Many people fish for recreation; they also eat what they catch. But the U.S. Geological Survey says they also could be ingesting unhealthy levels of mercury.
A multi-year study discovered mercury in all 291 streams it checked, including some of the most popular for fishing in Northern California -- the Sacramento, San Joaquin and Carson rivers.
"If you're doing sport fishing and you're planning to bring that fish home to have for dinner or to barbecue, I'd be very concerned, I'd be very cautious before I would ingest any fish that might be contaminated," Dr. Clarence Braddock said.
Levels in 25 percent of the samples were above criteria for safe eating.
Scientists said old mines are the source of California's contamination.
Tony Partal fishes and eats his catch. He is aware of the mercury issue.
"It is a concern, we watch the regulations and the warnings and avoid those species that has the most mercury in them," Partal said.
Many consumers say they have turned to fish for health reasons. Some say they have fish as often as three times a week.
At San Jose's Race Street Fish and Poultry Market, the mercury question comes up frequently.
"They're mostly concerned with the salt water fish rather than the fresh water fish and the amount of mercury; I don't think people actually consider the idea of mercury being in the fresh water supply," manager Eric Conover said.
The state office of environmental health says farm-raised fish typically has very low levels of mercury.
Braddock says mercury levels in fish are important because the symptoms can be overlooked at first.
"They may include fatigue, people may notice increased salivation, but by the time those symptoms come that are more specific to mercury poisoning, the poisoning's at a pretty high level," Braddock said.