Optimization of a GnRHa treatment for spawning commercially reared greater amberjack Seriola dumerili: Dose response and extent of the reproductive season

Reproduction control has been one of the main bottlenecks for the commercialization of greater amberjack Seriola dumerili, a promising species worldwide due to its rapid growth, large size and high market acceptability. Oocyte maturation, ovulation and spawning has been recently controlled successfully in a number of studies using hormonal therapies in the form of controlled-release delivery systems of Gonadotropin Releasing Hormone agonists (GnRHa). The objectives of the present optimization study were (a) to determine the most efficient GnRHa implant dose and (b) to examine the extent of the spawning induction period in greater amberjack reared in sea cages throughout the year, based on the resulting spawning kinetics, egg production and quality. Wild-caught greater amberjack were maintained in two commercial facilities, and were transferred to onshore tanks for spawning after treatment with GnRHa implants (a) of different doses (25 and 75 mu g kg(-1) body weight), or (b) at different times during the expected spawning season (June-July). Both doses were equally effective, and two consecutive implantations in 2 weeks resulted in mean ( +/- SD) total relative fecundity of 185,221 +/- 91,311 to 199,492 +/- 16,973 eggs kg(-1) in 11-18 spawns, without any significant differences in egg quality. Breeders maintained in sea cages were taken at different times and were induced to spawn successfully with a GnRHa implant of a recommended dose of 50 mu g kg(-1), during an effective period of at least 2 months when seawater temperature was 19-24 degrees C. The obtained information will contribute further to the delivery of an efficient egg production protocol to the aquaculture industry of the greater amberjack.