Induction of ovulation in captive-reared dusky grouper, Epinephelus marginatus (Lowe, 1834), with a sustained-release GnRHa implant

Captive-reared dusky grouper were induced to ovulate using a sustained-release delivery system (implant) loaded with gonadotropin-releasing hormone agonist [D-Ala(6), Pro(9), NEt]-GnRH (GnRHa). Thirteen females were implanted at doses ranging from 30.5 to 68.3 mug kg(-1) during three experiments between late June and early September. Of the injected females, 85% responded positively to the GnRHa implant and ovulated between 60 and 238 h after treatment, whereas none of control fish showed any sign of maturation. No spontaneous spawning was observed, and the eggs were manually removed from the females using gentle abdominal pressure. The mean number of ovulations per fish was 3.8, with a maximum of nine for one female. Overall, a total of 42 ovulations were obtained, resulting in the production of more than 5 million eggs. The average relative fecundity was 118.3 +/- 16.0 x 10(3) eggs kg(-1) BW, with a maximum of 202.2 x 10(3) eggs kg(-1) BW. Mean percentage fertilisation and hatching were 48.2% and 52.2%, respectively. The results demonstrate that GnRHa administration via controlled delivery systems is an effective method for producing good quality eggs in captive dusky grouper. (C) 2003 Elsevier Science B.V. All rights reserved.