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The southern bluefin tuna, Thunnus maccoyii, is a tuna of the family Scombridae found in open southern hemisphere waters of all the world's oceans mainly between 30°S and 50°S, to nearly 60°S. At up to 2.5 m (8.2 ft) and weighing up to 400 kg (882 lbs) it is among the larger bony fishes.

The southern bluefin tuna is a large, streamlined, fast swimming fish with a long, slender caudal peduncle and relatively short dorsal, pectoral and anal fins. The body is completely covered in small scales.

The body color is blue-black on the back and silver-white on the flanks and belly, with bright yellow caudal keels in adult specimens. The first dorsal fin colour is grey with a yellow tinge, the second dorsal is red-brown, and the finlets are yellow with a darker border.

Southern bluefin tuna, like other pelagic tuna species, are part of a group of bony fishes that can maintain their body core temperature up to 10 degrees above the ambient temperature. This advantage enables them to maintain high metabolic output for predation and migrating large distances. The southern bluefin tuna is an opportunistic feeder, preying on a wide variety of fish, crustaceans, cephalopods, salps, and other marine animals.

Harvesting

The onset of industrial fishing in the 1950s, in conjunction with ever improving technologies such as GPS, fishfinders, satellite imagery, etc., and the knowledge of migration routes, has led to the exploitation of southern bluefin tuna across its entire range. Improved refrigeration techniques and a demanding global market saw global SBT catch plummet from 80,000 tonnes a year during the 1960s to 40,000 tonnes a year by 1980.^[2] Australian catch peaked in 1982 at 21,500 tonnes, and the total population of SBT has since declined by about 92 percent.^[3]

Conservation

The southern bluefin tuna is now classified as Critically Endangered on the IUCN Red List of Threatened species.^[1] In 2010, Greenpeace International has added the SBT to its seafood red list. The Greenpeace International seafood red list is a list of fish that are commonly sold in supermarkets around the world, and which Greenpeace believe have a very high risk of being sourced from unsustainable fisheries.^[4]

There was a pressing obligation to alleviate some of the harvesting pressure on SBT populations, and increasing concerns about sustainability in the mid 1980s led the main nations fishing SBT at the time to manage catches. These nations imposed strict quotas to their fishing fleets, although no official quotas were put in place.

In 1994, the then existing voluntary management arrangement between Australia, Japan and New Zealand was formalised when the Convention for the Conservation of Southern Bluefin Tuna came into force. The Convention created the Commission for the Conservation of Southern Bluefin Tuna (CCSBT). Its objective was to ensure, through appropriate management, the conservation and optimum utilisation of the global SBT fishery. South Korea, Taiwan and Indonesia have since joined or are cooperating with the Commission. The CCSBT is headquartered in Canberra, Australia.

Current quota limits reflect the vulnerable nature of wild stocks, with quotas being reduced for the 2010/2011 seasons to 80% of years previous. Thus the global total allowable catch (TAC) has been reduced from 11,810 tonnes from the previously allocated global TAC to 9,449 tonnes.^[5] Australia currently has the highest "effective catch limit" with 4,015 tonnes, followed by Japan (2,261), Republic of Korea (859), Fishing Entity of Taiwan (859), New Zealand (709), and Indonesia (651).^[5] However, fishing pressure outside the allocated global TAC is still a major concern for instance, the Australian government stated in 2006 that Japan had admitted to taking more than 100,000 tonnes over its quota; the new quotas reflect this, as Japans was cut by half, as supposed punishment for overfishing.

The quota system has actually increased the value of the catch, where fisherman that once earned $600 a ton selling fish to canneries began making more than $1,000 per ton of fish, selling them to buyers for the Japanese market. The quotas are expensive and are bought and sold like stocks within their national allocations.^[6]

Aquaculture

Ranching

The rapidly declining fishery led to Australian tuna fishers investigating the potential for value-adding their catch through aquaculture. All SBT ranching occurs in a small region offshore of Port Lincoln, South Australia; the region comprising almost all of the SBT fishing companies in Australia since the 1970s.^[7] This industry was initiated in 1991 and has now developed to be the largest farmed seafood sector in Australia.^[7]

Southern Bluefin Tuna spawn between September and April each year in the only known spawning grounds in the Indian Ocean, between the north-west Coast of Australia and Indonesia. The eggs are estimated to hatch within two to three days, and over the next two years attain sizes of approximately 15 kilograms; this size is the principal wild catch of the Australian SBT industry. It is thought that SBT become sexually mature between 9 and 12 years in the wild,^[7] which highlights the major negative impact of removing pre-spawning populations from the wild.

