Between 2009 and 2013, the head region of 102 juveniles of Atlantic bluefin tuna (Thunnus thynnus L.) caught in four nursery areas of the Mediterranean Sea (Balearic Sea, Ionian Sea, Ligurian Sea and Tyrrhenian Sea) were analysed for parasites. Eleven parasite species were found: Capsala magronum, C. onchidiocotyle, C. paucispinosa, Nasicola klawei, Hexostoma thynni, Didymocystis sp. 2 (sensu Rodríguez-Marín et al., 2008), Didymosulcus sp. 2 (sensu Rodríguez-Marín et al. 2008), Didymosulcus wedli, Didymozoon pretiosus, Nematobothriinae gen. sp. and Wedlia sp. The prevalence of some food-borne parasites (Didymocystis sp. 2, D. pretiosus, Nematobothriinae gen. sp. and Wedlia sp.) had significant differences between localities (p ≤ 0.05). The results showed that the parasite fauna of juvenile tunas is not homogenously distributed in the Mediterranean Sea: parasite assemblages differed between hosts from the Balearic, Ionian, Ligurian, and Tyrrhenian seas, suggesting parasites as possible tags to identify the different tuna populations from the corresponding nursery areas.

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