Scientists with the University of California, Santa Cruz have detected a liver-damaging toxin in Bay Area shellfish.
The toxin, called microcystin, is produced by freshwater algae and is plaguing local lakes and rivers, including the Sacramento and San Joaquin Rivers, which flow into the San Francisco Bay Delta, and other area lakes.
"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," Raphael Kudela, a professor at UC Santa Cruz, said in a statement.
Researchers also detected low levels of the toxin in Tomales Bay oysters.
"There is monitoring of shellfish for marine-derived toxins, but because this is a freshwater toxin no one has been looking for it,” said Corinne Gibble, an environmental scientist with the California Department of Fish and Wildlife. “Now it seems microcystin is something we should be monitoring as well.”
State officials typically don’t allow mussels to be harvested for human consumption from May through October. They are now testing this freshwater toxin to determine the best way to respond to it.
"There is potential for this toxin to affect humans, but most of our samples are still below the recommended limits for human consumption, so people shouldn't panic and think they can't eat shellfish,” Kudela cautioned.
A larger concern, he said, is the potential impact on marine mammals such as sea otters, which consume large amounts of shellfish.
The production of microcystin has in the past forced officials to close public swimming areas at Lake Anza in Berkeley and Lake Temescal in Oakland.
According to Kudela, California’s historic ongoing drought has worsened the problem.
"The rains help by flushing things out,” he said. “Warm, dry conditions favor these blooms, so we've been seeing more of them lately than we would without the drought.”