Domoic Acid Monitoring in Razor Clams 2015-2021

The California Department of Public Health (CDPH) coordinates with the Office of Environmental Health Hazard Assessment (OEHHA) and the California Department of Fish and Wildlife (CDFW) to protect the public from biotoxins in bivalve shellfish and other seafood. This is a summary of the razor clam domoic acid data and health advisories, focusing on the 2015 to 2021 time period.

One of the naturally occurring marine biotoxins that CDPH monitors is domoic acid. Domoic acid, which is produced by microscopic algae of the genus Pseudo-nitzschia, is a nerve toxin which can cause amnesic shellfish poisoning. Mild symptoms include vomiting, diarrhea, abdominal cramps, headache, and dizziness. Severe symptoms include excessive bronchial secretions, difficulty breathing, confusion, disorientation, cardiovascular instability, seizures, permanent loss of short-term memory, coma, and death. Cooking does not destroy the toxin.

The CDPH Marine Biotoxin Monitoring Program has been frequently testing Pacific razor clams (Siliqua patula) for domoic acid in Del Norte and Humboldt counties since 2016. In California, the largest populations of razor clams are located in these two counties, providing recreational clamming opportunities. Razor clams are filter feeders which can accumulate naturally occurring toxins as they feed on  phytoplankton  that produce the toxins.

Figure 1 summarizes the razor clam domoic acid data and health advisories from 2015 to mid-2022. Annual detections of domoic acid in razor clams began in 2015 for Humboldt County and in 2016 for Del Norte County. CDPH increased razor clam sampling in response to the toxin detection, with assistance from volunteers and sampling partners, and issued health advisories to warn the public. Prior to 2015, razor clam sampling was less frequent, and domoic acid detections above the alert level occurred in 1992, 2004, and 2006 for Humboldt and in 2004 for Del Norte.

Razor clams are unusual in that they hold onto the toxin domoic acid in the meat and viscera much longer than other bivalve shellfish filter feeders, like mussels, oysters, and other clam species. The sample results show that the razor clams from Del Norte and Humboldt counties contained domoic acid levels above the 20 parts per million (ppm) federal alert level for about one year after each domoic acid event. The results slowly decrease over time, then annual domoic acid events in 2016 - 2020 between the months of June – November caused the results to increase again.

The data presented demonstrates that consistent monitoring of domoic acid levels in razor clams is needed to ensure the safety of recreational razor clamming due to the unique way these clams retain high levels for long periods of time. CDPH remains committed to monitoring biotoxin levels in this fishery to ensure public safety of domoic acid in razor clams in both Del Norte and Humboldt Counties.

The current Health Advisories issued by CDPH for bivalve shellfish are displayed in an  interactive map . CDPH also posts the recent  Shellfish Advisories online. 

Domoic acid levels greater than or equal to 20 ppm are a public health risk to consumers as established by the federal government. When domoic acid concentrations in bivalve shellfish meet or exceed this level, CDPH issues a health advisory for the County or counties with elevated results to warn recreational harvesters. Typically, an advisory is issued for all species of recreationally harvested bivalve shellfish (clams, mussels, and scallops). Advisories can also be issued for a specific species of shellfish, like razor clams, if the risk of poisoning is only related to that species.

Razor Clam Fishery Closures

The California Department of Fish and Wildlife (CDFW) and/or the Fish & Game Commission (FGC) have the ability to close the razor clam fishery to protect recreational harvesters from domoic acid.

Timeline of razor clam CDPH Health Advisories and CDFW/FGC fishery closures issued from 2015 to June 30, 2022 (Figure 1):

• August 26, 2015: CDPH Warns Consumers Not to Eat Bivalve Shellfish Caught in Humboldt or Del Norte Counties.

• December 9, 2015: CDPH Updates Warningabout Shellfish from Humboldt and Del Norte Counties. Health Advisory lifted for other species of shellfish, advisory remains for razor clams.

