Metazoan gill parasites of the Atlantic bluefin tuna Thunnus thynnus (Linnaeus) (Osteichthyes: Scombridae) from the Mediterranean and their possible use as biological tags

The gills of 63 specimens of the Atlantic bluefin tuna Thunnus thynnus (linnaeus) (osteichthyes: scombridae) from three
localities of the Mediterranean (sardinian, tyrrhenian and levantine seas) were examined for metazoan parasites. the parasite fauna
of T. thynnus from the sea of sardinia included 11 species: five didymozoid trematodes, three capsalid and one hexostomid monoge-
neans, and one caligid and one pseudocycnid copepods. Four didymozoids were found in fish from the levantine sea and only one
didymozoid was recorded in fish from the tyrrhenian sea. Dividing the hosts into four size-groups (small, medium-sized, large and
extra large), the pairwise comparison of prevalence and mean abundance of the new and literary data) showed differences according
to host size. the differences in the composition of the parasitic faunas and in the prevalence of parasites, observed between the small
tunas from the tyrrhenian sea and the medium-sized tunas from the Adriatic sea, levantine sea and the North-East (NE) Atlantic
ocean, indicated that these groups form discrete units. the parasite fauna of the large tunas from the sea of sardinia is the richest
among the bluefin tuna populations of the Mediterranean and the NE Atlantic, due to the presence of species not found elsewhere
in bluefin tunas, such as Caligus coryphaenae steenstrup et lütken, 1861, Capsala magronum (ishii, 1936) and C. paucispinosa
(Mamaev, 1968). this fact and the prevalence of some parasites of this group (lower than those of medium-sized fish from the NE
Atlantic and higher than the small and medium-sized tunas from the Mediterranean) suggest that the large-sized tuna group in the
western Mediterranean is formed by Mediterranean resident tunas (poorly infected), and by tunas migrating from the Atlantic ocean (heavily infected).