An ecological study of butter-clam (Saxidomus giganteus) toxicity in southeast Alaska

Abstract

Butter clams (Saxidomus giganteus) at South and North porpoise Islands and Pleasant Island, Southeast Alaska, were occasionally found to accumulate significant amounts (higher than the maximum human tolerance) of paralytic shellfish poison (PSP) at any season of the year, and to occasionally lose or regain PSP rapidly between two samplings. The fluctuations of toxicity levels were not similar at all stations and no consistent patterns were Observed. The toxicity of only 19 out of 53 samples collected at three high-toxicity stations exceeded the maximum human tolerance level for PSP (1200 MU), and clam samples taken from moderate and low-toxicity stations never exceeded this level. Neither phytoplankton populations nor hydrographic parameters had a consistently significant correlation with toxicity levels; however, fluctuations of phytoplankton numbers demonstrated an inverse relationship with fluctuations of inorganic nutrient concentrations. Dinoflagellate maxima tended to occur at relatively low salinity (22% - 29%) and relatively high temperatures (7°C - 16°C), whereas the diatom numbers did not significantly correlate with salinity or temperature. The three high-toxicity stations were all within Icy Passage; fluctuations of phytoplankton populations and the hydrographic conditions at these stations were similar, yet the fluctuations of toxicity levels were quite dissimilar. A number of possible sources may be responsible for the butter-clam toxicity in Southeast Alaska. However, more studies are needed to define the cause of the butter-clam poisoning problem in Southeast Alaska.