Induced sex reversal and spawning of blue-spotted grouper, Epinephelus fario

Abstract

Attempts were made to induce sex reversal of under 2-year-old blue-spotted groupers and to induce spawning of 4-year-old females. Mature males, through sex reversal, were obtained successfully by the oral administration of methyltestosterone (MT) at daily feeding doses of 0.5 mg and 1.0 mg MT/kg body weight for 5 months. The sperm concentration was 8×10 7 cells/ml milt. The 2-year-old fish attained a sexually mature state in June and July with an average gonadosomatic index (GSI) of 2.07%. Changes in GSI and serum profile of estradiol-17β and 17α-testosterone were correlated with stage of gonad development, including vitellogenesis, sex transition and spermatogenesis. The values of GSI, serum estradiol and testosterone were elevated towards the completion of vitellogenesis and depressed during the sex transition phase. An increase in sex steroid concentrations, testosterone in particular, was recorded during spermatogenesis. Serum testosterone concentrations, ranging between 150 and 200 pg/ml serum in the prespawning season, gradually increased as vitellogenesis progressed and reached a level of 633 pg/ml serum in the mature females during the spawning season. Under the influence of MT administration, serum testosterone concentrations increased to 1100 pg and 1630 pg/ml serum at the end of a 170-day period at daily feeding doses of 0.5 mg and 1.0 mg MT/kg body weight, respectively. Spawning was induced by multiple injections of human chorionic gonadotropin (Puberogen) at doses of 2000–3000 IU HCG/kg fish. The frequency of hormone injections depended upon the stage of sexual maturity, as indicated by the stage of oocyte development and initial oocyte diameter. Two injections, 24 h apart, were found adequate when the recipient females contained mature oocytes at the tertiary yolk globule stage and a mean diameter of greater than 0.55 mm. For fish containing oocytes in the range of 0.50 to 0.55 mm, three injections were required to achieve successful spawning.