The mercury in Brownlee is not from car batteries. It comes from natural sources. This problem is found in other Oregon lakes such as Cottage Grove, Owyhee, and Antelope Reservoirs and East Lake. Here's an excerpt from an OHD document found at
http://www.ohd.hr.state.or.us/esc/docs/esc0003.htm
"The mercury in the fish is thought to be from natural volcanic and geothermal sources in the upper drainage areas, possibly influenced by historical mining practices in the watershed. Once mercury enters the food chain it bioaccumulates and is not likely to diminish or disappear from organisms in the river. The Health Division, Department of Fish and Wildlife, and the Department of Environmental Quality continue to monitor contaminant levels and will update the advisory if significant changes occur."