1Fisheries Research Institute, Oita Prefectural Agriculture Forestry and Fisheries Research Center, Oita 879- 2602, 2National Research Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency, Hiroshima 739- 0452, 3Faculty of Agriculture Kyushu University, Fukuoka 812- 8581, Japan
Since 1996 blooms of the toxic dinoflagellate Gymnodinium catenatum Graham in Inokushi Bay and adjacent areas have recurrently caused paralytic shellfish poisoning (PSP) in the scallop Chlamys nobilis. Weekly plankton monitoring, toxin determination on natural suspended matter and toxicity of C. nobilis were investigated from December 2002 through April 2003 at a station in Inokushi Bay. Direct measurements of natural suspended matter showed that the cell density and estimated toxin content per cell of G. catenatum ranged for 17- 8,442 cells L-1 and 89- 1,688 f molcell-1, respectively. Overall, the toxicity of C. nobilis (digestive gland) rapidly increased with increasing cell density and toxin content in suspended matter. The maximum toxicity of C. nobilis was 675 MU g-1 on 31 March 2003. Rapid toxin accumulation in C. nobilis was closely related to not only the cell density of G. catenatum but also the increase in the toxin content of G. catenatum. We have succeeded in predicting C. nobilis toxicity (r=0.98), by applying two factors: toxin content in suspended matter and toxin content per G. catenatum cell.