2021 Marine Biotoxin Monitoring Annual Report
California Department of Public Health
Introduction
Historically, the majority of shellfish toxicity episodes have originated along the open coast, occasionally moving into bays and estuaries. Monitoring phytoplankton and mussels along the California coast is therefore essential for protecting the public that sportharvests shellfish. Coastal monitoring also provides an early warning of toxic conditions that may soon impact shellfish in bays and estuaries, which harbor the majority of commercial shellfish growers and recreational clam beds.
Paralytic Shellfish Poisoning (PSP) and domoic acid poisoning (also referred to as amnesic shellfish poisoning or ASP) represent serious ongoing public health threats that require year-round attention. The California Department of Public Health (CDPH) manages a prevention program comprised of six elements: (1) a coastal phytoplankton monitoring program for early detection of toxin-producing species that could impact shellfish resources; (2) a coastal shellfish monitoring program that serves to protect recreational harvesters and acts as an early warning for harmful algal blooms (HABs) that could be transported into the bays and estuaries used for commercial shellfish aquaculture; (3) frequent toxin and phytoplankton monitoring of commercial shellfish growing areas; (4) an annual statewide quarantine on sport-harvested mussels (from May 1 through October 31); (5) mandatory reporting of disease cases; and (6) public information and education activities.
This annual report summarizes the monitoring effort and results for the PSP toxins, domoic acid, and toxigenic phytoplankton for 2021. A summary of quarantine and health advisory activities is also provided.
Materials and Methods
Detailed information on sampling protocols, analytical methods, and other aspects of the CDPH marine biotoxin monitoring program can be found in the program’s Management Plan for Marine Biotoxins in California Shellfish, available upon request. The following is a brief summary of the materials and methods used for routine monitoring of phytoplankton and shellfish.
Shellfish Sampling
Mussel samples are routinely collected at one or more representative sampling stations in each coastal county. These coastal samples serve to protect the public that harvests shellfish recreationally and also act as an early warning for recreational (e.g., clamming) and commercial shellfish harvesting inside bays and estuaries.
Sea mussels (Mytilus californianus) are the predominant species collected along the coast, with bay mussels (M. galloprovincialis) more prevalent in calmer waters (bays, estuaries, marinas) and commercial aquaculture leases.
Coastal mussel samples are collected at least monthly by a number of participating agencies and volunteers. CDPH requests additional samples based on observed toxicity or the presence of a toxin-producing species of phytoplankton in the region. Coastal sampling is usually limited to two intervals per month when the daytime low tide is sufficiently low to allow safe access to mussel beds.
A valuable adjunct to the sampling of natural mussel populations is the use of sentinel mussels: plastic mesh bags filled with mussels and suspended from a pier or buoy. Both sea mussels and bay mussels may be used for this purpose. Samples from sentinel mussel bags can be retrieved regardless of tides and ocean conditions, providing a valuable tool for both routine and emergency response monitoring.
Commercial shellfish harvesting in California is conducted on private tidelands or aquaculture leases granted by state or local authorities. Commercial shellfish growing areas are sampled at least weekly. Sentinel mussels are strategically located in most commercial growing areas as an early warning tool; they can be retrieved regardless of tidal conditions and are often safer than sampling intertidal populations. In some cases there are natural sets of mussels in the desired location (e.g., on pilings) that can serve the same purpose. In most growing areas oyster samples from representative locations are also submitted weekly.
Shellfish sample handling and preparation is conducted following the CDPH sampling protocol provided to all collectors. This protocol is based on National Shellfish Sanitation Program (NSSP) requirements. A shellfish sample consists of at least 15-20 individuals, more if necessary, to provide a minimum of 250 grams (g) of tissue. In the case of certain larger or difficult to obtain recreational species (e.g., gaper clams and razor clams, respectively), fewer individuals may be collected to assess potential immediate risk to sportharvesters. Individual shellfish are cleaned of exterior sand and debris, opened while avoiding tissue damage and the byssal threads removed as needed, drained of excess liquid, and placed in a sample bottle provided by CDPH. The sample is immediately refrigerated, typically in an ice chest with ice packs (i.e., ‘blue ice’) and maintained at 0-10° during transport back to the collector’s base, where it is immediately frozen. The frozen sample and completed sample submission form are packaged and shipped to the CDPH laboratory in Richmond via 24-hour courier in an insulated shipping container provided by CDPH.
