Tunas are identified with Japanese cuisine and are usually served raw (as sashimi) or as broiled fillets (Teriyaki). Yellowtail tuna (Seriola quin-queiradiata) is one of the most popular speciescultured in Japan. The body size determines the name used commercially. Yellowtail tuna culture refers to the rearing of juvenile up to hamachi size (2–3 kg); commercial preference is for buri size (5–7 kg). Juveniles used for net-cage culture are from natural supplies. They are collected in May and June in the seas of southern Japan by fishermen specially licensed to catch them with hand nets or round haul nets. Fish farmers release them directly into the net-cages for on-growing. The yellowtail (tuna) culture cycle is around two years. Within a year, the fish can weigh more than 1 kg each and within a period of two years they may attain about 5–7 kg, which is a profitable market size. The optimum temperature for growth is between 24 and 26°C. A sudden drop in temperature due to heavy rains may adversely affect the health condition of the fish.
Eggs are collected from mature wild or cultured broodstock by stripping. It takes cultured brood stock three years to reach a well-matured brood stock weighing more than 5 kg, which produces 0.5 million eggs, and 6–8 kg, which may produce 1 million eggs. The spawning season starts in February and continues until June, depending on the ambient water temperature. Spawning can be induced by the injection of HCG (600 IV/kg) of fish into the dorsal muscles, which can achieve final maturation in two to three days. After fertilization viable floating eggs are separated and transferred into incubation or larval tanks. Hatching occurs after 70 hours under optimum temperatures of 18–21°C. Initial larval feeding starts with rotifers at three to four days. When the larvae reach 8–8.5 mm, Artemia nauplii and marine copepods are fed in addition to rotifers. Larvae larger than 12 mm are fed on adult Artemia or sometimes fish eggs or formulated microdiets. The larvae grow up to about 23 mm in length during May and survival rates are about 10 per cent when stocked in 100 m3 tanks at a water temperature of 22°C. Rotifers and Artemia are usually enriched with emulsified lipids rich in n-3 highly unsaturated fatty acids (HUFA). When Artemia without enrichment is fed, abnormalmortalities occur from the sixth day.
Recent studies have clearly shown that brood stocks of yellowtail tuna can be fed with extruded pellets (soft dry pellets in place of raw
fish or moist pellets). Various studies on the nutritional requirements using single moist pellets have been successful but the results do not appear suitable for application in dry pellets. Regardless of the type of diet, the digestibility of proteins in various feedstuffs is not different from other fish species. It is generally high, being more than 85 per cent for soybean, corn gluten, and feather and fish meals. Study of protein and energy requirements for maximum growth of yellowtail are generally higher than those of other fishes, probably due to their faster growth and higher swimming activity. Most of the lipids (highly unsaturated fatty acids) commonly used in fish feeds are highly digestible (90 to 99 per cent) in yellowtail tuna. Growth rate and feed efficiency
are dependent on the dietary HUFA content, reaching maximum at a level of 1.6 to 2.1 per cent. Yellowtail tuna does not seem to require supplemental phosphorus in the fish-meal-based diet.
Farming of northern bluefin tuna (Thunnusthynnus) is restricted now to a few Mediterrannean countries (Agius, 2002). The quota allowed by the ICCAT (International Commission for Conservation of Atlantic Tunas) restricts the individual transferable quotas based on a formula calculated on previous catch and investment history. Mediterranean countries are limited to a total 30 000 tons of allowable catch. Spain and Malta farmed 8000–9000 tons in the year 2001 and around 11 000–13 000 tons in 2002. Greece and Libya are now starting bluefin tuna farming.
Tuna farming in cages is restricted to a short season in May to July. They are caught in purse seine nets in stress-less condition. Farmed tuna has better meat quality and meets the requirements for sashimi markets. Farming is done in fattening cages made of strong, flexible polyethylene material. It is important that towing cages have a solid structure, as stocking sites may be located 200–300 miles away and the towing speed has to be slow to avoid too much stress on the young fish. Towing speed is reduced to 1 knot per hour for transport cages (90 m in circumference). Transport costs can be as high as US$ 2500 per tugboat per day. Insurance cover is ensured for towing and culture in cages. Mortality during farming once settled in cages or pens is usually not more than 1 per cent.
Tunas are fed with trash fish, horse mackerels or Atlantic mackerels, depending on the size of the tunas in the cages. When fed with sardines and squids large quantities may be needed, e.g. a 500-ton farm may require 20–25 tons of sardines per day. Tunas consume frozen feeds such as sardines, which sink to the bottom of cages.
Fish are ready for harvest by October. Some farmers complete the harvest by December, while others continue until February or March. Air transport to the market is preferred when available, as tunas for sashimi will be more attractive in the fresh state. If it is not possible to use aerial transport, the harvest may have to be sold frozen, but fresh fish have a better price in the market. Fish to be sold fresh are caught by stunning, using an electrical gadget. Fish
which have to be frozen before shipping are caught by the lowering a small net into the cage. The harvested fish are crowded into a square net with a V-shape projection on one side and an open window to allow the fish to swim into it. The fish are removed by pulling up the net/cage on a floating platform. Freezing of the harvest is often restricted by the blast freezing capacity of the processing ship, which is usually 20 tons per day and continues for 10 days. From the processing ship the frozen fish are moved to a reefer ship, which takes the cargo to the market, which is mainly in the Far East. The large tuna harvested are placed on a soft surface on board the ship. The price of fresh tuna depends on the market in Japan and on airfreight charges. Fresh fish are treated more carefully, and are cooled down quickly and individually placed in boxes for freighting.
