Methylmercury in Fish from Lake Pillsbury (lake County): Guidelines for Sport Fish Consumption

FOREWORD This report provides recommended guidelines for consumption of fish from Lake Pillsbury (Lake County). These guidelines are provided to the public as a result of findings of high levels of mercury in fish tested from Lake Pillsbury. These recommendations are provided to protect against possible adverse health effects that may result from consumption of mercury-contaminated fish. The report provides background information and a description of the data and criteria used to develop the guidelines. INTRODUCTION Mercury is a heavy metal that can be toxic to humans and other organisms. Mercury occurs naturally in the environment, and exists in various forms including elemental or metallic mercury, inorganic, and organic mercury (ATSDR, 1999; IARC, 1993). Cinnabar ores, naturally rich in mercury, are common in northern California. In fact, the California Coast Range contains one of the world's largest geologic deposits of mercury (SFEI, 1996). Extensive mining of this ore in the 1800's and early 1900's, largely to support gold mining operations, resulted in mobilized deposits of mercury in watersheds of the coastal mountains. Mercury continues to leach from scores of abandoned mines in the state. Thus, mercury contamination occurs as a result of both natural and anthropogenic sources. Mercury has also been commonly used in electrical equipment, medical supplies, dental amalgams, and as a fungicide in paints and on seeds and grains. Once mercury is released into the environment, it cycles through land, air, and water. Many streams and other water bodies in northern California have sediments containing mercury at relatively high levels. The deposition of mercury in aquatic ecosystems is a concern for public and environmental health because inorganic mercury can be converted by microorganisms (bacteria and fungi) in the sediments into methylmercury, a particularly toxic form of mercury. The physical, chemical, and biological characteristics of water bodies affect the propensity for mercury to be converted to methylmercury. Methylmercury is taken up by organisms dwelling in the sediments and then subsequently by larger aquatic animals, including fish, that feed on the smaller organisms. In this way, methylmercury accumulates or "biomagnifies" in the aquatic food chain, reaching the highest levels in fish and other organisms at the top of the food web. Concentrations of methylmercury in fish tissues can thus be orders of magnitude (e.g.,10, 100, or 1000 times) greater than concentrations in water. High levels of methylmercury in fish pose a health concern to humans and other organisms that consume …