The CDPH Microbial Diseases Laboratory conducts all PSP assays following the American Public Health Association (1970) methodology recognized by the NSSP. Samples are homogenized and extracted, with the majority of sample extracts screened for toxin presence/absence via an NSSP-approved lateral flow immunoassay (Scotia Rapid Testing). All positive samples are immediately tested with the mouse bioassay (APHA, 1970); negative samples do not require further testing. The PSP alert level is ≥ 80 micrograms per 100 g of shellfish tissue, abbreviated as ≥ 80 µg/100g, and the laboratory’s detection limit ranges from 32 to 37 µg/100g.
A subset of samples arriving each day are split after homogenization, with a portion of the sample being directed to the CDPH Food and Drug Laboratory for domoic acid analysis by high performance liquid chromatography (Dhoot, 1993; Quilliam, 1995). Staff of the marine biotoxin monitoring program determine which samples to analyze for domoic acid based on current trends in the presence and relative abundance of Pseudo-nitzschia and/or the detection of this toxin in shellfish from the region. The domoic acid alert level is ≥ 20 µg/g (= 20 parts per million [ppm]) and the laboratory’s minimum reporting limit is 2.5 ppm.
Phytoplankton Sampling
Phytoplankton samples are primarily collected from coastal piers, with some samples collected from the surf zone, boat docks, and offshore via vessel.
Qualitative samples are collected with a plankton net provided to each collector. Standardized equipment used by CDPH consists of a 1 meter length conical nylon mesh net, 25 centimeters wide at the mouth, with a 20 µm mesh size and a collecting bucket at the cod end to contain the concentrated sample. Vertical net tows are conducted at most sites, although horizontal tows are necessitated by shallow water sites. A standard tow consists of a total tow length of 50 feet (e.g., five vertical net retrievals in 10 feet of water). Adjustments can be made for deeper water sites (e.g., a single tow at 50 feet); collectors are advised to examine the sample and, if clear, do additional tows to increase the cell mass. The depth sampled and total tow length are recorded by the sampler.
For sandy beach surf zones (e.g., some coastal sites in Del Norte and Humboldt counties), volunteer samplers collect buckets of seawater and pour these through a supported net.
An estimate of the total volume sampled is recorded by the sampler. Each sample is thoroughly mixed before decanting into a sample bottle containing a small amount of fixative. The sample bottle and sample collection form are placed in a shipping canister and sent to the CDPH laboratory in Richmond. Marine Biotoxin Monitoring Program staff conduct all sample observations. A wet mount slide is prepared for each sample and observed using standard light microscopy. All cells present are identified to genera, with identification to species when possible, and an estimate made of the percent composition of each. A simpler abundance scale was developed primarily for use by volunteers trained to conduct field observations, consisting of the following ranks: Rare (< 1%), Present (1-9%), Common (10-49%), and Abundant (≥ 50%).
Estimates of abundance based on percent composition can be misleading because they do not account for varying cell densities (e.g., cells per liter of seawater) or sampling effort (i.e., the depth of water sampled and the number of times the phytoplankton net is retrieved through that depth). To adjust for the importance of cell mass and sampling effort, a Relative Abundance Index (RAI) was formulated[1]. The RAI data can provide perspective on the significance of the percent composition data for the species of interest.