Juvenile tuna are mainly caught on the continental shelf in the Great Australian Bight region from December to around April each year, and as mentioned weigh on average 15 kilograms. The tuna that are located are purse seined, and then transferred through underwater panels between nets to specialised tow pontoons. They are then towed back to farm areas adjacent to Port Lincoln at a rate of about 1 knot; this process can take several weeks. Once back at the farm sites, the tuna are transferred from the tow pontoons into 40–50 m diameter farm pontoons. They are then fed bait fish (usually a range of locally caught or imported small pelagic species such as sardines) six days per week, twice per day and "grown out" for three to eight months, reaching an average of 30 to 40 kilograms.^[2][7] Because SBT swim so fast and are used to migrating long distances, they are difficult to keep in small pens. Their delicate skin can be easily damaged if touched by human hands and too much handling can be fatal.

As with most aquaculture ventures, feeds are the biggest factor in the cost-efficiency of the farming operation, and there would be considerable advantages in using formulated pellet feed to supplement or replace the baitfish. However, as yet the manufactured feeds are not competitive with the baitfish.^[8]

A further future prospect in enhancing the ranching of SBT is the plan of Long Term Holding. By holding its fish for two successive growing seasons (18 months) instead of one (up to 8 months), the industry could potentially achieve a major increase in volume, greater production from the limited quota of wild-caught juveniles, and ability to serve the market year round.^[8] Undoubtedly, this presents several uncertainties, and is still in the planning stage.

At harvest time, the fish are gently guided into a boat (any bruising lowers the price) and killed and flash frozen and predominantly put on Tokyo-bound planes. They are so valuable, that armed guards are paid to watch over them; 2,000 tuna kept in a single pen are worth around $2 million.^[6] Australia exports 10,000 metric tons of bluefin worth $200 million; almost all is from penned stocks.^[6]

Complete aquaculture

Initially, the notorious difficulties in closing the life cycle of this species dissuaded most from farming them. However, in 2007, using hormonal therapy developed in Europe^[9] and Japan (where they had already succeeded in breeding Northern Pacific bluefin tuna to third generation^[10]) to mimic the natural production of hormones by wild fish, researchers in Australia managed for the first time to coax the species to breed in landlocked tanks. This was done by the Australian aquaculture company, Clean Seas Tuna Limited.^[11] who collected its first batch of fertilized eggs from a breeding stock of about 20 tuna weighing 160 kilograms.^[6] They were also the first company in the world to successfully transfer large SBT over large distances to its onshore facilities in Arno Bay which is where the spawning has taken place. This led Time magazine to award it second place in the 'World's Best Invention' of 2009.^[12]

The state-of-the-art Arno Bay hatchery was purchased in 2000, and undertook a $2.5 million upgrade, where initial broodstock facilities catered for Kingfish (Seriola lalandi) and Mulloway (Argyrosomus japonicas), along with a live-feed production plant. This facility has more recently been upgraded to a $6.5 million special purpose SBT larval rearing recirculation facility. During the most recent summer (2009/2010), the company completed its third consecutive annual on-shore Southern Bluefin Tuna spawning program, having doubled the controlled spawning period to three months at its Arno Bay facility.^[13] Fingerlings are now up to 40 days old with the grow-out program, and the spawning period has been extended from 6 weeks to 12, but as yet, grow-out of commercial quantities of SBT fingerlings has been unsuccessful.^[13] Whilst aquaculture pioneers Clean Seas Limited have not been able to grow out commercial quantities of SBT fingerlings from this season's trials, the SBT broodstock are now being wintered and conditioned for the 2010-11 summer production run.^[13]

With collaboration secured with International Researchers, in particular with Kinki University in Japan,^[13] commercial viability is likely to be achieved in the near future. The successful closing of the SBT life cycle by Clean Seas will therefore potentially produce unrestricted additional tonnages of this valuable fish; supplying the growing global demand for SBT.

Enhancing growth and production

Feeds

Scientists are currently trying to develop less expensive fish feed. One of main obstacles is creating a processed food that doesn't affect the taste of the tuna because what a tuna eats very much affects the taste of its meat. As previously mentioned, SBT are still largely fed fresh or frozen small pelagic fishes, and the use of formulated pellets is not yet viable.^[8] This cost is largely due to the expensive diet research costs (the annual costs of diet for research alone is approximately US$100,000^[14]), and the problems associated with working with such large, mobile marine animals. Farm raised tuna generally have a higher fat content than wild tuna. A one metre tuna needs about 15 kilograms of live fish to put on one kilogram of fat, and about 1.5 to 2 tons of squid and mackerel are needed to produce a 100 kilogram bluefin tuna.^[6] More research must be undertaken in evaluating the ingredients for use in SBT feed, and important information on ingredient digestibility, palatability and nutrient utilisation and interference can improve cost efficiencies.^[15]

Dietary supplements

The use of dietary supplements can improve the shelf life of farmed SBT flesh. Results of a study by SARDI (South Australian Research and Development Institute) indicated that feeding a diet approximately 10 times higher in dietary antioxidants raised levels of vitamin E and vitamin C, but not selenium, in tuna flesh and increased the shelf life of tuna.^[16] This is important as the frozen baitfish diets are likely to be lower in antioxidant vitamins than the wild tuna diet.

