Mercury in Fish Tissue

Summary

Mercury in fish below concentrations harmful to people eating traditional or subsistence amounts of fish and animals that rely fish.

General Information
What is it?

Mercury in fish tissue is an important measure of water and sediment quality. For mercury to increase in concentration in fish tissue, it must be available in the environment (water and/or sediment) and methylated, usually by bacteria in hypoxic/anoxic conditions. Trace metals such as mercury are natural components of rocks and soil and can enter the aquatic environments as a consequence of weathering and erosion (Garrett 2000). However, following gold-mining in California and global industrialization, unnatural quantities of many elements were and continue to be released into the riverine and coastal ecosystems, altering natural biological equilibrium (Haynes and Johnson 2000). Anthropogenic activities have altered both the distribution of metals in the environment as well as metal speciation, or biochemical form (Goyer and Clarkson 2001). Mercury primarily enters aquatic ecosystems in inorganic dissolved ionic or particulate form through wet and dry deposition from the atmosphere to water body surfaces, or via runoff from watersheds (Selin 2009). Although mercury occurs naturally in the environment, human activities such as gold mining and recovery, burning fossil fuels and waste, mining and smelting metals, and using mercury in products and industrial processes have added to the amount of mercury in the global environment. Ecosystems near point sources may be characterized by higher mercury concentrations, but due to global transport ecosystems that are far from any point sources may also have high mercury concentrations (Scheulhammer et al. 2007). Biological magnification is the process by which toxins become more concentrated in organisms at each successive trophic level up the food chain. Due to the tendency for mercury to bioaccumulate and magnify up the food chain, organisms at higher trophic levels on the food chain and with long life spans tend to be at greatest risk (Scheulhammer et al. 2007). Mercury concentrations in the tissues of top predators can be over one million times greater than those in the water column (Selin 2009). Furthermore, the proportion of methylmercury to total mercury increases at each trophic level in the food chain; in predatory fish, almost 100% of the mercury is methylmercury (EPA 1997, Morel et al. 1998).

Why is it important?

High enough concentrations are present in many waterways and wetlands to harm piscivorous birds and mammals, and groups of people that eat fish for traditional, subsistence or dietary reasons. Members of many California tribes historically ate large quantities of fish and in some cases maintain this legally-protected tradition (Shilling et al., 2014). Their ability to carry out traditional and subsistence use of fish is harmed by the presence of mercury. There are birds (e.g., terns) and mammals (e.g., otters) that naturally eat large quantities of fish. In areas of high concentrations of mercury in fish, they may be harmed, including reproductive failure, by eating their natural diet. Mercury is a trace metal of particular concern because it can exert negative toxic effects on humans and wildlife at low concentrations (Selin 2009). Methylmercury has been shown to cause massive neurological damage because of its ability to cross the blood-brain barrier, while inorganic forms can cause nephrotoxicity (Boelsterli 2007). The major pathway for human exposure to methylmercury is consumption of contaminated fish. Dietary methylmercury is almost completely absorbed into the blood and is distributed to all tissues including the brain; it also readily passes through the placenta to the fetus and fetal brain (EPA 2001). Based on available data, human exposures to methylmercury from all media sources except freshwater/estuarine and marine fish are negligible. Therefore, the EPA methylmercury water quality criterion is a concentration in fish tissue (EPA 2001).

What can Influence or Stress Condition?

There are several contributing factors to mercury accumulating in fish. One is availability of mercury, initially in the inorganic form and then in the organic-methylated form. Inorganic mercury can enter a waterway from atmospheric deposition (e.g., from fossil fuel burning) and from terrestrial sources (e.g., legacy mining areas). It can be transported and stored at various locations along a waterway. It may be eroded from legacy mines during rain events, transported at high flows downstream, and then stored in benthic sediments in reservoir. It may be stirred up and transported further downstream during storm or other events. Methylation of mercury usually is carried out by bacteria in benthic sediments. At higher temperatures or nutrient conditions, there may be higher rates of bacterial growth and methylation of mercury.

Target or Desired Condition

Mercury concentrations in fish were transformed into a corresponding score based on 1) low concentrations of mercury found in certain species (e.g., trout in montane systems) and smaller individuals of edible species and 2) EPA Reference Dose (RfD), Relative Source Contribution (RSC) and Tissue Residue Criterion (TRC) (EPA 2001). The quantitative health risk assessment for a non-carcinogen relies on an RfD, an estimate of a daily exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious health effects during a lifetime. The RSC is used to adjust the RfD to ensure that the water quality criterion is protective, given other anticipated sources of exposure. Finally, the TRC is the concentration in fish tissue that should not be exceeded based on a total fish and shellfish consumption weighted rate. The target condition is concentrations of <0.05 ppm mercury in fish. This concentration is roughly the background concentration expected in fish with low inputs of mercury in the environment and should protect traditional and subsistence rates of fish consumption. The target undesired condition is the concentration that would protect fish consumption at less than one 4-oz serving per day, >0.3 ppm mercury in fish. Score = 0 for concentrations > 0.3 ppm (corresponds to increased risk to human health via fish consumption) 100 for concentrations < 0.05 ppm (corresponds to little to no human health risk via fish consumption) Concentrations between 0.05 ppm and 0.3 ppm were given a corresponding proportional score using a 1:1 linear transformation.

References

Boelsterli, U. A. 2007. Mechanistic toxicology: the molecular basis of how chemicals disrupt biological targets. CRC Press, Boca Raton.

EPA (U.S. Environmental Protection Agency). 2006. EPA's Roadmap for Mercury. Office of Science and Technology. Washington, D.C.

EPA (U.S. Environmental Protection Agency). 1997. Mercury Study Report to Congress. Office of Science and Technology. Washington, D.C.

EPA (U.S. Environmental Protection Agency). 2001. Water quality criterion for the protection of human health: methylmercury. Office of Science and Technology, EPA-823-R-01-001. Washington, D.C.

FDA (Food and Drug Administration). 2001. Fish and fisheries products hazards and controls guidance, third edition. http://www.fda.gov/Food/GuidanceComplianceRegulatory Information/GuidanceDocuments/Seafood/ucm092041.htm. Accessed March 28th, 2010.

Morel, F.M., Kraepiel, A.M. and Amyot, M. 1998. The chemical cycle and bioaccumulation of mercury. Annual Review of Ecology and Systematics 29:543‐566.

Scheulhammer, A.M., et al. 2007. Effects of environmental methylmercury on the health of wild birds, mammals, and fish. Ambio 36(1):12‐18.

Selin, N.E., Global Biogeochemical Cycling of Mercury: A Review. Annual Review of Environment 177 and Resources, 2009. 34: p. 43‐63.

Shilling, F. White, A. Lippert, L. Lubell, M. 2010. Contaminated fish consumption in California’s Central Valley Delta. Environmental Research. Doi:10.1016/j.envres.2010.02.002

Shilling, F.M., A. Negrette, L. Biondini, and S. Cardenas 2014. California tribes’ fish-use: Final Report. Report for the State Water Resources Control Board and the US Environmental Protection Agency. 56 pages.