A PRELIMINARY STUDY ON THE PHENOMENON OF SPAWNING TWICE A YEAR IN THE GRASS CARP, SILVER CARP AND BIG HEAD UNDER THE CONDITION OF STRENGTHENED CULTIVATION

This paper summarizes the authors' practical experiences for years on making the Grass Carp (Ctenopharyngodon idellus), Silver Carp (Hypophthalmichthys molitrix) and Bighead (Aristichthys nobilis) to spawn twice a year under the condition of strengthened cultivation. A preliminary cyto-morphological and histological study of ovarian development demonstrates that immediately after the first spawning of the pond-cultured fishes, oocytes of Phase Ⅱ, Phase Ⅲ and a small number of residual Phase Ⅳ oocytes can be seen to coexist simultaneously. Furthermore, the early, middle, and advanced stages of Phase Ⅲ oocytes can be clearly distinguished. From these observations the authors come to the conclusion that Phase Ⅲ oocytes can develop into Phase Ⅳ oocytes in the same year under the condition of strengthened cultivation of the spawners. If the spent spawners are cultivated in an ordinary way without strengthening, their ovaries will degenerate into Stage Ⅱ. Thus the non-synchronous development of the oocytes in the ovaries of these fishes is the intrinsic factor which accounts for the success of a second or even a third induced spawning within the year.