Gonadal differentiation in relation to sex control of grass carp, Ctenopharyngodon idella (Pisces: Cyprinidae)

Some attempts to sex reverse grass carp (Ctenopharyngodon idella) have failed because androgen treatment was not administered during the period of gonadal differentiation. Early gonadal development was examined in 460 grass carp between the ages of 17 and 675 days (9 to 462 mm in standard length). Sex could not be determined for gonads of fish younger than 45 to 50 days old (less than 36 mm). Two phases of gonadal differentiation were distinguished; an anatomical change was seen for grass carp 50 to 75 days old (average length, 47 to 56 mm), but cytological differentiation occurred only in older and larger individuals. The transformation of gonial cells into gonocytes was easier to see in females than males. Development of gonocytes was delayed in slower growing individuals, but occurred in most females 180 to 232 days old (112 to 130 mm). The sex ratio (392 9: 379 6) was not significantly different from the expected 1:1 ratio.