In the Yatsushiro Sea, the harmful alga Chattonella antiqua red tides is becoming a major threat to the fishing industry, with damages exceeding 10 billion yen. In this study, a correlation analysis between the weather, marine conditions, and water quality factors which influence C. antiqua red tide, and the DCA I axis score was carried out. The amount of salinity of the surface layer, DIN, PO 4 -P, SiO 2 -Si, and stratification strength were abstracted. Next, an SI model was created from these

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... five factors and the diatom growth rate. Cell density was well reproduced by HSI model in the site sea area. This indicates that C.antiqua red tide can easily occur when stratification strength increases, and DIN and PO 4 -P exceed 0.3µ M, SiO 2 -Si exceeds 0.02µ M, and this species and the competing diatom growth rates are less than 0.1.