Occurrence of beko disease caused by Microsporidium seriolae (Microspora) in hatchery-reared juvenile yellowtail

Hatchery-produced juvenile yellowtail (Seriola quinqueradiata) were transferred from an indoor rank to sea netpens on June 20, June 30, July 10 and July 31, 1995 (4 groups), and each group was monitored at about 10 day intervals for Microsporidium seriolae, the causative agent of beko disease. Water temperature gradually increased during the study period, ranging from 20.8 to 30.2 degrees C. In the first 3 fish groups, almost all fish were found to be infected. However, prevalence of infection in the last fish group was much lower than in the other Groups which were transferred later attained their maximum prevalence of infection within a shorter Furthermore, we investigated the effects of water temperature and fish size on the development of M. seriolae in juvenile yellowtail. Two different fish size classes (average body weight 0.3 g and 22.0 g) were infected with M. seriolae to a similar extent. In the histological examination, the development of M. seriolae and host recovery were accelerated in fish reared at 25 degrees C compared to those kept at 20 degrees C.