Short communication GnRHa-induced spawning in cage-reared Atlantic bluefin tuna: An evaluation using stereological quantification of ovarian post-ovulatory follicles Guillermo Aranda a , Lourdes Aragón a , Aldo Corriero b , Constantinos C. Mylonas c , Fernando de la Gándara d , Antonio Belmonte e , Antonio Medina a, ⁎ a University of Cádiz, Department of Biology, Faculty of Marine and Environmental Sciences, Avda. Repúplica Saharaui, E-11510, Puerto Real, Cádiz, Spain b University of Bari, Department of Animal Health and Wellbeing, Str. Prov. per Casamassima Km 3, 70010 Valenzano (BA), Italy c Hellenic Center for Marine Research, Institute of Aquaculture, P.O. Box 2214, Heraklion 71003, Crete, Greece d Instituto Español de Oceanografía, Centro Oceanográfico de Murcia, Ctra. de La Azohía s/n, E-30860, Puerto de Mazarrón (Murcia), Spain e Tuna Graso, S.A., Carretera de la Palma Km 7, Paraje la Estrella, E-30593 Cartagena (Murcia), Spain abstract article info Article history: Received 28 September 2010 Received in revised form 7 April 2011 Accepted 15 April 2011 Available online 22 April 2011 Keywords: Thunnus thynnus Bluefin tuna GnRHa Induced spawning Fecundity Disector method 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. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The Atlantic bluefin tuna, Thunnus thynnus (L.), has been sustainably exploited in the Mediterranean Sea and East Atlantic Ocean for several centuries. During the last decades, however, increased fishing and farming activities have led to significant depletion of the stocks (Fromentin and Powers, 2005; Anon, 2009; MacKenzie et al., 2009). The first trials for rearing the congener Pacific bluefin tuna (Thunnus orientalis) were initiated in Japan in the 1970s (Masuma et al., 2008, 2011). Several years later, commercial rearing operations spread worldwide, mainly to Australia for southern bluefin tuna (Thunnus maccoyii), Mexico for Pacific bluefin tuna and the Mediterranean countries for Atlantic bluefin tuna (Farwell, 2001; Masuma et al., 2008). In particular, industrial farming of T. thynnus in off-shore cages has undergone a quick development in recent years throughout the Mediterranean Sea (Ticina et al., 2004; FAO, 2005; Mylonas et al., 2010a). A major drawback of bluefin tuna farming is that it relies on capture of wild stocks by purse seine and grow-out in floating sea cages. Thus, the expansion of Atlantic bluefin tuna farming activities in the Mediterranean Sea has generated a growing demand of wild specimens that has resulted in a negative impact on the natural resources (Mylonas et al., 2010a). This has raised increasing interest in developing spawning and larval rearing methods in order to produce fingerlings in support of a sustainable Atlantic bluefin tuna farming industry (Mylonas et al., 2007; De Metrio et al., 2010). Although considerable progress has been made in this regard, the economically feasible production of Atlantic bluefin tuna has not been achieved yet. Spontaneous spawning of captive Pacific bluefin tuna took place for the first time at the University of Kinki Fisheries Laboratory (Japan). Afterwards, the life cycle of this species was completed in 2002 when an artificially hatched and reared broodstock spawned spontaneously in an experimental net cage (Sawada et al., 2005). Recently, induced spawning of cage-reared Atlantic bluefin tuna and large-scale collection of fertilized eggs have been achieved in the southeastern coast of Italy (De Metrio et al., 2010) and the southeastern coast of Spain (Mylonas et al., 2007; De la Gándara et al., 2010), while spontaneous spawnings have been reported in different stocks in Spain (Gordoa et al., 2009; Mylonas et al., 2010a) and Croatia (Ivan Aquaculture 317 (2011) 255–259 ⁎ Corresponding author. Tel.: + 34 956016015; fax: + 34 956016019. E-mail address: antonio.medina@uca.es (A. Medina). 0044-8486/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2011.04.030 Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online