Types of Marine Biotoxins
We test for three biotoxins that can make people sick from eating shellfish:
Paralytic Shellfish Poisoning - aka "Red Tide"
Marine biotoxins are poisons produced by some microscopic plants or algae called phytoplankton. Algae that produce biotoxins occur naturally and are usually present in small amounts. When conditions are right, toxic algae can grow quickly or “bloom.” Toxin producing algae blooms are called harmful algal blooms or "HABs" because they can cause illness and death.
Most shellfish filter feed, pumping water through their bodies and filtering out algae to eat. If algae that produce biotoxins are in the water, shellfish will concentrate the toxins. If people, other mammals, or birds eat shellfish that have large amounts of biotoxins, they can become sick.
Cooking or Freezing Does Not Destroy Biotoxins
Cooking or freezing doesn’t kill or remove toxins. The toxins are part of the shellfish meat.
No Antidote for Biotoxin Poisoning
People who get sick from biotoxins must wait for the toxins to flush from their body. Ventilators and oxygen are used to keep people alive and stable. Learn more about the symptoms of Paralytic Shellfish Poisoning, Diarrhetic Shellfish Poisoning, and Amnesic Shellfish Poisoning.
Harmful Algal Blooms Rarely Color the Water
Toxic algal blooms have been called “red tides,” but most toxic blooms don’t change the color of the water and those that do, aren’t usually toxic. An area can have a toxic algae bloom when the water looks clear.
Check Before You Dig - Shellfish Safety Map
Toxic shellfish look, taste, and smell the same as safe shellfish. The only way to know that shellfish are safe from toxins is to test them. We test shellfish for biotoxins and close areas when shellfish are unsafe to eat.
Check the Shellfish Safety Map for biotoxin closures and advisories on the day you plan to harvest shellfish.
Other options to check that a beach is open or closed:
- List of Public Beaches and Status by County
- Recorded Hotline for Biotoxin Closures: 1-800-562-5632
- List of Biotoxin Beach Closures by County
- Look for signs at public beaches (not all beaches have signs)
Questions about closures? Call us at 360-236-3330 or the local county health department during weekday business hours.
Species Matters
Mussels accumulate toxins more quickly and to higher levels than other types of shellfish. Mussels are used as an indicator for toxins, alerting us that toxins are increasing or have become high enough to make people sick.
Varnish clams accumulate toxins to higher levels than other clams. Varnish clams are found shallower than other clams and live near fresh water sources that can be polluted, so harvesters should be especially aware of surrounding conditions when harvesting this type of clam.
Varnish clams and butter clams store toxins longer than other shellfish and can remain toxic for a year or more after a toxic algae bloom ends. This is why areas are closed for varnish and butter clam harvest but open for other shellfish.
Scallops accumulate dangerous and unpredictable levels of toxins compared to other shellfish. Recreational scallop harvesting is never safe and is listed as closed for biotoxins even when the harvest of other shellfish is open.
To help you know what you're harvesting, see how to identify bivalve shellfish.
Varnish Clams
Varnish clams, also known as purple varnish clams, dark mahogany clams, or savory clams, originate from Asia and are considered an invasive species. Varnish clams concentrate biotoxins to higher levels than other clams and they hold on to the toxins for longer periods of time. Varnish clams differ from other mollusks in that they are bi-modal feeders; they both filter feed and bottom feed, which may account for the higher toxin levels found in this species.
Scallops
Scallops can quickly accumulate dangerous levels of biotoxins—especially paralytic shellfish poisoning (PSP)—and may retain them for long periods. Unlike other shellfish, many scallops can swim, moving from toxic areas to open areas, which makes monitoring difficult. Non-swimming species like adult rock scallops can remain highly toxic even when nearby shellfish are safe. Contrary to popular belief, the adductor muscle can contain unsafe toxin levels. In 2024, all rock scallops tested for PSP collected by recreational divers in the Strait of Juan de Fuca had both adductor and viscera at unsafe levels. While other biotoxins like ASP and DSP pose risks, PSP is the most common and dangerous in scallops. The Washington State Department of Health advises that recreationally harvested scallops are not safe to eat due to biotoxin risks. Scallops commercially harvested in Washington are strictly tested and managed for biotoxins.
More Resources
Domoic Acid in Razor Clams Consumption Interim Advisory
Biotoxins - Myths and Misconceptions
Harmful Algal Blooms Fact Sheet, NOAA (PDF)
For information about marine biotoxin monitoring see Biotoxin Program.