Health survey of Atlantic Bluefin Tuna, Thunnus thynnus (Linnaeus, 1758), Reared in Adriatic Cages from 2003 to 2006

Tuna, Thunnus spp. (Linnaeus, 1758), is the most valuable finfish aquaculture product currently recognized around the world, with two main methods of farming presently employed. The most common is the fattening process where fish are kept in captivity for 6-10 months ; a method in wide use in the Mediterranean, Australia, Mexico and Japan. In the Adriatic Sea however, smaller specimens are caught and reared for a longer period of time (up to 3 years), in order to gain 50-80 kg of weight for marketable sale. Differences in the rearing technology are implicated in the isolation of different pathogen groups, as well as the diseases’ etiology. Between 2003 and 2006, parasitological examination was done on newly caught tuna specimens, fish reared for one or two years, harvested tuna and occasional mortalities. Bacteriological examination assessed by the incubation of bacterial colonies on different media and API system, and diagnosis of β nodavirus by immunohistochemistry, was done only when fish suffered elevated mortalities. The total number of isolated parasite species was 23, composed mostly of didymozoid digeneans, and showing variable impact on general tuna health. Secondary bacterial infections were contracted seasonally caused by Photobacterium damselae subsp. piscicida, to whom the highest losses were attributed during the spring epizootic outbreaks. During the study period, no nodavirus infections were diagnosed in fish suffering mortalities. Based on four years of findings, we argue that the major risk factor for the propagation of known and introduction of new pathogens in reared tuna is strongly related to the feeding methods employed. Imported frozen or fresh local baitfish represents a potential source of different pathogens, or if of poor quality, acts as a trigger for development of secondary bacterial diseases.