1Tamana Regional Promotion Bureau, Fisheries Division, Tamana, Kumamoto 865-0016, 2Kumamoto Prefectual Office Forestry and Fishery Department Fisheries Prompt Division, Kumamoto, Kumamoto862-0950, 3Kumamoto Fisheries Research Center, Ooyano, Kumamoto 869-3603, 4National Research Institute of Fisheries and Environment of Inland Sea, Ohno, Hiroshima 7390452, 5Hiroshima Fisheries Experimental Station, Ondo, Hiroshima 737-1207, Japan
A red tide due to an unarmored dinoflagellate occurred in Youkaku Bay, Kyushu Island, Western Japan in 1995. In order to evaluate the mechanism of red tide outbreaks, field surveys were carried out in this bay, from the initial outbreak until the end of the red tides of this species. Further, some biological assays on cultured finfish were conducted to clarify the toxic effects of this organism on marine animals. Maximum cell density of 9,000cells/mL was observed in 19 August, and seawater discolored to yellow brown. Cells of the red tide organism were 25-32μm in length, and 20-30μm in width and had a deep and wide cingulum. The Right lobe of the hypocone was markedly longer than the left one. Both ends of the cingulum displace 0.2-0.25 of cell length. Based on these morphological characters, this organism was classified in the genus Gyrodinium. It appeared that this organism has never formed any red tide in Japan. Water temperature and salinity during the red tide were 25.4-30.5°C and 31.22-33.65 psu, respectively. Heavy rainfall and subsequent nutrient input were observed before the red tide outbreak. Although mariculture of puffers Takifugu rubripes and pearl oysters Pinctada fucata are held in Youkaku Bay, there was no significant mortality in animals during the red tide. Exposure of young red sea bream Pagrus major to the red tide water (5,370cells/mL of Gyrodinium sp.) demonstrated no negative response and no lethal effect within 60 hours.