Mass mortality of the Japanese pearl oyster Pinctada fucata martensii in relation to water temperature, chlorophyll a and phytoplankton composition

Mass mortalities of the Japanese pearl oyster Pinctada fucata martensii hav e widely occurred in western Japan since 1994. The causes of these mass mor talities are at present not thoroughly understood. In this study, we invest igated oyster survival in relation to some environmental factors such as wa ter temperature, concentration of chlorophyll a and density or composition of phytoplankton. The examined mass mortality occurred from September to De cember 1998, and the color on the adductor muscle of the oysters was red-br own, suggesting an infectious disease. Oysters that became moribund during the experiment lost weight, while the weight of unaffected oysters increase d. The cell density of Nitzschia spp., an inedible algae for the oyster, in Uchiumi Bay increased before and during the mass mortality event, From the results of our study, we hypothesize that P. fucata martensii was weakened by starvation because of the dominance of inedible food and then contracte d an infectious disease that resulted in mortality.