engleski

Diseases of Atlantic bluefin tuna (Thunnus thynnus) farmed in the Mediterranean

Except for being one of the most physiologically fascinated large pelagic fish that exhibit partial endothermy, high metabolic rate and ability to expand and forage from tropical and subtropical through arctic waters (Block and Stevens, 2001), bluefin tuna’s particularity reflects as well in the surprising resistance to pathogens if reared in proper zootechnological conditions. Surprisingly small number of infective and noninfective diseases has been reported in a period of some 20-years of bluefin tuna cage rearing in the Mediterranean area (Miyake et al., 2003). At the early beginnings however, most mortalities were connected to the sharp changes of environmental in respect to meteorological conditions (e.g., lightening, storm, heavy rains, or forest fires) or inappropriate zootechnical manipulations. Once these obstacles were over passed, and more practical knowledge has been gained, diseases become seldom a part of the tuna aquaculture. Partially the reason lays in the fact that only large, robust and immunocompetent individuals are caught for the farming purposes. If compared with other finfish species, tuna farming cycle in most parts of the Mediterranean lasts short period of time (around 3-6 months), where rarely a development of the diseases is plausible under the good zootechnology practice. First “diseases filter” is suggested to be the towing process of fish by a purse seiner from the fishing ground to the cages, when an initial part of the least resistant or mechanically damaged animals die in the transport, dismissing a potential media for the diseases outbreak. The second “filter” is the period after the transfer to the farm cages, lasting couple of weeks, which leaves only the fittest and presumably the most resistant fish for the rearing. When small tuna averaging 30 kg of weight are caught for farming purposes in Croatia, they usually enter cages abundant with helminthic parasites unlike large tuna. However, it has been observed that they shed away the parasites in the first couple of months in the cages with adequate rearing practice. This fact is even more interesting if we take in consideration that tuna are in one proportion fed by the fresh baitfish, usually indicated as a parathenic or intermediate host for many helminthes isolated in tuna (Yamaguti, 1970 ; Lester, 1980). At the same time, feeding with frozen imported baitfish to cage-reared tuna initially raised a debate whether is possible that allochtone viral pathogens, mostly viral hemorrhagic septicemia (VHS) from the herring (Marty et al., 1998), became propagated through its susceptible host to reared tuna or even to the susceptible naïve population of baitfish. For the moment however, no firm scientific evidence exists in accordance to this viral transmission (Yamaguti, 1970 ; Jones et al., 1997). Nonetheless, tuna pathology is still a developing area of research that misses firmer links with elaborated physiological processes of the animal, and efforts have to be undertaken to understand tuna immunological background, stress response and adaptation to the life in sea-cages, in order to develop a sustainable and welfare-conscious environment for the rearing of one of the most precious aquaculture fish today.

bluefin tuna, Mediterranean, aquaculture, diseases

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