[1] The RAI is based on an estimate of cell mass as determined by settled cell volume (a), the percent composition of each species (b), and the sampling effort as determined by the total tow length (c; this is the depth sampled multiplied by the number of net retrievals over that depth)): RAI = (a*b)/c
Results
2021 Sampling Effort: Paralytic Shellfish Poisoning
Shellfish samples were collected at 70 different sites along the coast of California in 2021 for PSP toxin testing. Not all commercial sampling sites are shown due to the scale of the maps: several growing areas had multiple sampling sites, representing different harvest areas, within a small area.
Shellfish sampling sites in 2021.
There were 928 shellfish samples collected statewide for PSP toxin assay during 2021. The greatest number of samples (248) was collected at sites in Marin County; 76% of these samples were associated with commercial shellfish aquaculture companies operating in Tomales Bay.
Statewide, commercial shellfish growers accounted for 57% of all samples collected in 2021, followed by various state agencies and universities, citizen volunteers and nonprofit organizations, coastal county health departments, and federal and tribal agencies (23%, 11%, 5%, and 4%, respectively).
The majority of samples collected in 2021 consisted of mussels (74%), followed by Pacific oysters (25%). The Marine Biotoxin Monitoring Program uses mussels as a primary indicator species for PSP toxins because of their ability to bioaccumulate these toxins at a faster rate than other bivalve species (Shumway, 1990). Differential uptake in mussels versus oysters during a major PSP event in California was previously documented, with sentinel oysters accumulating only 12 to 35% of the total toxin concentration found in sentinel mussels (California Department of Health Services, 1991).
2021 Sampling Effort: Domoic Acid
Many of the samples arriving at the CDPH laboratory for PSP testing are also analyzed for domoic acid. Additionally, CDPH conducts targeted sampling for domoic acid based on phytoplankton observations; a key example of this is the need for routine monitoring of razor clam populations when an increase in Pseudo-nitzschia numbers is observed. Razor clams, in particular, can accumulate and retain dangerous levels of domoic acid. This was observed by the persistence of high concentrations of this toxin through 2021 following annual domoic acid events in Del Norte and Humboldt counties during 2015-2020.
There were 404 bivalve shellfish samples analyzed for domoic acid during 2021, with 46% of these samples originating from commercial aquaculture companies. State agencies and universities also accounted for a significant number of domoic acid samples (19%), with CDFW providing a large number of razor clam samples from Humboldt County. Shellfish from 55 different sampling sites were targeted for domoic acid analysis as a result of observations of elevated numbers of Pseudo-nitzschia spp. in samples from the volunteer monitoring network, as routine shellfish monitoring samples, or as a follow-up to a previous positive sample. The greatest numbers of samples were submitted from the counties of Santa Barbara (116, 29%), Humboldt (68; 7%), Del Norte (58; 14%), and San Luis Obispo (54; 13%). Mussels, oysters, and razor clams accounted for the majority of these samples (55, 25, and 19%, respectively).
2021 Sampling Effort: Phytoplankton
There were 1205 phytoplankton samples collected during 2021 by our volunteer-based monitoring effort. These samples were collected by 57 samplers at 106 sampling sites representing all 15 coastal counties and one county bordering San Francisco Bay. Several areas (e.g., commercial shellfish growing areas) had multiple sites that are not individually identified in the figures and some volunteers collect samples in multiple counties.
Phytoplankton sampling sites in 2021.
The distribution of toxin-producing species (Alexandrium and Pseudo-nitzschia) was widespread, being observed at 88 different sampling sites representing all counties sampled in 2021. Of the 1205 phytoplankton samples collected in 2021, 725 (60%) contained one or more toxigenic species. The percentage of total samples containing one or more toxin-producing species, by county, ranged from 32% (Orange County) to 85% (Marin County).
