Environmental features and the mass mortality of fish and shellfish during the Gonyaulax polygramma red tide occurred in and around Uwajima Bay, Japan, in 1994

Abstract

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 shellfish stocks worth more than 8 hundred million yen. The maximum density of G. polygramma reached 6.8 × 104 cells/ml in Uwajima Bay. From fluorometric measurements, it is supposed that G. polygramma cells existed in the surface layer in the daytime and migrated to the bottom layers 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 shellfish was mainly attributable to the anoxic waters with high sulfide and ammonia concentrations caused by the decomposition of G. polygramma 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. polygramma cells throughout the bay will help to establish a monitoring system 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.