Observations on health problems in hatchery reared tuna (Thunnus thynnus) in Italy.

In the last years, intense researches have been carried out in an effort to domesticate bluefin tuna (Thunnus thynnus) and develop a self-sustained aquaculture industry with the specific aim of alleviating the fishing pressure on the declining wild population. The low survival rates of hatchery reared tuna larvae currently represent a serious limiting factor in the development and achievements of aquaculture tuna production. Diseases can significantly contribute to increase the mortality rates during the first phases of larval production in mariculture. With the aim of studying the impact of infectious and parasitic diseases on production of hatchery reared bluefin tuna larvae, in the framework of the Italian Regional project ALLOTUNA (PS_085) aimed to improve the reproduction control of bluefin tuna in captive conditions, diagnostic analyses were performed on samples from different age groups and during mortality outbreaks in 2009 and 2010 (two production cycles). All the examined fish resulted negative for parasites and Betanodavirus. The bacteriological exam allowed to isolate Photobacterium damselae subsp. damselae, Vibrio sp. and Tenacibaculum sp. during mortality outbreaks. Histological observations showed the frequent occurrence of lesions referable to bacterial septicemia in gills, skin, muscle and internal organs. The most common pathological changes were observed in the gill, with severe hyperplasia, epithelial exfoliation, necrosis and telangiectasiae associated with the presence of abundant bacteria, and skin/muscular pathology. In particular, epithelial sloughing off was frequently observed in the skin with presence of deep ulcers in some cases reaching the dermis and the underlying muscle, with massive colonization by filamentous bacteria. The skeletal muscle often showed severe pathological changes characterized by hyaline degeneration and necrosis. The results of this preliminary survey seem to indicate as in the hatchery reared bluefin tuna stages the main health issues may be related to the occurrence of opportunistic bacterial infections triggered by environmental, husbandry and nutrition factors which are still the major problem to be solved in tuna hatchery production.