• April 25, 2016: The Fish & Game Commission  closed the recreational razor clam fisheries inHumboldt and Del Norte counties  in response to continued unsafe levels of domoic acid in razor clams posing a public health risk to consumers.

• April 29, 2021:  CDFW reopened the razor clam fishery for Del Norte County.  These actions were taken after two consecutive rounds of samples that met the reopening criteria.

• May 3, 2021*:  CDPH Lifts Warning aboutRazor Clams from Del Norte County.   

• August 9, 2021:  CDFW reopened the razor clam fisheryfor Humboldt County . These actions were taken after twoconsecutive rounds of samples that met the reopening criteria.

• August 11, 2021*:  ​CDPH Lifts Warning about Razor Clams from Humboldt County .

• December 16, 2021: ​ CDPH Warns Consumers Not to Eat Sport-Harvested Razor Clams From Del Norte County . 

• December 16, 2021:  CDFW closed the razor clam fishery in Del Norte County due to domoic acid .

• June 24, 2022:  CDFW reopened the razor clam fishery for Del Norte County . These actionswere taken after two consecutive rounds of samples that met the reopening criteria.

• June 27, 2022*:  CDPH Lifts Warning about Razor Clams from Del Norte County .

 *Note there are a few days between fishery re-opening and CDPH advisory liftings due to posting timing only.      

CDPH and CDFW receive guidance from the Office of Environmental Health Hazard Assessment (OEHHA) to determine sampling strategy and reopening criteria. 

The criteria for reopening the fishery or lifting the CDPH health advisory for razor clams, is two sets of samples collected at least one week apart testing below the 20 ppm alert level for domoic acid.  

Starting in 2016, samplers were directed to collect at least six razor clams per sampling date. The majority of the analyses were conducted on individual clam meats. Occasionally, small clams were pooled (combined) together to meet minimum sample weight requirements and for processing purposes. 

After the razor clam fisheries reopened in April 2021 for Del Norte County and August 2021 for Humboldt County, CDPH transitioned to a new sampling strategy with guidance from OEHHA. The samplers are instructed to collect 12 individual razor clams per sampling date. These clam meats will be analyzed by pooling four individuals together for three separate samples. The new sampling strategy was put into place because the pooled samples more accurately represent a meal-sized portion. The pooled sample result also makes it less likely that one individual clam would trigger an advisory.

The map shows primary sample sites for both razor clams (blue dots), mussels (orange diamonds), and phytoplankton (black dots) in Del Norte and Humboldt counties.

It should be noted that mussels and razor clams are collected from different locations and habitats, which may result in differences in exposure to the domoic acid producing phytoplankton Pseudo-nitzschia. The occurrence of Pseudo-nitzschia may vary along the coastline depending on ocean currents and water column characteristics

In California, the largest populations of razor clams are located in Del Norte and Humboldt counties. Razor clams can only be accessed at low tides when the portion of the beach where the clams live is exposed. The whole in-shell clams are dug out of the sand, frozen, and shipped to the CDPH laboratory in Richmond where they are processed and analyzed. At the laboratory, the clams are shucked and dissected to separate meat from viscera. The meat or viscera is ground up and analyzed by high performance liquid chromatography (HPLC), an analytical chemistry method.

Primary sample sites are Crescent Beach in Del Norte County and Clam Beach in Humboldt County, with Moonstone Beach in Humboldt County infrequently sampled. CDPH will continue testing razor clams from both counties.

Razor clam samples are collected for testing by partners from CDFW, Tolowa Dee-Ni’ Nation, and citizen volunteers.

Mussels are the main bivalve shellfish species sampled for routine biotoxin monitoring statewide. Mussels are found in the rocky intertidal and can be accessed at low tide. Mussels are collected and typically shucked, frozen, and shipped to CDPH. A sample consists of 12-30 shucked whole mussels pooled together and ground up. All mussel samples are tested for paralytic shellfish poison (PSP) toxins. Select mussel samples are tested for domoic acid based on risk assessment from phytoplankton data, during domoic acid events, and for commercial or other monitoring needs.

