We identify which species of fish and which harvest locations expose consumers the most to potentially toxic mercury. With this information, we seek to develop the ability to predict which species and areas (hot spots) present the greatest risk to human health, so that managers and legislators can develop effective means to reduce this risk.

Why We Care
Eating seafood is the main way you are most likely to be exposed to toxic mercury. If we know what species of fish and shellfish different groups of people eat, the amounts of mercury in these species, and where the fish and shellfish come from, we are able to better guide policies to protect people. Identifying hot spots—areas where mercury is at high levels in fish and shellfish—is a critical part of this effort. We are identifying hot spots by monitoring mercury levels in various seafood species and by conducting research to understand the processes and sources contributing to these high levels.

What We Are Doing
We measure mercury concentrations in the edible tissues of fish from regions of differing mercury exposure. We also measure mercury in some of the marine organisms eaten by these fish and in water and sediments in their habitats. We look for characteristics of these habitats where mercury can be transformed into methylmercury which is the chemical form of mercury that is most readily accumulated and is most toxic to marine life. We pursue this effort mostly in the Gulf of Mexico, but expect the results to have wider applicability.

What We Have Found
Not all of the fish we catch and eat from the Gulf of Mexico have the same mercury concentration. Among the characteristics of fish that seem to be associated with high mercury concentrations are:

• Fish at the top of the food web, such as king mackerel.
• Pelagic fish (live in the water column, away from the ocean bottom and the shore), such as bluefish.
• Older, larger fish with high energy requirements but slow growth, such as blue marlin caught in tournaments.

We have found exceptions, however. Dolphinfish, an energetic pelagic top predator, has relatively low mercury concentrations, probably because it grows rapidly and efficiently. Golden tilefish, a bottom dwelling fish living at the edge of the continental shelf, has high mercury concentrations averaging about 1 part per million, enough for the Food and Drug Administration (FDA) to include this species among the four groups that warrant greatest concern. Golden tilefish are slow growers and seem to acquire their elevated mercury levels over a lifetime of decades.

Some habitats, such as Lavaca Bay in Texas, have high mercury concentrations in resident fish and crabs because of local industrial mercury pollution; although, mercury discharges largely ended there more than 40 years ago. Mercury that still remains in Lavaca Bay bottom sediments seems be a continuing source of methylmercury that is transferred to sea life, enough so that harvesting around the former discharge area is still prohibited.

Areas that show up as hot spots, although there seems to be no local point source, are often characterized by a few critical attributes:

• Proximity to coastal wetlands where methylmercury can be produced efficiently.
• Slow flushing of the water column by river or tidal waters, which allows methylmercury concentrations to build up.
• Waters low in nutrients, which results in low biological productivity. This allows methylmercury concentrations to remain high in the water, suspended microalgae, and the associated food web, which supports top predator fish.
• Sediments that are not rich in organic matter, such that the buildup of sulfides in the sediments does not limit the production of methylmercury.

Many estuaries along the Gulf coast of Florida have these characteristics. We have found that the mercury concentrations in many species of fish here are higher than elsewhere, with the extreme south of Florida, in Florida Bay, having the highest concentrations.

Next Steps
We are using 20 years of research on mercury in the coastal environment to develop predictive models that will allow managers and researchers to predict mercury hot spots. These models, based on existing habitat characterizations and routine monitoring, will permit development of effective mitigation strategies that can reduce human exposure to mercury from seafood.