SHASTA LAKE, Calif. (AP) – Shasta Lake provides water to Sacramento and the crop-rich San Joaquin Valley more than 300 miles to the south.

It’s California’s largest reservoir and considered a jewel by anglers – many unaware of the mercury in the lake and in the fish they catch.

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Mercury, a legacy of the Gold Rush, has worked its way into the food chain of Shasta Lake, with levels beyond what is considered safe by the Environmental Protection Agency, state tests have found.

Yet no advisory has been issued to people who fish the lake, the San Francisco Chronicle reported Sunday.

Change may come this summer when officials at the California Office of Environmental Health Hazard Assessment, the state’s lead agency for environmental health risks, begins re-evaluating 2007 state tests to determine whether the reservoir’s game fish pose a danger to human health.

Officials could decide to list an advisory in the booklet given to all anglers who obtain a California freshwater fishing license, and the U.S. Bureau of Reclamation could then consider posting warning signs along the lake.

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Methlymercury limits set by the EPA for safe eating are 0.3 milligram per kilogram. Data from the 2007 state tests reveal bass with levels just under 0.5 milligram per kilogram and catfish at more than double the limit.

By comparison, testing of rainbow trout from 2002 and 2006 shows safe levels between 0.1 and 0.2 milligram per kilogram. Brown trout, a commonly eaten predatory fish typically higher in mercury than rainbow trout, could prove to be the deciding species as officials decide if fish in Shasta Lake should be listed as contaminated.

“That’s why we need more testing,” Brodberg said. “We don’t like to issue an advisory for one fish. We like to have at least three species of fish and nine samples of each of them.”

Runoff from sites once active with gold mining is the main source of mercury in the lake. Gold mining requires mercury, and there are many shuttered mines around the lake.

The widespread practice of gold panning is most to blame for leftover mercury in streams that feed into lake, said Jay Thompson of the Shasta Historical Society.

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Copyright 2015 The Associated Press.