BLUEFIN TUNA (Thunnus thynnus) FARMING IN TURKEY
The bluefin tuna (BFT) is a commercially important and heavily exploited teleost fish in the world's oceans and seas, and occupies a significant place in Turkish marine fisheries. Moreover, it is one of the most important species of aquaculture.
The BFT fishery to catch fish to be fattened is carried out by purse seines in the Eastern Mediterranean Sea between May and June. After tuna are captured by purse seines, divers join the seine to the towing cages underwater, and the live BFT are transferred to the towing cages. The live BFT in the towing cages are brought to fattening farms by tugging the towing cage with an average speed of 1-1,5 miles. During that time, fish are continuously monitored by the divers and dead fish are taken on board. It generally takes 7-10 days to bring the BFT in towing cages to the feeding cages depending on the fishing areas (Antalya, Levantine Sea). It takes 30-35 days to bring the towing cages to the farms in Izmir, Cesme (Aegean Sea), which is the farthest farm from the fishing area. The percentage of fish that die during the fishing operation is 1%, while the percentage of fish that die during transport to the farming area is 3%.
In the BFT fattening farms, cages are made of high density polyethylene (500 mm Ø) pipes with a diameter of 50 m. Nets with a depth of 20-30 m and a mesh size of 80-110 mm are bound to these round cages. These cages are installed in areas with high currents and depths of at least 50 m and 1000-1200 m away from shore. The cages are fixed to the base with an anchoring system which is dependent on the regional conditions, and which was investigated before installing the cages. Generally, plow-type anchors with a weight of 750-1000 kg are used. The volumes of the surface buoys used in towing cages are between 260 and 1250 lt. At a density of roughly 2-3 kg/m3, up to 130 tonnes of fish can be stocked in each cage.
After the transfer of the BFT to the feeding cages, fish were found to be under stress and some wouldn't feed for up to one month. BFT were fed twice a day 6 days a week. In some farms, BFT were fed once a day. The following species were used as feed: imported herring (Clupea harengus), capelin (Mallotus villosus), shad (Alosa alosa), sardine (Sardina pilchardus), and Atlantic mackerel (Scomber scomber) from Spain, Mauritania, Norway and Holland. Apart from these fish, horse mackerels, anchovies and calamares caught in Turkish waters were also provided as feed.
The BFT are stocked in the cages between May and June and are fed for 4-9 months. After January, the farming operations are negatively affected by high winds, currents and rainfall, and by storms and decreased water temperatures. Decreases in water temperature cause a decrease in food consumption and, accordingly a decreased growth rate of the fish. Generally, food consumption changes depending on the water temperature. For these reasons, the fattening operations are carried out only until January. Tuna are sold as fresh or frozen and most of the fresh tuna are sent to Japan market by air. Vessels from East Asia countries and those in the Mediterranean Sea buy tuna from the Turkish farms and sell it as frozen tuna in the Asian market.