Occurrence of Bivagina tai (Monogenea: Microcotylidae) on the Gills of Cultured Red Sea Bream Pagrus major

Abstract

Occurrence of a gill monogenean Bivagina tai was monitored monthly on a single stock of red sea bream Pagrus major artificially spawned and cultured in a net cage. The monogenean was found from the fish stock all the time except for the first few months, significantly in greater numbers on the first gills than on the others.
The infection fluctuated with seasons. There were three peaks of infection during the two-year study period. The O+ fish had the highest level of infection in winter, but the infection was modest in the next winter, suggesting the peak infection resulted from lowered resistance of the small fish to infection at low temperatures. A second and a third peak of infection were formed in May, when the fish was just one and two years from hatching; the rise of water temperature was probably responsible for the peak formation.
From the seasonal changes in the development of B. tai, using the clamp number as an in-dicator, it is deduced that the monogenean had three, possibly four, generations a year. The life-span is estimated as 3-5 months in winter and as 2-3 months in spring through autumn.