Seasonal cycle of gonadal development and serum levels of vitellogenin of the red porgy, Pagrus pagrus (Teleostei : Sparidae)

Histological examination of gonads of female and male red porgy, Pagrus pag rus, reared in captivity, was carried out in order to describe the main gon adal changes related to gametogenesis and the seasonal changes of environme ntal factors. The gonadosomatic index (GSI) and the concentration of serum vitellogenin (VTG) were also determined. The frequency distribution of gona d development stages and the GSI and vitellogenin concentration during the annual cycle indicated the separation of the female and male reproductive c ycles into three main periods. The autumn period when gametogenesis begins (October-November), the period of exogenous vitellogenesis (December-March) or spermiation (December-March), and the spawning season (March-May). The spawning period coincided with an increase in temperature (15-19 degreesC) and daylength. Serum levels of vitellogenin rose significantly in January, reached a peak at the beginning of the spawning period (March, 405.5 mug ml (-1)) and remained high until the end of the spawning period (May).