Histological investigation on the ovarian cycle of the bluefin tuna in the western and central Mediterranean A. C ORRIERO *, S. D ESANTIS *, M. D EFLORIO *, F. A CONE †, C. R. B RIDGES ‡, J. M. DE LA S ERNA §, P. MEGALOFONOU { AND G. D E M ETRIO * , ** *Department of Animal Health and Welfare – University of Bari, S.P. per Casamassima km. 3, I-70010 Valenzano (BA), Italy, †Department of Animal Biology – University of Sassari, via Vienna 2, I-07100 Sassari, Italy, ‡Institut fu ¨r Zoophysiologie, Lehrstuhl fu ¨r Stoffwechselphysiologie, Heinrich Heine Universita ¨t, D-40225 Du ¨sseldorf, Germany, §Centro Oceanografico de Malaga, Apartado 285, 29640 Fuengirola, 4 IEO, Malaga, Spain and {Department of Zoology, Marine Biology – University of Athens, Panepistimiopolis GR 15784 Athens, Greece (Received 6 August 2002, Accepted 24 April 2003) The histological analysis of eastern Atlantic bluefin tuna Thunnus thynnus ovaries caught from February to September 1999–2000, made it possible to distinguish the presence of seven oocyte developmental stages and allowed the characterization of six time-dependent ovary maturity stages. The ovaries of mature (fork length, L F  110 cm) bluefin tuna were non-active from August (spent period) to March (quiescent period) when they contained only perinucleolar- stage oocytes. Ovary development started in April to early May (recrudescent period) with the appearance of oocytes at the lipid stage. Vitellogenesis appeared in mid-May (ripening period) and post-vitellogenesis occurred in late May to mid-June (pre-spawning period). In late June to early July, hydrated oocytes, a sign of imminent spawning, were found only in specimens caught in Balearic waters. Females ranging between 100 and 110 cm L F , captured during the recrudescent and ripening periods, had the largest oocytes at the lipid stage, most of which were degenerating. An extensive vitellogenic atresia was observed in the ovaries of five females caught during the spawning period in non-spawning areas. # 2003 The Fisheries Society of the British Isles Key words: histology; oocytes; ovarian cycle; reproduction; Thunnus thynnus. INTRODUCTION The Atlantic bluefin tuna Thunnus thynnus (L.) is one of the most important commercial species among the large pelagic fishes living in the Atlantic Ocean and Mediterranean Sea (Susca et al., 2001). The International Commission for the Conservation of Atlantic Tunas (ICCAT) recognizes two management units of Atlantic bluefin tuna: west and east Atlantic, the latter unit including the Mediterranean. In the last few years, **Author to whom all correspondence should be addressed. Tel.: þ39 080 5443907; fax: þ39 080 5443908; email: g.demetrio@veterinaria.uniba.it Journal of Fish Biology (2003) 63, 108–119 doi:10.1046/j.1095-8649.2003.00132.x, available online at http://www.blackwell-synergy.com 108 # 2003 The Fisheries Society of the British Isles