The effect of a delivery system (implant) loaded with gonadotropin-releasing hormone agonist (GnRHa) on the spawning performance in captive-reared Atlantic bluefin tuna (Thunnus thynnus) was assessed by means of stereological quantification of ovarian post-ovulatory follicles (POF). Among untreated control fish no follicles were observed at the oocyte maturation (OM) stage; in contrast, 10 of the 16 GnRHa-treated fish showed follicles containing migratory-nucleus oocytes and/or hydrating oocytes. Post-ovulatory follicles were observed only in three of the 14 untreated individuals examined, while all but one GnRHa-induced fish showed POF in the ovary. Nevertheless, when POF were present in the ovary, the amount of ovulated oocytes was not found to be significantly different among GnRHa-treated and untreated control fish, and wild spawners. The relative batch fecundity (eggs g−1 of body mass) estimated from ovarian POF counts were 109.62±28.36 (control group), 68.92±13.05 (GnRHa-treated group) and 83.47±15.63 (wild group). It was concluded that treatment with GnRHa implant stimulated OM and spawning in captivity, thus enhancing the reproductive potential of the broodstocks; however, there appears to be a limitation in the number of eggs maturing in each batch, and the spawn fecundity was not increased by the hormonal treatment.