Parasites and pathology

So far the risk of parasite and disease spreading for SBT aquaculture is low to negligible; the modern SBT aquaculture industry has total catch to harvest mortalities of around 2-4%.^[17] A diverse range of parasite species has been found hosting the SBT, with most of the parasites examined posing little or no risk to the health of the farms - with some SBT actually showing antibody responses to epizootics^[18] - however, blood fluke and gill fluke have the greatest risk factors.^[19][20] Hypoxia is also a significant issue, and can be escalated due to unforeseen environmental factors such as algal blooms.^[17] So it seems that pathological risks are low now, however, this is seen as a dynamic process, therefore ongoing monitoring should take place to ensure its control, especially if farming intensifies and stocking levels increase.

Negative impacts

Sustainability is the key issue here, and with feed conversion ratios (feed to tuna growth) of approximately 10:1 or higher, though this is purely a consequence of the carnivorous diet and high metabolic costs of the species. Removing tuna from the wild before they have spawned is another obvious impact, which hopefully the closed life cycles of SBT at Clean Seas will alleviate some of the pressure on declining stocks. Tuna farms are point sources of solid waste onto to the benthos and dissolved nutrients into the water column. Most farms are more than a kilometre off the coast, thus the deeper water and significant currents alleviate some of the impact on the benthos. Due to the high metabolic rates of SBT, low retention rates of nitrogen in tissue is seen, and there are high environmental leaching of nutrients (86-92%).^[17]

Other environmental impacts include the use of chemicals on the farms, which can leach into the surrounding environment. These include antifoulants to keep the cages free from colonial algae and animals, and therapeutants to deal with disease and parasitism. Toxicants, such as mercury and PCBs (polychlorinated biphenyls), can build up over time, particularly through the tuna feed, with some evidence of contaminants being more elevated in farmed fish than in wild stocks.^[21]

Flesh

Southern Bluefin Tuna is a gourmet food, which is in demand for use in sashimi and sushi. It has medium flavoured flesh and is regarded by both Japanese and Western chefs as the best raw fish to eat in the world.^[2]

Market

By far the largest consumer of SBT is Japan, with USA coming in second, followed by China. Japanese imports of fresh bluefin tuna (all 3 species) worldwide increased from 957 tons in 1984 to 5,235 tons in 1993 [7].^{[full citation needed]} The price peaked in 1990 at $34 per kilogram when a typical 350 pound fish sold for around $10,000.^[2] As of 2008, bluefin was selling for $23 a kilogram.^[2] The drop in value was due to the drop in the Japanese market, an increase in supply from northern bluefin tuna from the Mediterranean, and more and more tuna being stored (tuna frozen with the special "flash" method can be kept for up to a year with no perceivable change in taste).

The Tsukiji Market in Tokyo is the largest wholesale market of SBT in the world. Tsukiji handles more than 2,400 tons of fish, worth about US$20 million, a day, with pre-dawn auctions of tuna being the main feature.^[22] No tourists are allowed to enter the tuna wholesale areas, which they say is for purposes of sanitation and disruption to the auction process.^[23] Higher prices are charged for the highest quality fish; bluefin tuna worth over $150,000 have been sold at Tsukiji. In 2001, a 202-kilogram wild tuna caught in Tsugaru Straight near Omanachi I Aomori Prefecture sold for $173,600, or about $800 a kilogram.^[2]

References

• Froese, Rainer, and Daniel Pauly, eds. (2006). "Thunnus maccoyii" in FishBase. March 2006 version.
• Tony Ayling & Geoffrey Cox, Collins Guide to the Sea Fishes of New Zealand, (William Collins Publishers Ltd, Auckland, New Zealand 1982) ISBN 0-00-216987-8
• Clover, Charles. 2004. The End of the Line: How overfishing is changing the world and what we eat. Ebury Press, London. ISBN 978-0-09-189780-2
• Bye bye bluefin: Managed to death The Economist. 30 October 2008. Retrieved 6 February 2009.