Seed populations of a harmful unarmored dinoflagellate Cochlodinium polykrikoides Margalef in the East China Sea

An unarmored dinoflagellate Cochlodinium polykrikoides has formed red tides responsible for fish mass mortalities especially in coastal areas of western Japan and southern Korea almost every summer to autumn. In laboratory culture, the optimum temperature for growth of the species is ca. 27 °C. Since the species cannot survive in water of temperatures of less than 10 °C, it was considered to over-winter in some certain regions as a motile form or resting cyst, and expand its distribution after the temperature increases to a level tolerable for growth. To determine the over-wintering regions and migration pattern of C. polykrikoides, occurrences of the motile cells were surveyed in the coastal and offshore areas of western Kyushu, Japan and south coast of the Korean Peninsula from April 2006 to August 2008. Cells of C. polykrikoides were found at 14 sites during the investigated period. Motile cells occurred throughout the year in Usuka Bay, Hirado of West Japan. From offshore regions of the Goto Islands and off Shin-Nagasaki Fishing Port, motile cells of C. polykrikoides were first detected from late May, and continuously occurred until February in Nama Bay of the Kami-Goto Islands. This first appearance was before red tides of C. polykrikoides reported at coastal areas in western Kyushu. In Korea, this species was first observed in May and disappeared after October in 2007. These occurrence patterns imply that Usuka Bay in Hirado is one of the over-wintering regions in western Kyushu, and also this species is possibly transported into the northern part of the East China Sea by the Tsushima Warm Current every year.