Induction of spawning of captive-reared Senegal sole (Solea senegalensis) using different administration methods for gonadotropin-releasing hormone agonist

Captive-reared Senegal sole were treated with intramuscular injection or sustained-release implants of gonadotropin-releasing hormone agonist [D-Ala6, Pro9, NEt] GnRHa (GnRHa) to induce spermiation and spawning. Fish were treated with GnRHa during spring (April-May) or autumn (October), the natural spawning seasons known to occur in wild Senegal sole. During spring, females injected with a dose of 5 μg GnRHa kg− 1 three times a week, or treated with a single GnRHa-loaded implant (50 μg kg− 1) showed multiple ovulations and spawns within a period of approximately 30 days. Plasma levels of 17β-estradiol (E2) were elevated at 7 days post-treatment, preceding the spawns with the highest fecundity, concomitant with a decrease in the levels of plasma testosterone (T). During autumn, however, females changed their responsiveness to GnRHa treatment, a dose of 5 μg kg− 1 being ineffective in inducing spawning, while a dose of 1 μg kg− 1 induced a few spawns. Accordingly, plasma E2 levels showed a small increase after injection of 1 μg kg− 1 GnRHa. In contrast to females, GnRHa injection or implantation in males was ineffective in enhancing spermiation or milt production, regardless of the season when the treatments were applied. However, GnRHa treatment in males increased plasma levels of T and 11-ketotestosterone (11-KT) during spring, while in autumn the levels of plasma androgens decreased after GnRHa injection. These results show that administration of GnRHa during spring either by injection or sustained-release implants induces multiple spawns in captive Senegal sole, although these treatments were ineffective in inducing or enhancing sperm production in males.