engleski

Farming northern bluefin tuna (Thunnus thynnus L.) in the Adriatic sea

Technology of northern bluefin tuna (Thunnus thynnus) farming in the Adriatic Sea originated from southern bluefin tuna (Thunnus maccoyii) aquaculture experiences gained in Australia. Eventually, the transfer of knowledge and investments by ex Croats living in Australia enabled a rapid increase of northern bluefin tuna (BFT) farming in the eastern Adriatic. The first pilot program was established in 1996 and the amount of 39 MT of gutted and gilled bluefin tuna product was exported to Japan. In the 2003 and 2004 production was 4.700 and 3000 MT respectively. In principle, two different operational modes in BFT aquaculture are practiced and should be distinguished on the basis of duration of the rearing period. Firstly, BFT “ fattening” for periods less than 1 year, where the principal aim is to increase fat content in the flash in order to meet the requirements of sashimi market in Japan, consequently increasing the price. Secondly, BFT “ farming” for a period of a couple of years with principal aim to increase the BFT biomass without increasing fishing mortality rate. Croatian BFT farming is based on capture of wild specimens of different sizes, which are transferred alive to floating cages. The fish are then kept in large cages for variable periods, depending on the fish size and farming conditions. Tuna are normally fed baitfish six days a week. During the farming period BFT is normally overfed and conversion rations (CR) are about 15-20 : 1, varying with the culture season, quality of baitfish, and age of fish caged. However, BFT industry is facing two main obstacles. Firstly, environmentalists are saying that BFT farming and fattening have disastrous effects on marine environment and that are incompatible with other sectors such as tourism and related coastal activities. Secondly, biologists are worried that young tuna placed in cage before its sexual maturity does not contribute to recruitment of natural stock. Due to the fact that northern BFT farming technology is still in the initial development stage, additional improvements of zoo-technical measures (i.e. feed and feeding, stocking density) are needed to minimise its adverse impact on the environment. Additionally, positive attitude towards such a practice as a legitimate way to increase capturing fish biomass that is limited by TACs is needed. One of the most challenging research goals would be the development of manufactured feeds to replace the use of baitfish as to benefit both the economy and environmental performances of BFT industry. Furthermore, capture-based aquaculture has been proved in culturing number of marine finfish such as eels, groupers and yellowtails. The fact is that no aquaculture that depends on seed fish from natural stocks can be considered as a fully sustainable aquaculture activity. The most severe problem for a capture-based aquaculture is supply in fingerlings, as natural stocks are known to fluctuate considerably year-by-year. At this stage, BFT farming may facilitate research and enable experiences that are essential for development of economically feasible BFT aquaculture with fully controlled fish life cycle. Encouraging result was reached for the first time in 2002 in Japan, where capture-based farming of juvenile BFT (150-500 grams) for 3-4 year rearing period has been practiced since 1970, when Japanese researchers at the Kinki University successfully completed the full life cycle of BFT.

Bluefin tuna; farming; Adriatic Sea

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