Infection dynamics of Microsporidium seriolae causing the beko disease of yellowtail Seriola quinqueradiata and amberjack S. dumerili were investigated at fish farms and an experimental facility. A 4-year survey (2006-2009) showed a sharp increase in prevalence of infection with M. seriolae in June or July soon after stocking net cages with yellowtail and amberjack wild fingerlings. Most cysts blackened and finally disappeared in November in the following year. However, in 2006 when the levels of infection were relatively high, some visible cysts still remained even in autumn of the following year. Infective periods and effects of fish size on M. seriolae infection were investigated via natural exposure of hatchery-reared yellowtail juveniles by transferring to an open sea cage in an endemic area. Invasion of the parasite into fish occurred in June and July but not after the mid-August. A comparison of M. seriolae infection among four size classes of yellowtail showed that the smallest fish (1 g in mean weight) had the lowest prevalence of infection.
2011 The Japanese Society of Fish Pathology