STUDIES ON THE TYPE OF SPAWNING OF THE GRASS CARP,CTENOPHARYNGODON IDELLUS

This paper deals with the histological structure of the ovaries of the sexually mature pond-cultured Grass Carp as well as river Grass Carp both before and after spawning. The results of our examination demonstrate that the Grass Carp is a type of fish that spawns only once a year.The fact that some of the female fish, after induced ovulation,spawn perfectly while the others spawn partially is owing to the spawners' different degrees of maturity. The female fish caught from the natural spawning ground of the Hsiang River (in Hunan Province) also spawn perfectly or partially. The histological structure of the ovaries, after perfect spawning through induced ovulation, contains oocytes of phase Ⅰ and phase Ⅱ. The ovaries after partial spawning through induced ovulation contains phase Ⅰ, phase Ⅱ and phase Ⅳ (Ⅳ+, Ⅳ++ ). The histological structure of the ovary reaching full maturity without induced ovulation consists of phase Ⅰ, phase Ⅱ and phase Ⅳ(Ⅳ++). In the ovary of the Grass Carp spawner, no matter whether subjected to induced ovulation or not, there are no oocytes in phase Ⅲ. The above results are also true of the ovaries of the gravid Grass Carp caught from the Hsiang River which spawned perfectly, partially, or which had not yet spawned at the time of being caught. It is demonstrated that oocytes develop from phase Ⅲ into phase Ⅳ synchronously. Oocytes in the ovary of the pond-cultured Grass Carp, after perfect spawning in May, remains in phase Ⅱ during June, July, August and September and no new oocytes of phase Ⅳ are found in the spent ovary. Hence the ovary of the mature fish, after perfect spawning in late spring or early summer, cannot complete the course of development from phase Ⅱ through phase Ⅳ in the same year. The coefficients of maturity of the ovaries in the pond-reared Grass Carp exhibit only one peak during one reproductive cycle.