Primary mussel sampling sites in Del Norte County are Wilson Creek and Hunter Rock, and less frequently sampled sites include Pyramid Point, Point St. George, and Crescent City locations. Primary sample sites in Humboldt County are inside Humboldt Bay and at Trinidad Head, and less frequently sampled sites include Baker Beach, Moonstone Beach, Luffenholtz Beach, Shelter Cove, Stone Lagoon, and other Trinidad locations.

Del Norte County mussel sample collectors include the Yurok Tribe, Tolowa Dee-Ni’ Nation, CDFW, and volunteers. Humboldt County collectors include Pacific Shellfish Humboldt, Humboldt County Environmental Health, Yurok Tribe, Humboldt State University Marine Lab, CDFW, and volunteers.

Phytoplankton samples are collected using a 2 µm mesh net to concentrate seawater. Sample bottles are mailed to CDPH where the types of phytoplankton are identified using light microscopy. CDPH estimates the relative abundance of phytoplankton (like Pseudo-nitzschia) as the percent observed out of 100% of both living and non-living matter.

Primary phytoplankton sample sites are Wilson Creek and Hunter Rock in Del Norte County and Trinidad Pier and Indian Island Channel in Humboldt County.

Collectors include Pacific Shellfish Humboldt, Yurok Tribe, Tolowa Dee-Ni’ Nation, Humboldt State University Marine Lab, and volunteers.

Below is a data summary for January 2015 to December 2021. A table of recent domoic acid results in razor clams is  posted online .

Figures 2 and 3 display the domoic acid sample results in razor clams and mussels for each year from 2015 to 2021 collected in Del Norte and Humboldt counties. 

The majority of data points are for concentrations of domoic acid in individual clam meats without viscera. A small portion of data are pooled samples of two or more clam meats, and a small portion are individual or pooled whole clams (meat and viscera). Samples are represented by transparent blue dots which appear darker where there are overlapping data points. Pooled mussel samples are represented by orange triangles. The red line represents the 20 parts per million (ppm) alert level. Samples which tested below the detection limit of 2.5 ppm are plotted at 1 ppm. Domoic acid is plotted on a non-linear logarithmic scale, to show the lower range of the values.

Due to the timing of ocean conditions and sampler availability, there are data gaps and changes in sampling frequency.

Mussels do not retain domoic acid for very long and detection in mussels can indicate when a domoic acid event occurred in the region. Note that mussel samples were collected infrequently in Del Norte County.

For Del Norte County, domoic acid was detected in pooled mussel samples every year from 2015-2019 and in 2021. For Humboldt County, domoic acid was detected in pooled mussel samples every year from 2015-2020. Detection of domoic acid in mussels occurred between the months of June to November. When mussel samples exceeded the alert level for domoic acid, they returned to levels below the detection limit (2.5 ppm) within two to seven weeks.

From 2015-2021, for every instance of domoic acid detection in pooled mussel samples, there was an even higher concentration in razor clams that slowly declined over the following months such that another domoic acid event would cause the razor clams to re-accumulate the toxin before reaching safe levels. The domoic acid levels in all razor clam samples didn’t fall below the alert level until early 2021 for Del Norte County and mid-2021 for Humboldt County.

For both counties, Figures 2 and 3 show samples above the alert level occurred every year, when samples were tested. There was variability in razor clam meat domoic acid concentrations between individuals collected on the same date from 2016 to 2020. Humboldt County razor clams accumulated higher levels of domoic acid than razor clams from Del Norte County with a maximum domoic acid concentration in individual meat of 450 ppm which occurred in April 2019 at Clam Beach. In Del Norte County the highest concentration of 280 ppm occurred in January 2018 and March 2019, at Crescent Beach.

