Induction of spawning of captive-reared Senegal sole (Solea senegalensis) using different administration methods for gonadotropin-releasing hormone agonist Maria J. Agulleiro a,c , Victoria Anguis d , José Pedro Cañavate d , Gonzalo Martínez-Rodríguez e , Constantinos C. Mylonas f , Joan Cerdà a,b,c, ⁎ a Center of Aquaculture-IRTA, 43540-Sant Carles de la Ràpita, Tarragona, Spain b Lab IRTA-ICM, CMIMA (CSIC), Passeig Marítim 37-49, 08003-Barcelona, Spain c Reference Center in Aquaculture, Generalitat de Catalunya, Spain d Centro de Investigación y Formación Pesquera y Acuícola (CIFPA) “El Toruño”, Junta de Andalucía, Apartado 16, 11500 Puerto de Santa María, Cádiz, Spain e Instituto de Ciencias Marinas de Andalucía (CSIC), Apartado Oficial, Puerto Real, Cádiz 11510, Spain f Institute of Aquaculture, Hellenic Center for Marine Research, P.O. Box 2214, Heraklion 71003, Crete, Greece Received 9 September 2005; received in revised form 17 December 2005; accepted 2 February 2006 Abstract Captive-reared Senegal sole were treated with intramuscular injection or sustained-release implants of gonadotropin-releasing hormone agonist [D-Ala 6 , Pro 9 , 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 7days 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. © 2006 Elsevier B.V. All rights reserved. Keywords: GnRHa; Sexual steroids; Spawning; Sperm; Implant 1. Introduction During the last decades, marine fish aquaculture in Southern Europe has been mostly concentrated on Aquaculture 257 (2006) 511 – 524 www.elsevier.com/locate/aqua-online ⁎ Corresponding author. IRTA Lab, Room B46, CMIMA-CSIC, Passeig Marítim, 37-49, 08003-Barcelona, Spain. Tel.: +34 93 230 95 31; fax: +34 93 230 95 55. E-mail address: jcerda@icm.csic.es (J. Cerdà). 0044-8486/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2006.02.001