Skip to main content Skip to site navigation

Shellfish Safety

General information

San Mateo County’s Mussel Sampling Program

This page was last updated 5/8/24

IMPORTANT NOTICE: ANNUAL MUSSEL QUARANTINE IS IN EFFECT FROM MAY 1st – OCTOBER 31st. For more information, read California Department of Public Health’s press release

Approximately every two weeks Environmental Health Services staff collects mussel and plankton samples along San Mateo County’s coastline as part of the State’s Mussel Sampling Program.

The mussel samples are sent to the California Department of Public Health and tested for unsafe levels of naturally occurring toxic materials known as “biotoxins.” Results are published once a month to keep the public informed on when it’s unsafe to eat mussels and other shellfish.

Why are biotoxins in shellfish dangerous?

Certain species of algae (phytoplankton) produce naturally occurring biotoxins that can become too abundant in shellfish like mussels, clams, oysters, crabs, and lobsters. Shellfish consumed with unsafe levels of biotoxins can cause paralytic shellfish poisoning, domoic acid poisoning, and other health issues that may pose a serious threat to humans and wildlife, including death.

When is it safe to eat recreationally harvested mussels in San Mateo County?

Never assume mussels are safe to eat. While a quarantine on mussels takes effect annually from May 1st - October 31st as a safety precaution, the quarantine may become extended or take effect at any time of the year. Please call the biotoxin hotline at 1-800-553-4133 before including mussels or any shellfish on your menu.

Quarantine Boundaries

The quarantine extends from the California-Oregon boundary south to the California-Mexico boundary and applies to all bays, inlets, and harbors.

Using Mussels as Bait

Mussels for use as bait shall be broken open at the time of taking, or prior to sale at the discretion of the enforcing agency. Mussels used as bait should be placed and sold in containers adequately labeled in large bold faced letters that are at least a half an inch in size.


Clams should be washed thoroughly and cooked. All dark parts of the clams and scallops must be discarded because biotoxin is concentrated in the dark parts if present. Only the white meat of clams and scallops should be prepared for human consumption.

For the latest quarantine and health advisory updates by the California Department of Public Health’s Marine Biotoxin Monitoring Program, please call 1-800-553-4133 or click here.

Related Content

Picture of Fish Smart SF Bay Fishing Guidelines

Fish Smart Program
Promoting Guidelines for Safe-to-Eat Fish from the San Francisco Bay and Pacific Ocean

We are fortunate to have access to fresh seafood from the San Francisco Bay and Pacific Ocean. Many fish in the bay and ocean are healthy and safe to eat, but some fish have high levels of toxins that can harm your health and are NOT safe to eat. Below are guides to help you choose the safest fish for you and your loved ones to consume. You can also sign up for guideline updates