Draft Fish Consumption Guidelines for Trinity Lake and Selected Waterbodies in the Trinity River Watershed (Trinity County)

Oct 1, 2005

Note: this advisory was finalized in October 2005.

The United States Geological Survey (USGS) conducted a reconnaissance survey of mercury contamination in edible fish tissue from Trinity Lake and selected water bodies in the Trinity River watershed, an area possibly affected by historic gold and mercury mining. These data were evaluated by the Office of Environmental Health Hazard Assessment (OEHHA), together with fish samples collected in this region through the Surface Water Ambient Monitoring Program (SWAMP), in an effort to determine whether there may be potential adverse health effects associated with the consumption of sport fish from these water bodies.

More than 95 percent of the mercury found in fish occurs as methylmercury, which is a highly toxic form of the element. Consumption of fish is the major route of exposure to methylmercury in the United States. The critical target of methylmercury toxicity is the nervous system, particularly in developing organisms such as the fetus and young children. Significant methylmercury toxicity can occur to the fetus during pregnancy even in the absence of symptoms in the mother. In 1985, the United States Environmental Protection Agency (U.S. EPA) set a reference dose (RfD, that is the daily exposure likely to be without significant risk of deleterious effects during a lifetime) for methylmercury of 3x10 -4 mg/kg-day, based on central nervous system effects (ataxia and paresthesias) in adults. In 1995, and confirmed in 2001, this RfD was lowered to 1x10 -4 mg/kg-day, based on developmental neurologic abnormalities in infants exposed in utero , using the Iraqi and Faroe Island data, respectively. Because OEHHA finds convincing evidence that the fetus is more sensitive than adults to the neurotoxic effects of mercury, but also recognizes that fish can play an important role in a healthy diet, OEHHA chooses to use both the current and previous U.S. EPA reference doses for two distinct population groups. In this advisory, the current RfD based on effects in infants will be used for women of childbearing age and children aged 17 and younger. The previous RfD, based on effects in adults, will be used for women beyond their childbearing years and men.

In order to determine whether the issuance of fish consumption advice for a site is justified, mean mercury concentrations in fish from that site are compared to OEHHA guidance tissue levels for methylmercury, which are designed so that individuals consuming no more than a preset number of meals should not exceed the RfD for this chemical. Although Trinity Lake and the Trinity River watershed include separate water bodies, with the exception of rainbow trout at two sites, mercury levels in fish species from the various water bodies were sufficiently similar to justify combining data from the different sites so that any advice could be unified in order to facilitate communication with the public. Data for rainbow trout, however, showed that this species contained considerably lower levels of mercury at Lewiston Lake and Carrville Pond compared to other water bodies evaluated within the Trinity River watershed. Thus, rainbow trout from those two sites were evaluated separately. After combining fish mercury data for other species from all sites in the Trinity River watershed, a statistically representative sample size was available to assess the need for consumption guidelines for largemouth bass, smallmouth bass, white catfish, brown trout, and rainbow trout. Supporting data (such as contamination data for a closely related species at a similar trophic level) were used to develop additional consumption guidelines for other sport fish, as appropriate.

Evaluation of data and comparison with guidance tissue levels for methylmercury indicated that development of fish consumption advisories was appropriate for Trinity Lake and other Trinity River watershed water bodies. Consumers should be informed of the potential hazards from eating fish from these water bodies, particularly those hazards relating to the developing fetus and children. All individuals, especially women of childbearing age and children aged 17 and younger, are advised to limit their fish consumption to reduce methylmercury ingestion to a level as close to the reference dose as possible. To help sport fish consumers achieve this goal, OEHHA has developed advisories for all black bass species (largemouth, smallmouth and spotted bass), white catfish, Chinook salmon, and all trout species. For other fish species not included in this evaluation, but potentially found in these water bodies (e.g., green sunfish, Kokanee salmon, brown and black bullhead), OEHHA advises that women of childbearing age and children aged 17 and younger follow the recent U.S. EPA and U.S. Food and Drug Administration (U.S. FDA) Joint Federal Advisory for Mercury in Fish. This advisory recommends that pregnant women or women who may become pregnant, nursing mothers and young children consume no more than one meal per week of locally caught fish, when no other advice is available, and eat no other fish that week. OEHHA recommends that children through age 17 also follow this advice because of continued nervous system development during adolescence. Additionally, OEHHA recommends that women beyond their childbearing years and men consume no more than three meals per week of sport fish, if other advice is not already in place for the water body where they fish. These advisories and additional guidelines are provided in this report. Meal sizes should be adjusted to body weight as described in the advisory table.

For general advice on how to limit your exposure to chemical contaminants in sport fish (e.g., eating smaller fish of legal size), as well as a fact sheet on methylmercury in sport fish, see the Fish Advisories on OEHHA's website, www.oehha.ca.gov. It should be noted that, unlike the case for many organic contaminants, various cooking and cleaning techniques will not reduce the methylmercury content of fish.