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April 24, 2012 > Don't be shellfish: California mussel quarantined

Don't be shellfish: California mussel quarantined

By Catherine Kirch

Starting in May, seafood lovers will find themselves without a certain sport-harvested shellfish: the mussel. Don't worry - wild mussels are not going away; they just will not be gracing your plate until November.

California Department of Public Health (CDPH) institutes annual mussel quarantines, usually from May 1 to October 31, in order to protect the public against paralytic shellfish poisoning (PSP) and domoic acid poisoning (DAP). The quarantine applies to sport-harvested - not commercially grown mussels along the California coastline.

PSP toxins and domoic acid are both naturally-occurring marine toxins produced by certain phytoplankton. These phytoplankton are a natural food source for filter-feeding animals, including bivalve shellfish like the mussel. When mussels filter the food from the seawater, they can accumulate these nerve poisons, which do not harm the mussels directly, but can remain in their system and harm their consumers.

The rise and fall of these toxins are likely related to population trends of the dinoflagellate, a small, single-celled plant and member of the phytoplankton category. Dinoflagellates can rapidly increase in number - a period called a "bloom." Peak periods of PSP and DAP toxins coincide with such blooms; the environment during the quarantine months likely provides the most desirable growth conditions for toxin-producing dinoflagellates.

A sudden increase in dinoflagellate populations can sometimes impart a brown-to-red color in the water, which is why such periods are often called "red tides." Many believe red tides cause shellfish to be toxic - this is a myth. Though red tides can coincide with toxin levels, they can also be harmless. Additionally, the absence of red-colored waters does not mean that toxins are absent as well. The color and clarity of the water is not a reliable predictor of toxin levels; both dinoflagellates and toxins can increase without a water color change.

"The occurrence of these toxins is unpredictable," says Matt Conens of CDPH, "and their increase in concentration is rapid. The annual quarantine period provides the best approach for protecting the public."

There have been 542 reported illnesses and 39 deaths attributed to mussel-related poisoning since 1927. The May-through-October timeline encompasses 99 percent of these illnesses and deaths. That being said, Conens reminds the public that "CDPH may begin the quarantine early or extend it if monitoring results indicate the presence of dangerous levels of toxins outside of the normal quarantine period."

CDPH monitors these toxins year round, measuring toxin levels in both shellfish and phytoplankton, including dinoflagellate. The efforts of the monitoring program have been successful, Conens affirms, "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 the CDPH laboratories in Richmond."

"There are currently no predictive tools for these events," says Conens. "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."

Conens reminds the public to pay attention to CDPH warnings and to not harvest wild mussels during the quarantine period: "It is very important that the public honor this quarantine because there are no known antidotes to the toxins found in mussels." PSP toxins and domoic acid have no taste or odor, and there is no way to visibly determine safe versus unsafe shellfish. Cleaning does not remove the toxins, nor does cooking destroy them.

Even during non-quarantine months, it is important to remain aware of the dangers surrounding marine toxins. Before sport-harvesting mussels on the California coast at any time of the year, contact CDPH's Shellfish Information Line at (800) 553-4133 for updated information on quarantines and shellfish toxins.

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