CAUTION: Humboldt Shellfish Poisoning

Caroline Griffith, NEC Executive Director

On November 3, the California Department of Fish and Wildlife closed the recreational razor clam fishery in Del Norte County due to findings by the Office of Environmental Health Hazard Assessment (OEHHA), in consultation with the California Department of Public Health (CDPH), which determined that consumption of razor clams taken from Del Norte County poses a significant threat for domoic acid exposure.

Domoic acid, a potent neurotoxin, is produced under certain ocean conditions by the naturally-occuring marine algae Pseudo-nitzschia, which benefits from nitrogen loading in the environment. Domoic acid poisoning in humans may occur within minutes to hours after consumption of affected seafood. Symptoms range from vomiting and diarrhea to permanent loss of short-term memory (Amnesic Shellfish Poisoning or ASP), coma or death. There is no way to prepare clams for consumption that will remove the toxin – cooking and freezing have no effect. 

This closure comes on the heels of the notification released by CDPH on October 27 warning of dangerous levels of Paralytic Shellfish Poisoning (PSP) toxins that were detected in mussels from Humboldt and San Luis Obispo counties. CDPH is advising consumers not to eat sport-harvested mussels, clams, or scallops from these counties. PSP toxins are caused by the algae Alexandrium catenella. Much like with domoic acid, PSP toxins are not affected by cooking or freezing. Also like domoic acid poisoning, symptoms of paralytic shellfish poisoning can occur within minutes to hours after consuming affected shellfish. Symptoms include tingling and numbness of the lips, tongue, and extremities; nausea, vomiting, and diarrhea; loss of coordination, difficulty speaking, difficulty swallowing, and, in severe cases, total muscular paralysis with respiratory arrest.

Both PSP toxins and domoic acid are naturally occuring marine toxins, but certain conditions can increase their growth and cause Harmful Algal Blooms (HABs). One cause is an increase in levels of nutrients like nitrogen and phosphorus in the water. This can come from fertilizers, sewage or run-off from city streets. Increases in water temperature can also contribute to blooms. The State of California imposes an annual quarantine on sport harvesting of mussels for food from May 1 through October 31, the period when they are most likely to accumulate PSP toxins. On October 31 it ended the quarantine for all counties except Humboldt, Monterrey, San Mateo and San Luis Obispo. 

The State of California has one of the oldest marine biotoxin monitoring programs in the country, started after an outbreak of PSP in 1927 that affected over 100 people. The Department of Public Health collects and tests shellfish and also utilizes volunteers to monitor algae conditions to detect HABs.

Animals can also be affected by ASP and PSP. If you see seals, birds or other marine animals acting “drunkenly” or strangely it may be because they have ingested shellfish containing the toxins. If you are in Humboldt County and suspect an animal has been affected by ASP or PSP you can call the Marine Mammal Center of Cal Poly Humboldt at 707-465-6265 or Bird Ally X at 707-822-8839.

To get the latest information on current fishing season closures related to domoic acid, call CDFW’s Domoic Acid Fishery Closure Information Line at (831) 649-2883. For shellfish advisories call (800) 553-4133. Shellfish advisories and a link to the Recreational Shellfish Advisory Map can be found at