Spawning induction of individual European sea bass females ( Dicentrarchus labrax) using different GnRHa-delivery systems

Abstract

The present study evaluated the effect of different delivery systems loaded with gonadotropin-releasing hormone agonist (GnRHa) on spawning induction, and egg and larval quality in female European sea bass ( Dicentrarchus labrax). Mature females (mean±S.E.M. oocyte diameter=905±10 μm) were treated with [ d-Ala 6, Pro 9 NEt]-mGnRHa using GnRHa-loaded microspheres, Mc (60 μg kg −1 body weight, bw) GnRHa-loaded implants, EVAc (50 μg kg −1 bw) or received a single injection of GnRHa in saline, IN (15 μg kg −1 bw). Controls (C) were injected with saline. Females were allocated individually in 2-m 3 tanks with two GnRHa-treated males and were allowed to spawn without further handling. All GnRHa treatments induced spawning 3 days after treatment (d.a.t.), whereas control fish did not spawn during the 21-day experimental period. GnRHa microspheres and implants induced up to four consecutive spawns in one female, whereas GnRHa-injected fish spawned only once. Egg quality seemed relatively lower in all GnRHa-treated groups compared to spontaneously spawning fish (natural spawns, NAT group) maintained in large populations (80 fish in 15-m 3 tanks), possibly due to both the isolation of individual females in separate tanks and the stress from handling. Mean egg quality of the first spawns was variable, but similar in all GnRHa treatments (6.6–18.2% buoyancy; 21.7–27.4% hatching). In subsequent spawns, the Mc-treated females produced eggs of higher buoyancy than the EVAc group (20.5±6.0% vs. 2.87±1.14%), and similar to the first spawn of the GnRHa-injected group. Our results indicate that, compared to a single GnRHa injection, GnRHa-loaded microspheres can induce multiple spawns in European sea bass, without a negative effect on egg quality.