Induction of oocyte maturation and ovulation with gonadotropic hormone injection in wild Japanese Sea Bass, Lateolabrax japonicus

Sequential changes in body weight and oocyte development after injection of gonadotropic hormones were examined for wild Japanese Sea Bass collected d uring spawning season in 1991 and 1992. Just before hormonal treatment oocy te samples were sucked out with a catheter and maturational stage of oocyte s was examined by microscopic and histological observations. Twenty mature females (3.7 to 5.9 kg) in 1991 and 13 females (2.3 to 7.2 kg) in 1992 spaw ning seasons, collected by longlines near the coast of Kujukuri in Chiba Pr efecture, were used, They were injected with both human chorionic gonadotro pin (HCG) and chum salmon pituitary extract (CP). Based on changes in the f ish body weight and maturational stage of oocytes, artificial inseminations were conducted. The period from injection to ovulation varied depending on maturational stage of oocyte at the time of injection: 3-4 days needed fro m oocytes at the secondary yolk globule stage (oocyte diameter ranged 560-7 00 mu m), 2-3 days from oocytes at the tertiary yolk globule stage (690-800 mu m), 2 days from oocytes at the migratory nucleus stage (800-880 mu m), 1 day from prematuration and mature stages (890-1080 mu m and 1100-1190 mu m). The present results combined with previous results suggest that time at which optimum egg quality is achieved is within one day after ovulation an d optimum hormonal doses/kg fish are ca. 500 IU of HCG and ca. 20 mg of CP.