EFFECTS OF EXTENDED DAYLENGTH ON OVARIAN MATURATION AND HCG-INDUCED SPAWNING IN YELLOWTAIL FED MOIST PELLETS

The present study examined the effects of extended daylength (18:00-24 :00) on ovarian maturation and HCG-induced spawning in yellowtail fed moist pellets. In the experiments conducted in 1991, 1992, and 1993, t he daylength was extended by 6 h for 20 or 28 days followed by an inje ction with hormone, HCG. Female yellowtail brood stock kept under exte nded daylength were induced to mature more rapidly than those kept und er natural lighting conditions. The mean number of eggs produced per f ish was more than 200 x 10(4) in the extended daylength groups, compar ed to about 100 x 10(4) in the control groups. The rate of normal eggs , rate of hatching, rate of normal larvae, and survival activity index (SAI) of larvae obtained from the extended daylength groups were sign ificantly higher (p < 0.01) than those of the control fish. These resu lts demonstrate that manipulation of daylength is an effective method for acceleration of final maturation in female brood stocks of yellowt ail.