EVALUATION OF cGnRH IIa FOR INDUCED SPAWNING OF Synodontis nigriventris

Efficacy, reliability, and safety are of principal concern for spawning aids used in reproduction protocols for aquaculture species. Ovaprim®, a salmon gonadotropin releasing hormone analog (sGnRHa, D-Arg6-Pro9-Net, 20 μg/mL) and a dopamine antagonist (Domperidone, 10 mg/mL) is currently the preferred choice for induction spawning of ornamental fishes, however, this preparation may be unreliable or completely ineffective in some cases. cGnRH IIa (D-Arg6, Pro9-NHet) has garnered recent interest as an alternative GnRH subtype which offers increased biological activity, reliability, and may ultimately help to increase on farm productivity and expand the diversity of species able to be cultured. The objective of this study was to evaluate the efficacy of cGnRH IIa and Ovaprim® on various quantitative and qualitative measures of spawning performance in Synodontis nigriventris.

Ovarian biopsies were collected from female brood stock and those which exhibited >50% germinal vesicle migration were selected for use in the study. Four hormone doses were evaluated (50, 100, 200 µg/kg cGnRH IIa; 10 µg/kg sGnRHa), each of which also contained an equal concentration of a dopamine antagonist (5 mg/kg domperidone). Propylene glycol served as negative control. All injections were administered intramuscularly near the base of the dorsal fin. Fish were palpated to check for ovulation at 16, 20 and 24 hours post injection. Upon successful ovulation, eggs were manually stripped, weighed, and fertilized. A subsample was photographed under a dissecting microscope 2 to 3 hours post fertilization for subsequent determination of fertilization success and egg diameter. Hatching success for individual spawns was calculated from subsamples of fertilized eggs stocked into hatching containers. Hatching generally occurred 24 to 36 hours post fertilization, at which point the number of larvae present in each container was recorded.

Results of logistic regression models indicated no significant differences in ovulation success among the four hormone treatments at the 16, 20 and 24 hour time periods (P = 0.1643, 0.4851, 0.6801 respectively). Total fecundity (egg per gram body weight) was also not significantly different among treatments (P = 0.213). Likewise, fertilization and hatching success did not differ significantly among cGnRH IIa and Ovaprim® treatments (P = 0.435 and 0.892, respectively). Diameters of spawned eggs did vary significantly (P <0.05); however this was not considered to be biologically significant with the largest observed difference in mean diameter of only 63 µm. Taken together, these results suggest that cGnRH IIa exhibits comparable spawning performance to the industry standard Ovaprim®, for induced spawning of S. nigriventris.