Metazoan gill parasites of wild albacore Thunnus alalunga (Bonaterre, 1788) from the Balearic Sea (western Mediterranean) and their use as biological tags

Metazoan gill parasites of 30 albacore Thunnus alalunga caught in the Balearic Sea (western Mediterranean) were examined for parasites with the aim to evaluate their possible use as biological tags. A total of 9 species of parasites were found: 1 capsalid monogenean, 6 didymozoid trematodes and 2 crustaceans. Most of the parasites collected were didymozoids (95.8% of all specimens) and Didymozoon longicolle was the dominant species. Albacore is a new host record for Capsala paucispinosa and Didymozoon pretiosus, while Didymosulcus aahi, Didymosulcus dimidiatus, Nematobothrium latum and Rocinela sp. are for the first time reported from the Mediterranean Sea. Significant differences were found grouping data by host size, with lower infection levels in the larger sized fish, whereas no differences were found between host sex. Most of the parasites showed a high site selection: D. aahi, D. dimidiatus and D. longicolle had significant differences of prevalence between internal and external margins of gill filaments, and almost all specimens of Pseudocycnus appendiculatus were attached to the gill filaments of the second and third holobranchs. The usefulness of parasites as biological tags is discussed; particularly, D. longicolle and D. pretiosus could be used to separate Mediterranean and northeast Atlantic stocks of albacore.