Toxin forces order to stop eating recreationally caught shellfish in Bay Area

An unseasonably warm winter is putting a pause on the Bay Area’s recreational seafood catch, bringing elevated levels of a toxin found in shellfish much earlier in the year than usual.

The California Department of Public Health warned residents in Marin, San Francisco and San Mateo counties to refrain from harvesting and consuming mussels, clams, oysters and other bivalve shellfish due to harmful levels of paralytic shellfish poison.

The warning only applies to recreational harvests, as commercially harvested shellfish are regularly screened for toxicity levels before sale.

Paralytic shellfish poison, also called PSP, is a biotoxin produced by algae. Shellfish that eat these algae can retain the toxin, which can affect the central nervous system if consumed by humans.

Marin County recorded particularly alarming toxin levels. Recent mussel samples taken in Marin County from the Chimney Rock sentinel station, in Point Reyes National Seashore, contained 37 times the level of PSP that would be considered harmful for human consumption.

“We were really surprised by the levels,” Marin County public health officer Matthew Willis said. “It’s the first time I’ve sent out a public health advisory about this.”

The levels are the highest recorded in Marin in 20 years and are potentially fatal, Willis said, but no PSP-related illnesses have been reported recently.

Symptoms of PSP poisoning include tingling around the mouth and fingertips, followed by loss of balance and muscle control, slurred speech and difficulty swallowing. In severe cases, complete paralysis and death from asphyxiation can occur.

High PSP levels can be brought on by changes in water temperature, flow and salinity. The state imposes an annual quarantine on mussel harvesting from May to October due to elevated PSP risk from the warmer conditions, but Willis said it was unusual for such high levels to be present this early in the year.

“Right now it’s at a time of year when people night not be expecting levels to be so high, even higher than in the summertime previously,” Willis said. “It’s just another sign of the extreme weather events and the changing climate.”

Over the past 90 years, state statistics show 542 reported illnesses and 39 deaths related to PSP in California.

Willis said he had reached out to the National Park Service to post warning signs along Drakes Bay to warn against recreational shellfish harvest while the toxin levels are elevated.

Anyone who experiences symptoms of PSP poisoning, including numbness, tingling, headaches, dizziness, nausea, rapid pain or respiratory problems after eating any type of shellfish should seek medical attention immediately, Willis said.

Annie Ma is a San Francisco Chronicle staff writer. Email: Twitter: @anniema15

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