The quarantine of sport-harvested mussels is a yearly event run by the California Department of Public Health to isolate mussels containing dangerous amounts of biotoxins. This quarantine encompasses all mussels harvested for sport from harbors, inlets and bays as well as the open ocean, and is normally in effect from May 1 through Oct. 31. 

The quarantine is in place to protect the public against paralytic shellfish poisoning and domoic acid poisoning. These toxins are highly dangerous and have been responsible for many deaths worldwide. Death can occur within 30 minutes of consuming mussels with high quantities of these biotoxins, and there are no known antidotes. Cooking does not reliably destroy toxins.

In California, deaths from paralytic shellfish poisoning are extremely rare on account of the strict measures enforced by the California Department of Public Health. There have been no reported human cases of domoic acid poisoning in California.

Mussels are at a particularly high risk for accumulating these toxins, but other shellfish such as oysters and clams are not.

Ben Ruttenberg, the director of Cal Poly’s Center for Coastal Marine Sciences, is the lead scientist on the Pismo Clam project, evaluating factors controlling the decline and recovery of Pismo Beach clams.

“The toxins generally don’t harm the shellfish, so as far as we know, they don’t impact the clams,” Ruttenberg said. “Our monitoring data show consistent abundance trends across winter, spring and summer surveys, which reinforces the idea that the blooms don’t hurt the clams.” 

The months from May to October are considered the high-risk period for marine toxins. However, the California Department of Public Health can begin the quarantine early or extend it, if the monitoring systems in place indicate dangerous levels of biotoxins outside the quarantine window. 

For more information, view the California Department of Public Health’s page on the Annual Mussel Quarantine.