The following is a brief summary of major trends in the distribution and relative abundance of toxic phytoplankton and the associated distribution and magnitude of marine biotoxins in shellfish. More detail can be found in the monthly reports produced by the CDPH Marine Biotoxin Monitoring Program. The monthly reports contain detailed maps of shellfish toxin and phytoplankton data that illustrate the weekly domoic acid and PSP toxin concentrations and the distribution and relative abundance of Alexandrium and Pseudo-nitzschia. In addition, information is provided on the current status of quarantines and health advisories and lists of program participants are provided. These reports are available at the following Internet site:
2021 Monitoring Results: Alexandrium Observations and Paralytic Shellfish Poisoning Toxicity
Alexandrium occurred in 21% of samples representing all coastal counties. The highest frequency of occurrence for Alexandrium (41%) was observed in samples from San Luis Obispo County.
Observations of Alexandrium were recorded in each month of the year at multiple sites along the California coast. Increases in the percent composition of Alexandrium occurred between July and November, the majority of which involved multiple sites in San Luis Obispo County.
Spikes in the percent composition of Alexandrium occurred on July 12 (10%) and August 8 (10%) in mid Morro Bay, with an October 21 spike in the outer bay (10%). In August there were additional increases of Alexandrium at sites in other counties, including Sonoma, Marin, San Francisco, San Mateo, and Santa Barbara. By the end of September Alexandrium was also common at the Monterey Commercial Wharf (10%, September 22), persisting at slightly lower levels through November. This dinoflagellate continued to be observed at elevated levels in samples from San Luis Obispo and San Diego through December.
The increase in the relative abundance index for Alexandrium was consistent with the percent composition data, with the greatest cell masses observed between July and September.
The July to September increase in Alexandrium relative abundance was observed in samples from various sites in San Luis Obispo County and, to a lesser extent, in San Mateo and Marin counties. Elevated cell masses of Alexandrium were observed in samples collected between May and August from San Mateo, Monterey, Marin, and San Luis Obispo counties.
Of the 249 observations of Alexandrium, 184 were ranked as “Rare”, 64 were “Present”, and four were “Common” with respect to the percent composition. As noted above, the “Common” occurrences of Alexandrium were observed at one sampling location in Monterey and two locations in Morro Bay.
There was a significant decline in PSP activity in 2021 compared to the previous eight years.
There were 133 shellfish samples containing PSP toxins, only one of which exceeded the alert level. PSP toxicity was detected in shellfish from each coastal county between Del Norte and Santa Diego counties, excluding San Francisco, Los Angeles, and Orange.
In the one incident of elevated toxicity, the PSP toxin concentration increased above the alert level in mussels from the Drakes Bay sentinel station on August 25 (131 µg/100 g) following several weeks of sporadic low levels of toxicity. Toxin concentrations at this site declined below the alert level by September 14 and below the detection limit by October 15.
The majority of positive PSP samples were from sites in Marin, San Luis Obispo, and Sonoma counties.
2021 Monitoring Results: Pseudo-nitzschia Observations and Domoic Acid Toxicity
Pseudo-nitzschia was observed in samples from all coastal counties and was most frequently observed in samples from Marin (116) and San Luis Obispo (90). This diatom was also observed in samples from multiple counties during each month of the year.
Of the 618 observations of Pseudo-nitzschia, 256 were ranked as “Rare”, 298 as “Present”, 60 as “Common”, and four as “Abundant”. The “Common” occurrences of Pseudo-nitzschia were observed at 32 different sampling locations representing all coastal counties except Mendocino, Sonoma, and Monterey. “Abundant” occurrences of this diatom were observed at two Los Angeles sites offshore of Palos Verdes in May and two Del Norte sites in July.
All but one of the 50 samples containing domoic acid were razor clams from Del Norte and Humboldt counties; the exception was a mussel sample from northern Del Norte County. Of the 50 positive samples, 10 exceeded the alert level.
The magnitude of domoic acid in shellfish declined significantly in 2021 compared to previous years.
There were three noticeable increases in the percent composition of Pseudo-nitzschia observed in the spring (May), summer (July), and fall (September) of 2021.
Although each of these periods of increase included samples from multiple counties, often geographically discontinuous, the peak percentages occurred in single counties.
