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Marine Biotoxin Monitoring Program

Annual Mussel Quarantine - Frequently Asked Questions (FAQs)


What is the annual mussel quarantine?

The mussel quarantine is a yearly event prohibiting the public from harvesting mussels for human consumption due to potentially dangerous levels of biotoxins that may be present in shellfish anywhere on the California coast, including bays, inlets and harbors. The annual mussel quarantine is in place to protect the public against paralytic shellfish poisoning (PSP) and domoic acid poisoning, also known as Amnesic Shellfish Poisoning.

The quarantine applies only to sport-harvested mussels. Other types of bivalve shellfish, including oysters and clams, and commercially harvested mussels from certified companies are not included in the quarantine. 

When is the annual mussel quarantine?

The annual quarantine is normally in effect from May 1 through October 31.

However, the California Department of Public Health (CDPH) may begin the quarantine early, or extend it, if monitoring results indicate the presence of dangerous levels of biotoxins outside of the normal quarantine period. The May through October quarantine period encompasses more than 99 percent of all PSP illnesses and deaths reported in California since 1927.

Where is the annual mussel quarantine in effect?

The mussel quarantine is in effect along the California coast, from the Oregon border to the Mexican border. All bays, inlets and harbors are included.

Why is there an annual mussel quarantine?

The annual quarantine is in place so the public does not collect mussels during this high-risk period for marine toxins. The occurrence of biotoxins in mussels is unpredictable and they can increase in concentration very rapidly. Therefore, the annual quarantine period is the best approach for protecting the public from these potentially deadly biotoxins.

Why do these biotoxins occur at this time?

The peak period for the toxins can coincide with the most desirable growth conditions for the naturally occurring phytoplankton, or algae, that produce these toxins.

Certain single-celled algae species that produce toxins usually occur at very low concentrations and pose no problem. However, when the algae "blooms" the concentration of the toxin producing species can increase dramatically. The increased amount of algae becomes a greater food source for bivalve shellfish (mussels, clams, oysters, and scallops). As the shellfish filter and eat the toxic algae, they accumulate biotoxins in their tissues. Biotoxins don't harm shellfish, so the level in their tissue continues to increase until the bloom subsides. When the number of toxin-producing algal cells returns to low levels or disappear, the shellfish eventually flush the toxin from their systems. It can be several days to several months or longer before they're safe to eat again.

How dangerous are PSP and domoic acid?

These toxins are very dangerous and have been responsible for many deaths worldwide.  Since 1903, 582 total cases including 543 illnesses and 39 deaths have been attributed to PSP in California.  Death can occur within 30 minutes of consuming toxic shellfish. There are no known antidotes to these toxins found in mussels. Cooking does not reliably destroy the toxins.  

In California, PSP is extremely rare due to the efforts put forth by CDPH to protect the public against poisoning.  There have been no reported human cases of domoic acid poisoning in California.

What are the symptoms of PSP?

Early symptoms include tingling of the lips and tongue, which may begin within minutes of eating toxic shellfish or may take an hour or two to develop. Symptoms may progress to tingling of fingers and toes and then loss of control of arms and legs, followed by difficulty in breathing. Some people feel nauseous or experience a sense of floating. If a person consumes enough toxin, muscles of the chest and abdomen become paralyzed, including muscles used for breathing, and the victim can suffocate. Death from PSP has occurred in less than 30 minutes.

What are the symptoms of domoic acid poisoning?

Symptoms include vomiting, nausea, diarrhea, and abdominal cramps within 24 hours of ingestion. In severe cases, more serious symptoms develop within 48 hours and include headache, dizziness, confusion, disorientation, loss of short-term memory, muscle weakness, seizures, breathing difficulty, heart rhythm problems, coma and possibly death.

What should I do if I think I have eaten toxic mussels?

If you think you have eaten toxic mussels and have any of the symptoms described above, seek immediate emergency medical care at a hospital or call 911. Even suspected cases should be reported immediately by telephone to the local health department and to the nearest poison control center. If any of the mussels remain, they should be refrigerated for later testing by CDPH or another authorized laboratory.

Can I tell if the mussels along the coast are toxic?

There is no way to visibly determine if mussels or any other seafood contain dangerous levels of these toxins. The toxins do not affect the appearance or behavior of the shellfish in any way. The only way to tell if the shellfish are toxic is through testing by an approved laboratory.

How are the toxins detected or predicted?

CDPH maintains a year-round monitoring program for these toxins in shellfish, as well as for the phytoplankton species that produce these toxins. This effort has been successful thanks to the participation of many of the coastal county health departments, State and federal agencies, and citizen volunteers who collect the shellfish and plankton samples for analysis by scientists at the CDPH State Public Health Laboratory in Richmond. There are currently no predictive tools for these events, so routine monitoring of the California coastline provides the best approach for the early detection of a toxic bloom so that the public can be alerted.

Why aren't other shellfish, like oysters and clams, included in the annual quarantine?

Mussels are a particularly high risk because they can concentrate these toxins very quickly. When dangerous levels of toxin are detected in mussels, CDPH will begin testing other shellfish species for these toxins if samples are readily available. If samples are not easily obtained, or if toxin levels are increasing rapidly, CDPH will issue a press release announcing a special health advisory for the potentially toxic seafood species in the affected area. CDPH is assisted by local county health departments to make sure the public receives this information.

Can I eat mussels that are commercially grown during the quarantine?

Yes. Commercially grown mussels from certified companies are exempt from this quarantine.

Why are commercially grown mussels exempt from this quarantine?

CDPH regulates all commercial bivalve shellfish harvesters in the state. All of these certified companies are required to submit frequent shellfish samples to CDPH for toxin testing. If any sample exceeds the federal alert level for the PSP or domoic acid toxins, then that area is immediately closed to harvesting until conditions become safe.

All commercial shellfish harvesters in California are aquaculture operations, growing mussels, oysters and clams from 'seed' that are produced in hatcheries. Aquaculture operations occur on defined leases, making it easier to monitor and track toxin levels.

Is it safe to eat sport-harvested mussels when the annual quarantine is not in effect?

No, you should never assume that sport-harvested mussels are safe to eat when the annual quarantine is not in effect. The toxins can occur at any time, which is why CDPH continues to monitor for them throughout the year. If dangerous levels of toxins are detected when the quarantine is not in effect, CDPH will issue a health advisory to warn the public to avoid consuming certain seafood in the affected areas.

The public can view an interactive recreational bivalve shellfish advisory map or call the Biotoxin Information Line 1-800-553-4133 to get updates on current quarantines and health advisories throughout the year.

How can I get involved with the biotoxin monitoring program?

CDPH is always interested in having new volunteers join the shellfish and phytoplankton monitoring programs, especially if they are in an area that needs additional coverage. Our ability to protect the public from these dangerous toxins is due in large part to the numerous organizations and citizen volunteers that contribute their time and effort to provide samples to our program for testing. CDPH can provide the necessary training and equipment for collecting and shipping samples at no cost to the volunteer. Program participants are acknowledged in our monthly biotoxin reports and are added to our distribution list for reports and updates. 

To learn more about volunteer opportunities, email or call 1-800-553-4133.


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