Japanese researchers achieved the first full-scale breeding of endangered bluefun tuna in late June 2002, in Fisheries Laboratory of Kinki (FFI, 2002). The matured tuna spawned around one million eggs, and the scientists expect around 800 000 to hatch. Breeding success means a breakthrough for a stable, inexpensive supply of bluefin tuna. Adults can measure up to three metres and weigh about 550 kg.
Southern bluefin (Thunnus maccoyi) (fig. 24.12) have attracted prices of upto US$ 50/kg in Japan. Attempts to breed bluefin are taking place in Australia and the Mediterranean countries where captured bluefin are being raised in captivity. Environmentalists have been calling for restrictions on the fisheries of Australian southern bluefin tuna, which accounts for limiting the bluefin catch to a permitted quota of 5265 tons, utilised for stocking tuna cage farms. The benefits of farming are not only to the owners; the spin-off industries such as transport and freight are also beneficiaries.
By now most people know of the success of tuna farming in the regional South Australian Town of Port Lincoln, where a complete industry, almost on the verge of destruction, was completely turned around by the techniques of fish farming. In 1991 the Australian Tuna Boat Owners Association (ATOA) agreed not to catch 3000 of its allocated 14 500 tons quota in return for payment. In the same year,ATOA set up its first experimental tuna farm, a joint venture with the Japanese Government.
Twelve months later commercial tuna farming began in Port Lincoln, although it was limited by the old catching and towing system. In 1993 fish caught by purse seine nets were towed at extremely slow speeds of 1 knot per hour, taking about 14 hours for the vessels to return to unload their catch in Port Lincoln. A year later huge storms effectively wiped out 70 per cent of fish held in the farms. Since then there have been continued improvements in grow-out systems and lower fish mortalities, which have resulted in a production of over 8000 tons. There are at present about 15 southern bluefin farms on 18 sites, of approximately 20 to 30 hectares in area. There are about 130 pontoons of 40 metres in depth, governed by the toughest environmental monitoring funded by the industry on a quarterly basis from 2001 through the South Australian Government. This quarterly regional monitoring has been expanded to include a more intensive water quality and bottom fauna survey of both farm and non-farm sites. Regulations governing the farming include (a) a maximum stocking rate of 4 kg per cubic metre of water, (b) a maximum of 400 tons of fish in a 30 ha site, (c) minimum of 1 km distance between sites, and (d) a minimum clearance of 5 metres between the fish net and the seabed.
The technology of farming involves finding the fish and getting it on board while causing a minimal amount of stress in order to ensure premium quality to the overseas sashimi market. Understanding of the transfer of southern bluefin tuna, the towing of fish, the movement of pontoons and subsequent transfer on to farms have improved gradually over the years. The industry is now able to understand better the impact of stress on grow-out mortalities and growth rates and the effect of water quality deterioration. The industry was able to use the old-style boats and factories, so that the financial strain during the early days was reduced. Over the years, the industry has developed dedicated equipment such as onshore freezing plants. These have now been joined by the new nitrogen freezing systems, which most farmers have built on to their vessels for processing at sea. Many companies now have new multipurpose vessels which can both tow and service the pontoons. High-strength cages are now being manufactured locally – a 126 m circumference, 450 mm double collar seacage wasbuilt incorporating new tuna stanchions up to 1.6 m high. The strong, sturdy and stable stanchions are required to protect the employees against the threat of sharks which are keen to take advantages of the tempting supply of tuna on the other side of the cages. The latest advance has been to develop a stanchion that acts as a float (FFI, 2002).
Japan has been trying to grow out northern bluefin larvae now since the 1980s. Progress has been minimal due to problems caused by collision and cannibalism within the tanks. In addition, southern bluefin tunas do not spawn in the wild until 10–11 years of age, while northern bluefin spawn at four to five years of age. There is an indication that solutions to the problems of fish collision and cannibalism within the tanks will demand heavy research investment in both time and money. Given the delicate nature of tuna propagation, it will take some time before propagation could make any significant contribution to southern bluefin tuna production. This does not prevent concentrated effort on globally distributed species like the tunas if scientists decide on a species such as yellowtail tuna for which some of the essential data are available.
The Inter-American Tropical Commission’s Achotines Laboratory, established to investigate the culture and captive spawning of tunas and billfishes, worked between 1993 and 2001 with several Japanese scientists on the spawning and rearing to sexual maturity of tuna species. Black skipjack captured in the wild spawned for extended periods. Eggs and larvae hatched in captivity were used in various experiments through 1994; early juvenile black skip-jack (Euthynnus lineatus) collected at sea were reared at the lab. By 1996 research emphasis was shifted to yellowfin tuna that were familiar to the scientists. Brood stock yellowfin tuna are fed squids, herring and anchovy supplemented with vitamins and minerals at about 2 to 4.5% of their body weight per day. Two to three year old yellowfin brood stocks in tanks have been spawning almost daily since October 1996 (FFI, 2002).
Spawning can be intermittent during February and March, and generally occurs from early afternoon to late evening. The number of fertilized eggs collected after spawning ranges from several hundred to several million. Fertilized eggs are hatched in 300 cylindrical incubation tanks. Juveniles are reared for five to six weeks and have survived up to about 100 days.