The spring event was represented by increases in the percent composition of Pseudo-nitzschia at multiple sites in Los Angeles County, with lesser activity in Ventura and Santa Barbara counties. For the summer event, Pseudo-nitzschia was common to abundant at two sampling sites representing north and south Del Norte County. The fall event was the lesser of the three, with Pseudo-nitzschia common at several sites offshore of Marin County.
Fluctuations in Pseudo-nitzschia cell mass, represented by the relative abundance index, mirrored the patterns of increase and decrease in percent composition for the spring and summer events, but not for the fall event.
There was an additional solidary, but significant, spike in cell mass in February and March involving single sites in Sonoma and Santa Cruz counties. Overall, the observed cell mass was low, with the exception of the July 26 sample from Wilson Creek in southern Del Norte County, which was significantly higher than any other sample.
Domoic acid concentrations in razor clams ranged from nondetectable to well above the alert level through June.
Razor clams from Del Norte County continued to contain elevated levels of domoic acid in February, declining below the alert level in March. Follow-up samples collected in April were also below the alert level, resulting in the rescission of the CDPH health advisory and the CDFW fishery closure for razor clams in Del Norte County. In contrast, clam samples from Humboldt County continued to contain variable levels of domoic acid, with some samples exceeding the alert level through May. By late June the toxin concentrations had declined to safe levels for all samples. Domoic acid levels remained below the alert level in follow-up samples collected in late July and early August, resulting in the rescission of the CDPH health advisory and the CDFW fishery closure for razor clams in Humboldt County.
Clam samples collected in Del Norte County in December once again exceeded the alert level for domoic acid, resulting in a new CDPH health advisory and CDFW fishery closure (see the Health Advisory section for details).
CDPH would like to acknowledge the tremendous help provided by CDFW biologists in the Eureka office, and the Tolowa Dee-ni’ Nation and volunteer Ken Graves in Crescent City, in providing razor clam samples from Humboldt County and Del Norte County, respectively. Their efforts were essential in protecting the public from gathering and consuming highly toxic shellfish.
Quarantines, Health Advisories, and Reported Illnesses
Annual Mussel Quarantine
CDPH issues the annual quarantine on sport-harvesting of mussels for human consumption on May 1, which normally extends through October 31. Toxin activity outside of the normal quarantine period may result in an early start or an extension to the quarantine. The annual mussel quarantine applies only to sport-harvested mussels along the entire California coastline, including all bays and estuaries. Routine biotoxin monitoring is maintained throughout the quarantine period. Additional health advisories are issued for other affected species when toxins are detected at dangerous levels. The annual quarantine does not affect the certified commercial shellfish growing areas in California. Shellfish sold by certified harvesters and dealers are subject to frequent mandatory testing, with harvest closures directed by CDPH as needed.
Health Advisories
- On January 12 CDPH issued a press release rescinding the November 18, 2020 PSP health advisory for all sport-harvested mussels, scallops, and clams in Mendocino County.
- On February CDPH issued a press release rescinding the December 31, 2020 PSP health advisory for all sport-harvested mussels, scallops, and clams in Marin County.
- On May 1 CDPH issued the annual quarantine on sport-harvesting of mussels for human consumption.
- On May 3 CDPH issued a press release rescinding the August 2015 warning related to dangerous levels of domoic acid in sport-harvested razor clams in Del Norte County.
- On August 11 CDPH issued a press release rescinding the August 2015 warning related to dangerous levels of domoic acid in sport-harvested razor clams in Humboldt County.
- On August 27 CDPH issued a health advisory warning the public not to eat sports-harvested mussels, clams, or scallops from Marin County due to elevated levels of the PSP toxins.
- On October 5 CDPH issued a press release rescinding the August 27 PSP health advisory for sport-harvested clams and scallops in Marin County.
- On October 31 CDPH rescinded the annual quarantine on sport-harvesting of mussels for human consumption.