For Del Norte County, in 2016, 84% of razor clam samples were at or above the 20 ppm alert level, including individual and pooled samples (n=50). In 2020, this number had fallen to 52% (n=83), and from January to April 2021, only 7% of samples were at or above 20 ppm (n=28). Two sets of samples collected in March and April 2021 had all individuals below the alert level and allowed for re-opening.

For Humboldt County, in 2016, 90% of razor clam samples were at or above the 20 ppm alert level, including individual and pooled samples (n=49). By 2020 this number had fallen to 64% (n=33), and from January to April 2021, 31% were at or above 20 ppm (n=16). Samples were collected more consistently over time in Humboldt County and Figure 3 shows more clearly the differences in razor clam and mussel DA concentrations. Two sets of samples collected in June and July 2021 had all individuals below the alert level and allowed for re-opening.

Figures 4 and 5 show the razor clam maximum domoic acid result per sample date and mussel domoic acid results with the percent composition of Pseudo-nitzschia, the diatom which can produce domoic acid. The dataset spans from 1996-2021. Note that shellfish data are sporadic prior to 2015.

The razor clam maximum domoic acid sample result per date are represented as blue circles and the pooled mussel domoic acid sample results are represented by orange triangles. The red line represents the 20 parts per million (ppm) domoic acid alert level. Samples which tested below the detection limit of 2.5 ppm are plotted at 1 ppm. Domoic acid is plotted on a non-linear logarithmic scale, to show the lower range of the values.

Bivalve shellfish such as mussels and clams can accumulate domoic acid from filter feeding on Pseudo-nitzschia cells. Note: Pseudo-nitzschia cells do not always produce the biotoxin domoic acid, so the monitoring of shellfish is necessary to confirm a toxic event. CDPH considers detection of elevated abundance or blooms of Pseudo-nitzschia cells in the water column to be a high risk for domoic acid production. Percent composition is an estimate of Pseudo-nitzschia relative abundance, the percent observed out of 100% of both living and non-living matter in a concentrated water sample determined via light microscopy. The grey lines show the percent composition of Pseudo-nitzschia observed in a sample. A spike in the percent composition represents a period of elevated relative abundance of Pseudo-nitzschia cells in the water column.

The spikes in mussel DA indicate which Pseudo-nitzschia blooms contain toxigenic species and are an indicator of the magnitude and length of time of a DA event. The figures show the rapid response of mussels for both accumulating and purging the toxin during and after a domoic acid event. The razor clams also appeared to uptake domoic acid rapidly, but in contrast, took much longer to cleanse. Consequently, the razor clams maintained elevated domoic acid concentrations due to repeated exposure to toxigenicPseudo-nitzschia activity.

The historical data, starting in 1991, show episodic domoic acid occurrences in shellfish samples. Prior to 2015, domoic acid was detected over the alert level in shellfish in 1991, 2002, 2004, 2005, and 2006. When razor clams were sampled, they generally contained higher levels of domoic acid than the mussels. There may be some domoic acid events that were missed due to gaps in the data set. This shows a need for constant monitoring of both plankton and shellfish.

•  CDPH Marine Biotoxin Monitoring Program 
•  Domoic Acid Frequently Asked Questions 
• California Department of Fish and Wildlife:  Health Advisories and Closures for California Finfish, Shellfish and Crustaceans 
•  Food and Drug Administration: Fish and Fishery Products Hazards and Controls Guidance  
•  Office of Environmental Health Hazard Assessment (OEHHA) Harmful Algal Blooms (HABs) 

Before harvesting or consuming recreationally harvested shellfish please check the  Recreational Bivalve Shellfish Advisory Map  or call the Biotoxin Information Line at 1-800-553-4133.

For more information about volunteer opportunities sampling shellfish or phytoplankton please contact CDPH at  redtide@cdph.ca.gov .