Environmentalists call it "legacy mercury" - the pollution left behind from the Gold Rush mining days when mercury was used to purify gold.
"In our rivers and creeks you can suck mercury up with a turkey baster, you don't have to measure it at parts-per-billion."
Elizabeth Martin heads The Sierra Fund. The Nevada City based group conducted a survey of anglers at Sierra waterways like Rollins Reservoir and Lake Englebright.
Science Director Carrie Monohan says more than 90% of the people surveyed said they intended to eat the fish they caught.
"Half of the people that were going to eat the fish that they were going to catch that day were indeed taking it home to women and children. Now this is important because women and children are a known sensitive population."
The Sierra Fund is pushing for more signage warning anglers about mercury in Sierra rivers and streams, including state-issued safe eating guidelines.