Mercury is a metal that comes from natural sources, mining, and air fallout from burning coal and other fuels. The Northern California Coast Range, where Clear Lake is located, is naturally rich in mercury and other ores. The Sulphur Bank Mercury Mine, located on the shore of Clear Lake, has caused mercury contamination of the lake sediments
Once mercury gets into the water, it settles to the bottom where bacteria in the mud or sand change it to the organic form methylmercury. Methylmercury, a more toxic form of mercury, gets into fish and shellfish through the food they eat. It is passed up the food chain from small plants and animals to larger, older fish, where it builds up.
For more information visit the Environmental Health Mercury in Fish page.