Shellfish Biotoxins

Photo: Danger Sign - Shellfish Area CLosed

Eating contaminated Shellfish can be life threatening: Always check for Marine Biotoxin and Sanitary Contamination Closures if you plan on harvesting and consuming any shellfish.

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Species and Seasonality Aspects of Marine Biotoxins

In B.C., the Canadian Food Inspection Agency (CFIA) is responsible for analysing samples of shellfish for paralytic shellfish poisoning toxin (PSP - saxitoxins) and amnesic shellfish poisoning (ASP - domoic acid). The action levels for PSP and Domoic Acid Poisoning are 80 micrograms per 100 g of meat and 20 ppm of domoic acid respectively.

As the basis of the marine biotoxin monitoring program in the Pacific Region, large mussels are hung in plastic mesh sacks in shellfish growing areas (in addition to a lesser number of commercial samples of all species). Samples are withdrawn on a weekly or biweekly basis and shipped to the CFIA laboratory for analysis. Shellfish species differ in their tendencies to accumulate and eliminate marine biotoxins in their tissues. Mussels tend to be indiscriminate feeders of phytoplankton and are therefore much quicker to pick up biotoxins than many other shellfish species. The levels of biotoxin in mussels are frequently up to 10 times higher than oysters and manila or littleneck clams growing in the same area. This allows the CFIA to recommend harvest restrictions (eg. area closures) to Fisheries and Oceans Canada while species other than mussels, such as clams, oysters and scallops, are still safe. Mussels also tend to rid their tissues of biotoxins faster than other species. Therefore, although harvest restrictions are usually based on mussel analysis results, samples of the other species in the area are tested before the harvest restrictions are lifted.

Some bivalve species cause additional concerns. For example, butter clams are able to retain high levels of PSP toxin for over a year. For this reason, Fisheries and Oceans Canada, on the recommendation of the CFIA, put in place a blanket harvest ban for this species. If a fishery for butter clams is planned pre-harvest samples are collected to ensure that the levels of toxin are acceptable prior to lifting the restriction for the area in question. Education and awareness with respect to shellfish biotoxins is very important. There have been incidents in the past in which individuals have harvested and consumed butter clams under the harvest ban and been hospitalized as a result of PSP. Many non-commercial harvesters do not take the time to check with their local Fisheries and Oceans Canada office, the Pacific Region Fisheries and Oceans website or the 24 hour information line at 604-666-2828 to insure that the area and time of year are safe for harvest.

The CFIA is also concerned about scallops and geoduck clams. These species are generally found sub-tidally, well below the intertidal zone, and they may accumulate biotoxins at different rates than intertidal bivalves in adjacent areas. Reasons for this may be that currents expose them to different amounts of the toxic phytoplankton during a bloom (can be greater or less) or they may be exposed to greater numbers of dormant cysts of the toxic dinoflagelates (e.g. Alexandrium catanella.) These cysts are a part of the algae's life cycle and they are found resting on the bottom in sediments. Strong currents or storms may resuspend them and they may then be ingested by filter feeding bivalves such as scallops or geoduck clams.

In scallops, biotoxins can be accumulated to high levels and be retained for a considerable period of time. They may still be toxic even after oysters and manila clams in the area are safe. The toxins are particularly concentrated in the mantle, viscera, roe, etc. where we have seen levels considerably greater than 1000 micrograms. In most species, the adductor muscle rarely accumulates levels of toxins that would cause illness. An exception is the native purple hinge rock scallop that has been found to have elevated levels of biotoxins in the adductor muscle. These levels are still much lower than in the other portions of the animal. With smaller species, such as the pink and spiny scallops, this does not apply as the whole animal is marketed and consumed and the removal of the adductor muscle from the rest of the animal is not usually practised.

In geoduck clams the toxin is almost never detectable in the siphon and body meat. In this species, the toxin is found in the viscera which is consumed in some markets. These large subtidal clams are also prone to picking up toxins in the winter as storms can stir up the cysts of toxic dinoflagelates.

Additional note:

Crabs and predatory molluscs (eg. moon snails) may feed on toxic bivalves and thus become toxic themselves even though they are not filter feeders. In crabs the toxins are found in the hepatopancreas (part of the viscera, under the carapace) which the CFIA periodically analyses for biotoxins. Although the action level may be exceeded at times and harvest restrictions have been instituted, there have been no illnesses reported from any species other than bivalves. Likewise, there have been no recorded instances of illness as a result of Domoic Acid Poisoning from any species harvested from B. C. waters although the CFIA has found levels of domoic acid in excess of the action level and closed areas to harvest as a result.


  1. Different bivalve species may accumulate and eliminate biotoxins at different rates and levels.
  2. Toxin levels may be very different in intertidal and adjacent subtidal bivalves due to differences in their growing environment.
  3. Some bivalve species will concentrate toxins in a particular part of their bodies, making some parts much more toxic than others.
  4. Long term retention and resuspension of cysts may result in toxic shellfish in B. C. at any time of the year even though the toxic algae blooms generally occur during the warmer periods of the year.
  5. Some non-bivalve molluscs and crustaceans may accumulate biotoxins that could result in illness.

For more information, please visit the Canadian Shellfish Sanitation Program page (Canadian Food Inspection Agency site).