- On December 16 CDPH issued a health advisory warning the public not to eat recreationally harvested razor clams from Del Norte County due to the detection of dangerous levels of domoic acid. In addition, the California Department of Fish and Wildlife closed the recreational razor clam fishery in Del Norte County on this date, following a recommendation from the Office of Environmental Health Hazard Assessment.
Reported Illnesses and Deaths
There were no reported illnesses or deaths associated with either PSP or domoic acid in 2021.
Additional Information
CDPH Office of Public Affairs press releases for the above health advisories:
CDPH Food and Drug Branch domoic acid health advisories for Dungeness and rock crab:
Acknowledgements
The information in this report represents the efforts of many individuals, including program participants that collect shellfish and phytoplankton samples; California Department of Public Health (CDPH) laboratory scientists responsible for domoic acid and paralytic shellfish poisoning testing; environmental scientists in the CDPH Preharvest Shellfish and Marine Biotoxin Monitoring Program that manage the monitoring programs and examine phytoplankton samples; food inspectors in the CDPH Food and Drug Branch that collect a variety of seafood samples during a biotoxin event; communications experts in the department’s Office of Public Affairs who issue press releases to alert and educate the public; and many others that support these efforts. All of those involved in the CDPH Marine Biotoxin Monitoring Program play a vital role in protecting the public’s health.
CDPH would like to thank the California Department of Fish and Wildlife for their support of a portion of the paralytic shellfish poisoning monitoring effort of the Marine Biotoxin Monitoring Program. This program provides protection and guidance to sportharvesters and commercial shellfish growers. The CDPH Marine Biotoxin Monitoring Program would also like to acknowledge the dedicated work and expertise of the scientists of the Department’s Microbial Diseases Laboratory and the Food and Drug Laboratory for their efforts in conducting PSP assays and domoic acid analyses, respectively. Due to the unpredictable nature of marine biotoxin activity, the laboratories are often called upon to respond immediately to an influx of samples resulting from these events. It is due to their efforts that we are able to provide rapid feedback to field samplers and notify the public of potential health risks.
Shellfish toxicity data are generated on a daily basis by the Marine Biotoxin Monitoring Program thanks to the continuing efforts of our program participants. Additionally, volunteers are collecting phytoplankton samples on a routine basis, increasing their sampling frequency during periods of concern and providing near real-time observations of the occurrence of toxin producing species. These efforts are critical to our ability to monitor phytoplankton distribution and abundance along the California coast. As with all such endeavors, our success in protecting the public is due in large part to the numerous people who contribute their time and effort to collect samples at representative sites along the coast. The listing of our program participants, provided in each monthly report and in this annual report, illustrates the diversity of groups and individuals that contribute to these efforts.
We would like to express our sincere appreciation to our program participants for all of their efforts. It is through their active participation that CDPH is able to protect and improve the health of all Californians.
The following is a list of program participants that collected shellfish samples during 2021:
The following is a list of program participants that collected phytoplankton samples during 2021:
References
American Public Health Association, 1970. Recommended Procedures for the Examination of Sea Water and Shellfish, 4th Edition, APHA, New York, N. Y.
California Department of Health Services Marine Biotoxin Monitoring Program, 1991. Shellfish biotoxin monitoring program annual report, 1991, 50 pp.
California Department of Public Health, Monthly Biotoxin Monitoring Program reports, 2019.
Dhoot, J.S.; del Rosario, A.R.; Appel, B.R.; Tamplin, B.R.1993. An improved HPLC procedure for domoic acid analysis in seafood, Int. J. Environ. Anal. Chem. Volume 53, 4, 269-279.
Quilliam, M.A., Xie, M., and Hardstaff, W.R. 1995. Rapid extraction and cleanup procedure for the determination of domoic acid in tissue samples. NRC Institute for Marine Biosciences, Technical Report #64, National Research Council Canada #33001. Journal of AOAC International, 78: 543–554.
Shumway, S. E. 1990. A review of the effects of algal blooms on shellfish and aquaculture, J. World Aquaculture Soc., 21: 65 – 104.
