Tuna are several species of ocean-dwelling fish in the family Scombridae, mostly in the genus Thunnus. Some tuna are able to inhabit freshwater environs as well. Tunas are fast swimmers—they have been clocked at 77 km/h (48 mph)—and include several species that are warm-blooded. Unlike most fish species, which have white flesh, the flesh of tuna is pink to dark red. This is because tuna muscle tissue contains greater quantities of myoglobin, an oxygen-binding molecule. Some of the larger tuna species such as the bluefin tuna can raise their blood temperature above the water temperature with muscular activity. This enables them to live in cooler waters and survive a wider range of circumstances. Some tuna species and fisheries are overfished and there are risks of some tuna fisheries collapsing.^[1]

Commercial importance

Tuna is an important commercial fish. Some varieties of Tuna, such as the bluefin and bigeye Tuna, Thunnus obesus, are threatened by overfishing, dramatically affecting Tuna populations in the Atlantic and northwestern Pacific Oceans. Other areas seem to support fairly healthy populations(for example, the central and western Pacific skipjack tuna, Katsuwonus pelamis), but there is mounting evidence that overcapitalization threatens Tuna populations world-wide. The Australian Government alleged in 2006 that Japan had illegally overfished southern bluefin to the value of USD $2 billion, by taking 12,000 to 20,000 tonnes per year instead of the internationally agreed 6,000 tonnes. This has resulted in severe damage to stocks. "Japan's huge appetite for tuna will take the most sought-after stocks to the brink of commercial extinction unless fisheries agree on more rigid quotas, wildlife campaigners warned today" stated by the WWF.

Increasing quantities of high-grade tuna are entering the market from operations that rear tuna in net pens and feed them on a variety of bait fish. In Australia the southern bluefin tuna, Thunnus maccoyii, is one of two species of bluefin tunas that are kept in tuna farms by former fishermen. Its close relative, the northern bluefin tuna, Thunnus thynnus, is being used to develop tuna farming industries in the Mediterranean, North America and Japan.

Due to their high position in the food chain and the subsequent accumulation of heavy metals from their diet, mercury levels can be relatively high in some of the larger species of tuna such as bluefin and albacore. As a result, in March 2004 the United States FDA issued guidelines recommending pregnant women, nursing mothers and children limit their intake of tuna and other types of predatory fish ^[2]. However, most canned light tuna is skipjack tuna and is lower in mercury. The Chicago Tribune reported that some canned light tuna such as yellowfin tuna^[3] is significantly higher in mercury than skipjack tuna, and caused Consumer Reports and other health groups to advise pregnant women to refrain from consuming canned tuna.^[4] Further, the Eastern Little Tuna (Euthynnus affinis) has been available for decades as a low-mercury, less expensive canned tuna. Of the five major species of canned tuna imported into the United States, however, it is the least commercially attractive, mostly due to its dark color and more pronounced 'fishy' flavor. Its use has traditionally been exclusively restricted to institutional (non-retail) commerce.

Recreational importance

In the 1930s and the 1940s, bluefin tuna were abundant in the waters of Cuba, Bimini and Cat Cay just a few miles off the Florida coast, targeted by fishermen such as Ernest Hemingway, and Habana Joe aboard his 1938 40-foot Wheeler named Pilar. Word spread quickly among fishermen around the world about the exciting new sport of big game fishing. Despite the growing popularity of the sport, however, the boats of the day were hardly ideal for the challenges of fighting the prized fish. Most boats utilized in those days were converted cabin cruisers and all were relatively slow and hard to maneuver.

The Rybovich family of South Florida eventually constructed a boat in 1946 that catapulted the sport and gave birth to a new industry. This boat, the Miss Chevy II, was the first Sportfish boat the world had ever seen.[1]

Merritt gained particular notoriety during the 1950s through the 1970s with its 37- and 43-foot custom sport fishing boats, which together with boats like those being built at the same time by Rybovich gave birth to a new category of fishing yachts and helped fuel the growth of big game tuna fishing in the United States and around the world with captains such as Habana Joe.

Canned tuna

Canned tuna was first produced in 1903, and quickly became popular.^[5] In the United States, only Albacore can legally be sold in canned form as "white meat tuna"; in other countries, Yellowfin is also acceptable as "white meat tuna."

While in the early 80s canned tuna in Australia was most likely to be Southern bluefin, as of 2003 it is usually yellowfin, skipjack, or tongol (labelled "northern bluefin").^[5].

As tuna are often caught long distances from where they are processed, poor quality control may lead to spoilage. Tuna are typically eviscerated by hand, then pre-cooked for 45 mins to 3 hours. The fish are then cleaned and filleted, packaged into cans, then sealed. The second cooking of the tuna meat (called retort cooking) is carried out in the cans, this time for 2 to 4 hours. ^[6] This process kills any bacteria, but retains the histamine produced by the fish and rancid flavors. The international standard sets the maximum histamine level at 200 mg per kg. An Australian study of 53 varieties of unflavored canned tuna found none to exceed the maximum histamine level, although some had "off" flavors^[5].

