But another at-risk population in the Bay Area, she said, are lower income folks, who do spend time fishing out on the piers in Martinez, Berkeley, Pinole and other East Bay cities every season not only for recreation, but to supplement the family dinner table. The striped bass, sturgeon and halibut they bring home can be loaded with mercury, which is widespread in the bay but impossible to detect with the naked eye.

“Mercury is invisible and prevalent throughout the bay system,” said Sejal Choksi, executive director of San Francisco Baykeeper, an environmental group that works to reduce pollution in the bay.

Once known as “mad hatter disease” after the afflicted Victorian hatmakers who used mercury to produce the felt in their wares, the creeping symptoms of mercury include tremors, problems with vision, hearing, nausea and vomiting, as well as stranger effects like pathological shyness and irritability. The toxin can cause permanent damage to the central nervous system.

Anyone with an immune-compromised system is at greater risk for deleterious effects of mercury, which is also neurotoxic to developing brains, making it especially dangerous for pregnant and nursing women, babies, and small children.

Mercury is found primarily around the bay in a red rock known as cinnabar. When it settles in waterways, bacteria transform it into a highly toxic form known as methyl mercury, which is easily absorbed by marine plants and the tiny aquatic organisms that eat them.

“In wildlife, mercury in high concentrations can cause developmental problems, just as it does in humans,” said Choksi. “If you’ve got mercury impairing wildlife and their immune systems, then they’re more susceptible to infectious diseases; they can have cancerous growths. It’s pretty much the same as in the human population.”

It doesn’t take much to constitute a problem. Mercury pollution is measured in parts per billion – the amount contained in a drop of water in a backyard swimming pool.

“So the amount you might find in an old thermometer is enough to cause significant contamination,” said Bruce Wolfe, executive officer with the San Francisco Bay Regional Water Quality Control Board, the state agency that oversees water pollution in the Bay Area.

So where does all this mercury come from? Mercury enters the bay watershed from a number of sources, including stormwater and wastewater runoff from local oil refineries and cement kilns. Significant quantities also drift through the air from coal-burning power plants in China.

But the biggest culprit can be found at very root of California’s history and prosperity. In the 19th century, Gold Rush miners also mined mercury in copious amounts in the cinnabar-rich hills just south of San Jose. To extract mercury, crushed ore was heated in furnaces and transformed into a vapor. As the gas cooled and condensed, it turned into a liquid form known as quicksilver, which is naturally attracted to gold. Sierra miners used it to separate gold from crushed rock.

By the early 1900s, miners had switched to cyanide to extract gold, but mercury still had many uses – in industry, medicine, dentistry (it was used for fillings) and common household products. Even though the mines in the Almaden Hills near San Jose closed decades ago, all that mining left behind a legacy -- rocky deposits from the old furnaces are still leaching mercury into the surrounding creeks and rivers, which eventually drain into San Francisco Bay.

Roughly 2,000 pounds of mercury enter the bay each year from all these different sources. The bay is slowly cleaning itself, washing an estimated 3,100 pounds a year out to sea. But at the present rate, it will take generations for the bay to flush out so much mercury that fish are no longer contaminated.

To speed up the process, in 2008 the regional water board launched an ambitious, multi-billion dollar cleanup plan called a Total Daily Maximum Load. The multifaceted plan aimed to reduce both the mercury entering the bay and the amount of the toxin that converts to its poisonous methylmercury form. The plan also provided for advanced monitoring to better understand how mercury makes its way through the watershed.

Seven years after the TMDL plan went into effect, progress has been made in reducing urban wastewater runoff. Most of the contaminated South Bay mining waste sites have been, or are being, cleaned up, and efforts are underway to remove toxic sediment within the Guadalupe River and its tributaries and reservoirs.

But this accounts for only a small fraction of the total load entering the Bay. The greatest source is the legacy poison on the bay floor, which steadily erodes over time and is nearly impossible to clean up. Seven years after the TMDL went into effect, toxic levels in fish and wildlife remain as high as ever.

There is a tentative revision of the TMDL planned for 2018. In the meantime, the Water Board estimates that it will take more than 100 years for the Bay to recover. At a minimum, three generations will be impacted by the potent and long-lasting poison still lingering in the bay mud.

Environmental groups say that’s too long to wait for cleaner waters. They want to see enforceable urban stormwater limits for mercury, an accounting of mercury pollution from crude oil refineries, and a full inventory of old mining sites.

“This process gets you a lot of planning and paperwork but not tangible reduction of mercury in the bay,” said Choksi. “We want to see zero mercury in the bay and we want to see it soon.”

Click here for the state’s advisory on eating fish from the San Francisco Bay.

Click here to listen to tips from Dr. Jane Hightower about how to avoid mercury in your diet.