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An exceptionally extensive bloom of the dinoflagellate Karenia mikimotoi caused mass mortality of aquaculture fishes leading the severe economic loss in 2017 in Imari Bay, Japan, Development of the 2017 bloom completely differed from other recent blooms (in 2012 and 2014). Bloom dynamics and its controlling factors were examined by field survey, numerical modeling and satellite data analysis. The bloom initially developed in the eastern area of Imari Bay as in the 2012 and 2014 blooms. Subsequently, the bloom broadly expanded to the western area and cell density increased to over 10,000 cells ml(-1). Horizontal expansion of the bloom resulted in mass mortality of aquaculture fishes. Horizontal expansion and increase of cell density occurred accompanying a temperature decrease and nutrient increase in the mid and lower layers of the central area. Results of the numerical model indicated two factors related to the passage of a typhoon that promoted the bloom; horizontal bloom expansion induced by the wind-driven current and nutrient supply from the typhoon-induced coastal upwelling. This implies that the effect of the typhoon differentiated the 2017 bloom from previous blooms. The risk of harmful blooms as in the 2017 Karenia bloom in Imari Bay potentially will increase due to the effects of global warming.