Mass Mortalities of Japanese Pearl Oyster in Uwa Sea, Ehime in 1997-1999

Since 1996, mass mortalities of cultured Japanese pearl oyster Pinctada fucata martensii have been occurring in the western Japan including the Uwa-Sea, Ehime Prefecture. Mortalities were accompanied with reddish-brown discoloration of the soft body, especially the adductor muscle of oysters. The condition of the disease was investigated through monitor of farmed and experimentally cultured pearl oysters and histopathological examinations of these samples in 1997-1999. The extent of reddish-brown discoloration of the adductor muscle was expressed as Δa-value using a colormeter. The discoloration started to occur from the southern area which experienced high temperatures in June, then extended to all the culture areas in August November. The mortalities occurred approximately one month after the appearance of the discoloration. As water temperature declined in October-November, the mortalities tended to decrease. These results indicate that the disease of pearl oysters is highly dependent on water temperature. Further, low temperatures in the previous winter appeared to suppress the occurrence of the disease in the following growing season, while high temperatures accelerated the occurrence and intensify the severity of the disease.