Blooms of the dinoflagellate Karenia mikimotoi cause mass mortality of aquaculture fishes and have become a worldwide environmental problem in coastal waters, especially in Asia. One area is Imari Bay, Japan, where blooms have recurrently formed in recent years. This study examines the meteorological and oceanographical conditions causing the recent increase in occurrence. The recent frequency of occurrence (2012-2018) is nearly four-fold previous years (1999-2011). In recent years, total nitrogen increased in the eastern area where the bloom initially develops, and The Tsushima warm current (TWC) have intensified during spring. Spring total nitrogen has increased as a result of climate change involving increasing rainfall and weakening monsoon rather than eutrophication because of the asynchrony between increasing and total nitrogen in rivers. Intensity of TWC changed in synchrony with M-2 amplitude with lunar nodal cycle. Potential methodology for the medium term prediction of blooms is suggested in Imari Bay using environmental parameters. (C) 2020 Elsevier B.V. All rights reserved.