Paralytic shellfish toxins in Puget Sound, Washington State

The first illnesses and only deaths in Washington State resulting from paralytic shellfish poisoning were documented in the 1940s, resulting in the establishment of one of the longest monitoring programs for paralytic shellfish toxins in commercial and recreational shellfish in the United States. An analysis of the Washington Department of Health's monitoring data for the Puget Sound area has allowed us to examine temporal changes in shellfish toxin levels and geographical distribution of shellfish harvesting closures. The values of toxins in shellfish were normalized to control for variable levels of toxin accumulation in different shellfish species by dividing individual values by the yearly average for a given species. These normalized values increased significantly over the past five decades, indicating that the observed increase in paralytic shellfish toxin levels in Puget Sound shellfish was not caused by the shift in species monitored. A geospatial map of the first shellfish closures or paralytic shellfish-poisoning event in each Puget Sound basin suggests that over time, toxigenic Alexandrium cells have been transported from northern to southern Puget Sound. Shallow sills that restrict the exchange of water between adjacent basins have hindered the transport of toxic dinoflagellates, especially because these cells generally do not prosper in mixing conditions that are characteristically found at sills. Large-scale events, such as the bloom that occurred in the Whidbey and Central basins in 1978, may have been induced by global climate changes or shifts, such as the Pacific Decadal Oscillation. Although greater numbers of closures have been observed over time in basins of Puget Sound, closures as a percentage of total samples analyzed have decreased or remained constant in all basins, indicating that the Washington Department of Health has established an effective monitoring program to protect public health while allowing for maximum harvest potential.