Reproductive status of Atlantic bluefin tuna, Thunnus thynnus, during migration off the coast of Sardinia (western Mediterranean)

From late April until mid-June Atlantic bluefin tuna, Thunnus thynnus (Linnaeus, 1758), migrates along the western coast of Sardinia to reach spawning grounds of the Mediterranean. Despite the substantial information on the reproductive biology of the species, the tracking of its reproductive parameters along the migratory pathways is far to be completed. This study provides some reproductive parameters for adult bluefin tuna collected from the last trap fishery Tonnara of the Mediterranean. The following points summarize our results: (i) gonadosomatic index (GI) was positively correlated with fork length (FL) and varied significantly between sexes, having higher values in males; (ii) ovaries contained a mixture of oocyte stages with primary growth oocytes representing 72%, cortical alveoli 16%, primary and secondary vitellogenic stages 4.6% and 6% respectively, only a small proportion of late vitellogenic oocytes and atretic follicles; (iii) batch fecundity was 95-125 oocytes per gram of body mass and varied from 1.6 million for a female of 105 cm FL, to 29.5 million eggs for a 233 cm FL specimen; (iv) sex ratio differed significantly from 1:1, with males being predominant both for the size classes and years analyzed. These findings suggest that bluefin tuna from western Sardinia exhibits intermediate reproductive characteristics between specimens migrating through the Strait of Gibraltar and those from the spawning grounds of the Balearic Islands.