ENVIRONMENTAL FEATURES AND THE MASS MORTA LITY OF FISH AND SHELLFISH DURING THE GONYAULAX-POLYGRAMMA RED TIDE OCCURRED IN AND AROUND UWAJIMA BAY, JAPAN, IN 1994

A red tide of long duration (late August to mid November) of Gonyaulax polygramma Stein occurred in and around Uwajima Bay in 1994. This red tide caused mass mortalities of cultured and natural fish and shellfi sh stocks worth more than 8 hundred million yen. The maximum density o f G. polygramma reached 6.8 x 10(4) cells/ml in Uwajima Bay. From fluo rometric measurements, it is supposed that G. polygramma cells existed . in the surface layer in the daytime and migrated to the bottom layer s at night. During the red tide, oxygen deficient water was formed and eventually the whole water column became anoxic in the coastal waters of Uwajima Bay and Hoketsu Bay. It seems that the death of fish and s hellfish was mainly attributable to the anoxic waters with high sulfid e and ammonia concentrations caused by the decomposition of G. polygra mma cells. The bloom of G. polygramma in Uwajima Bay was initiated in the Yoshida area, and then the bloom gradually spread to the southern areas of the bay. Analysis of this mechanism of the spread of G. polyg ramma cells throughout the bay will help to establish a monitoring sys tem for future blooms. Meteorological and hydrographical data suggest that little rainfall, high water temperature and a low water exchange rate enhanced the initial occurrence and long duration of the red tide of G. polygramma.