Ecological Features and Mass Mortality of Pearl Oysters during Red Tides of Heterocapsa sp. in Ago Bay in 1992

Abstract

Large-scale red tides of Heterocapsa sp. (Dinophyceae) occurred in Ago Bay during the summer and autumn of 1992. The seawater was discolored to yellow brown, and the maximum cell density was 87,420 cells/ml. The summer red tide occurred from mid to late August, and the autumn red tide from late October to late November. Water temperatures and the salinities were 23.8 to 26.9°C and 33.2 to 33.4 during the summer red tide, and 16.9 to 22.2°C and 30.5 to 32.2 during the autumn red tide. Thus, Heterocapsa sp. can form red tides over a wide range of water temperature. The autumn red tide developed slowly as compared to the summer one. This may be due in part to the lower water temperature in autumn. Both red tides were preceded by heavy rainfall and/or vertical mixing of the seawater associated with a typhoon. It is suggested that these physical events promoted and supported the growth of this species. The red tides caused mass mortality of pearl oysters (Pinctada fucata); however, mortalities of cultured and feral fish were not observed. During the red tides, pearl oysters closed their shells tightly and died despite the high concentration of dissolved oxygen. This is the first report of Heterocapsa sp. red tides causing mass mortaliy of pearl oysters in Japan. © 1995, The Japanese Society of Fisheries Science. All rights reserved.