The action levels for PSP and ASP are 80 micrograms per 100 g of meat and 20 ppm of domoic acid respectively. Samples are withdrawn on a weekly or biweekly basis and shipped to the CFIA laboratory for analysis.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.
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.