Juvenile yellowfin tuna raised in sea cages for 8 months gain a lot of body fat, approaching the value of bluefin tuna's fatty belly meat
Yellowfin tuna is also used for sashimi, but of course it cannot match the high price of bluefin tuna, and because it is caught year-round, it is considered a common source of sashimi in Taiwan, China. Yellowfin tuna is currently all caught by fishing boats at sea, but yellowfin tuna can actually be farmed. The Fisheries Research Institute of the Council of Agriculture has just concluded a 4-year yellowfin tuna farming project. The Coastal and Offshore Resources Research Center of the Fisheries Research Institute analyzed the farming results, and they are actually quite good. The fat of yellowfin tuna is no longer inferior to the fatty belly meat of bluefin tuna, with a fat rate of nearly 8%, more than 15 times that of wild yellowfin tuna, and the farming period is only 8 months, which can both reduce capital costs and avoid the farming risks of typhoon season.
Wu Longjing, director of the Coastal and Offshore Resources Research Center of the Fisheries Research Institute, pointed out that the 4-year project began in 2014 and ended in 2018. After juvenile yellowfin tuna were caught at sea, they were placed in sea cages for farming. After about 3 years of farming, they could reach a market size of 36 kg. The feed cost during the rearing period for 300 juvenile yellowfin tuna was about NT$8 million. However, from the perspective of the market value of yellowfin tuna and consumer preference, operators are lacking interest in farming yellowfin tuna.
However, researchers found that there is no need to raise them to 36 kg; raising them to only 10 kg is enough. This not only greatly reduces financial pressure but can also achieve a commercial effect, though this depends on changing consumers' thinking. According to research records, when wild yellowfin tuna had a body length of 50 cm and a weight of 2.1 kg, the fat ratio was 0.2%; when the body length reached 136 cm and the weight reached 36 kg, the fat ratio was 0.4%, so yellowfin tuna is a low-fat source of fish meat.
However, the situation of yellowfin tuna farmed in cages in the research project was completely different. Juvenile yellowfin tuna started being raised from 1.5 kg. After a 4-month fattening process, when the body length reached 66.2 cm and the weight reached 5.44 kg, the fat ratio was as high as 7.47%, already more than 15 times that of wild 36 kg yellowfin tuna, and higher than the 3.99% lipid content of the commercially sold fatty belly meat of bluefin tuna. This also highlights that yellowfin tuna farmed through a fattening process should have commercial value.
The farming report of the research center pointed out that the fatty belly meat of bluefin tuna has a lipid content of 3.99% and is a sashimi grade selling for NT$1,500 per kg. If yellowfin tuna is farmed for 12 months, each fish can reach a weight of 7 to 11 kg, and the fat content can reach 20%. Moreover, the fat of yellowfin tuna is evenly distributed throughout the whole fish, unlike bluefin tuna, which differs greatly in fat ratio and price among different parts.
Wu Longjing believes that for yellowfin tuna farmed in sea cages, raising them to 10 kg is just right, because it takes only 8 months to raise juveniles to 10 kg, and the 8-month fattening period can just avoid typhoon season. Sea cage farming is conducted at sea and is greatly affected by sea conditions, and damage to sea cages due to typhoons is also common. Therefore, an 8-month fattening period can avoid the risks that typhoons bring to marine farming, and the financial pressure on operators is also relatively smaller.
Although farming operators currently have little interest in farming yellowfin tuna, with environmental protection and health awareness continuing to strengthen, perhaps one day yellowfin tuna will become one of the species farmed in sea cages. Why is this related to health? Because the concern about eating large migratory fish is that methylmercury accumulates in the bodies of large fish high up in the food chain. Wu Longjing said that actual measurements of the methylmercury content in the meat of artificially farmed yellowfin tuna were much lower than that of wild yellowfin tuna, and also far below the allowable value set by the Ministry of Health and Welfare.
The Coastal and Offshore Resources Research Center of the Fisheries Research Institute set up a sea cage with a diameter of about 20 m at a water depth of about 27 m in the Hengchun sea area for yellowfin tuna farming. The experimental sea cage adopted a submersible design and could sink to more than 10 m underwater when a typhoon struck.