Induction of Spawning in Captive Female Thicklip Grey Mullet (Chelon labrosus) Treated with Different GnRHa Delivery Systems

Thick-lipped grey mullet (Chelon labrosus) has important characteristics that make it a promising candidate species for diversifying Mediterranean aquaculture. However, spontaneous spawning in captivity has not been documented, mainly due to failure of females to spawn, highlighting the need for further research on reproduction control. This study evaluated the efficacy of GnRHa administration, using repeated intramuscular injections or slow-release Ethylene–Vinyl Acetate (EVAc) implants combined with a dopamine antagonist (metoclopramide, Met), in terms of spawning performance and egg quality. Three groups were established: (a) saline injection (0.9% NaCl; Saline-INJ), (b) GnRHa [Des-Gly10, D-Ala6-ProNEth9-mGnRHa] injection at 10 μg kg−1 BW (GnRHa-INJ), and (c) EVAc implant containing GnRHa at 50 μg kg−1 bw (GnRHa-IMP). Over four weeks, Saline-INJ and GnRHa-INJ females received weekly saline or GnRHa, respectively. GnRHa-INJ and GnRHa-IMP females also received weekly Met (15 mg kg−1 bw). GnRHa induced 11 spawns (1,768,680 eggs), nearly triple the Saline-INJ group (4 spawns, 394,400 eggs). Daily relative fecundity (DRF) and fertilization success were highest in GnRHa-INJ (56,982 eggs kg−1 day−1; 59.7%), followed by GnRHa-IMP (20,375; 18.8%) and Saline-INJ (13,061; 9.1%). Multiple injections showed a trend toward higher spawning performance and egg quality compared to implants, although variability was high and further replication is needed. Nevertheless, optimizing both GnRHa delivery methods could further enhance their effectiveness while maintaining operational benefits for aquaculture.