Season-, age-, and sex-related variation in the prevalence of bonamiasis in flat oysters (Ostrea edulis L.) on the south coast of Ireland

Monitoring of two age groups of native fiat oysters Ostrea edulis L. was undertaken in Cork harbour on the south coast of Ireland over 24 months to determine prevalence and intensity of infection of Bonamia ostreae (Piehot et al. 1980) and levels of mortalities occurring on the beds. Seasonal occurrence, as well as age- and sex-related variation were investigated. Two years appears to be a critical age in relation to disease development; bonamiasis was not detected in oysters younger than this, but once the infection appeared in this group, the disease quickly spread, and prevalence and intensity of infection and mortalities rose rapidly in the following months. Gonad smears indicated that males and females in both age groups were equally affected by the parasite.