San Francisco Bay mussels contain high levels of a toxin that can cause liver damage, according to a study announced by UC Santa Cruz.
The toxin called microcystin is produced by a type of cyanobacteria, or blue-green algae, that thrives in warm, nutrient-rich water. It has also been found in the Sacramento and San Joaquin rivers, which flow into the San Francisco Bay Delta, and in Lake Anza and Lake Temescal.
Researchers in UC Santa Cruz professor of ocean health Raphael Kudela's lab tested mussels collected from five sites in the Bay, and did experiments with mussels and oysters in tanks to test how quickly the shellfish take up the toxin, as well as how long it takes to clear it from their tissue.
"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," Kudela said in a statement.
Microcystin was also found in commercial oysters from Tomales Bay, but at low enough levels that it did not pose a risk, according to university officials.
"There is monitoring of shellfish for marine-derived toxins, but because this is a freshwater toxin, no one has been looking for it. Now it seems microcystin is something we should be monitoring as well," environmental scientist Corinne Gibble, who worked on the study as a graduate student, said in a statement.
Typically a state quarantine on harvesting mussels for human consumption is put into place from May to October.
"The agencies have been very responsive. 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 said.
However, sea otters and other marine mammals are at risk because of the large amount of shellfish they eat.
This really highlights the connectivity between what people do on land and what happens in the ocean. A lot of shellfish farms are downstream from freshwater sources, so we want to raise awareness of this issue," Kudela said.
"The rains help by flushing things out. Warm, dry conditions favor these blooms, so we've been seeing more of them lately than we would without the drought," he said.