Standards once required cans of tuna to contain at least 51% tuna in Australia, but these regulations were dropped in 2003. ^[7] The remaining weight is usually made up with oil or water.

Major tuna producers

According to Foodmarket Exchange, total tuna catching stood at 3,605,000 tons in 2000, down about 5.7 percent from 3,823,000 tons in 1999. The main tuna catching nations are concentrated in Asia, with Japan and Taiwan as the main producers. Other important tuna catching nations in Asia are Indonesia and South Korea.

Spain and France are important tuna fishing countries, mainly catching in the Indian Ocean.

Japan remains the main nation fishing for tuna in the Pacific. In 2000, total tuna caught by Japanese vessels stood at 633,000 tons, about 17 percent of the world tuna catch. Taiwan was the second biggest tuna producer at 435,000 tons, or about 12 percent of the total tuna catch. Spain supplies most of the yellowfin to European canneries, accounting for 5.9 percent of the total tuna catch, while Ecuador and Mexico dominate the Eastern Pacific Ocean.

Management and conservation

There are 5 main tuna fishery management bodies. The five are the Western Central Pacific Ocean Fisheries Commission, the Inter-American Tropical Tuna Commission, the Indian Ocean Tuna Commission, the International Commission for the Conservation of Atlantic Tunas and the Commission for the Conservation of Southern Bluefin Tuna.^[8] They met for the first time in Kobe in Japan in January 2007. Environmental organisations made submissions ^[9] on risks to fisheries and species. The meeting concluded with an action plan drafted by some 60 countries or areas. Concrete steps include issuing certificates of origin to prevent illegal fishing and greater transparency in the setting of regional fishing quotas. The delegates will meet again at the second joint meeting in January or February 2009 in Europe.^[10]

Association with dolphins

Many tuna species associate with dolphins, swimming alongside them. These include yellowfin tuna in the eastern Pacific Ocean, but not albacore and skipjack. The reason for the association is believed to be the avoidance of dolphins by sharks, which are predators of tuna. Swimming near dolphins reduces the likelihood of the tuna being attacked by a shark [2].

Fishing vessels can exploit this association by searching for herds of dolphins. They encircle the herd with nets to catch the tuna beneath [3]. The nets are prone to entangling dolphins, thus injuring or killing them. Amidst public outcry about this, methods have become more "dolphin friendly", generally involving lines rather than nets. However, there are neither universal independent inspection programs nor verification of "dolphin safeness" to show that dolphins are not harmed during tuna fishing. According to the Consumers Union, this gives little credibility to the claims that tuna is "dolphin safe".

Methods of capture

• Andalusian method of Almadraba, in which nets are used, creating a maze in which the tuna are secured.
• big-game fishing
• longline fishing
• purse seine - the major threat to dolphins
• pole-and-line

Species

There are eight tuna species in the Thunnus genus:

• Albacore, Thunnus alalunga (Bonnaterre, 1788).
• Yellowfin tuna, Thunnus albacares (Bonnaterre, 1788).
• Blackfin tuna, Thunnus atlanticus (Lesson, 1831).
• Southern bluefin tuna, Thunnus maccoyii (Castelnau, 1872).
• Bigeye tuna, Thunnus obesus (Lowe, 1839).
• Pacific bluefin tuna, Thunnus orientalis (Temminck & Schlegel, 1844).
• Northern bluefin tuna, Thunnus thynnus (Linnaeus, 1758).
• Longtail tuna, Thunnus tonggol (Bleeker, 1851).

Species of several other genera (all in the family Scombridae) have common names containing "tuna":

• Skipjack tuna Katsuwonus pelamis (Linnaeus, 1758)
• Slender tuna Allothunnus fallai (Serventy, 1948)
• Bullet tuna Auxis rochei (Risso, 1810)
• Frigate tuna Auxis thazard (Lacepede, 1800)
• Kawakawa (little tunafish or mackerel tunafish) Euthynnus affinis (Cantor, 1849)
• Little tunny (little tunafish) Euthynnus alletteratus (Rafinesque, 1810)
• Dogtooth tuna Gymnosarda unicolor (Rüppell, 1836)
• Black skipjack tuna Euthynnus lineatus (Kishinouye, 1920)

References

• Clover, Charles. 2004. The End of the Line: How overfishing is changing the world and what we eat. Ebury Press, London. ISBN 0-09-189780-7
• FAO Species Catalog Vol. 2 Scombrids of the World. FAO Fisheries Synopsis No. 125, Volume 2. FIR/S125 Vol. 2.ISBN 92-5-101381-0

External links

• Nutritional benefits of tuna
• The Sushi FAQ - tuna use in sushi: akami & toro
• The slide show - How to cut Maguro (Tuna)
• U.S. Code of Federal Regulations 21CFR161 Fish and Shellfish