Recent trends in paralytic shellfish toxins in Puget Sound, relationships to climate, and capacity for prediction of toxic events

Temporal and spatial trends in paralytic shellfish toxins (PSTs) in Puget Sound shellfish and their relationships with climate are investigated using long-term monitoring data since 1957. Data are selected for trend analyses based on the sensitivity of shellfish species to PSTs and their depuration rates, and the frequency of sample collection at individual sites. These criteria limit the analyses to the shellfish species Mytilus edulis at 20 sites from 1993 to 2007. Blue mussel toxicity is highly variable, but typically exceeds the regulatory limit for human consumption from July to November annually, with most closures occurring early in fall. Using blue mussel data only, we find no robust evidence to suggest that the frequency, magnitude, duration, or geographic scope of PST events in Puget Sound increased between 1993 and 2007. However, there is a significant basin-wide trend for closures to occur earlier in the year. There are no significant correlations between annual indices of mussel toxicity and aspects of the local and large-scale climate. Case studies of daily variations in local environmental factors leading up to exceptionally toxic events identify a combination of conditions that generally precedes most closures from 1993 to 2007. These results suggest that periods of warm air and water temperatures and low streamflow on sub-seasonal timescales may facilitate toxin accumulation in mussels. No relationships were found between water residence times in the surface layer and either streamflow or mussel toxicity. Recommendations are made for future monitoring to improve forecasting of PST risks in Puget Sound, an important region for recreational, commercial, and tribal subsistence shellfish harvesting.