Anyone who has wandered San Francisco's waterfront has likely seen the men and women, poles and nets in hand, who fish from the city's many piers. And just as likely, they've also seen the California Department of Public Health signs warning that some Bay fish contain high levels of mercury and should not be consumed. Now, thanks to a University of California at Santa Cruz study picked up by KRON4, we can add a liver-damaging toxin to seafood consumption concerns.

In this case it's mussels that have caught the attention of scientists. According to the study, researchers pulled samples from five sites around the Bay and tested them for a toxin by the name of microcystin. Microcystin is the byproduct of a type of algae that thrives in the Sacramento and San Joaquin rivers — rivers that flow into the Bay Delta.

“We found that this freshwater toxin accumulates in shellfish, both mussels and oysters, and that in San Francisco Bay, the toxin levels in some mussels exceed the recommended guidelines for consumption by quite a bit,” UCSC professor Raphael Kudela, who led the study, said in a statement. High levels of the toxin can cause liver damage in humans. Marine mammals like sea otters that eat large amounts of shellfish are particularly at risk.

So why are we just learning about this now? "There is monitoring of shellfish for marine-derived toxins, but because this is a freshwater toxin, no one has been looking for it [in the Bay]," Corinne Gibble, who worked on the research project, said in a statement. "Now it seems microcystin is something we should be monitoring as well.”

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