greater threat to human health, particularly women of childbearing age and children younger than 7.

Earlier this month, the FDA announced new, more stringent guidelines for fish consumption, advising women of childbearing age, nursing mothers and children less than seven to avoid shark, swordfish, king mackerel and tilefish, while limiting intake of fish lower in mercury levels.

Although adults are normally not permanently affected by low levels of mercury exposure, mercury is known to be toxic to the kidneys of adults and children.

The U.S. Environmental Protection Agency began issuing safe eating guidelines for fish in 1993. The state DEQ produces a similar annual report. Currently, mercury warnings are in effect for 30 Louisiana waterways and bodies, including the Blind, Bogue Falaya, Pearl, Tchefuncta and Tickfaw Rivers, the Toledo Bend Reservoir, Henderson Lake, Lake Vernon and the Gulf of Mexico.

A complete list of Louisiana advisories and the fish included in those alerts is available at

Kohl said the state began more stringently identifying problematic mercury sites in 1995. Unfortunately, since that time, none of the advisories has been lifted, he said.

“I would like to say that there’s no need for any fish advisories,” Kohl said. “The big concern we have in Louisiana is trying to reduce the known sources of mercury.”

He said a multi-pronged approach – including continued data collection, public awareness efforts and attention to the old and new sources of contamination – could significantly improve the mercury situation.

But Kohl said he does not believe the federal government is putting enough pressure on mercury-producing industries in the state and across the country to reduce discharges.