A tuna (also called tunny) is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max. length: 50 cm (1.6 ft), weight: 1.8 kg (4 lb)) up to the Atlantic bluefin tuna (max. length: 4.6 m (15 ft), weight: 684 kg (1,508 lb)). The bluefin averages 2 m (6.6 ft), and is believed to live up to 50 years.
The "true" tunas are those that belong to the genus Thunnus. Until recently, it was thought that there were seven Thunnus species, and that Atlantic bluefin tuna and Pacific bluefin tuna were subspecies of a single species. In 1999, Collette established that based on both molecular and morphological considerations, they are in fact distinct species.
The tuna is a sleek and streamlined fish, adapted for speed. It has two closely spaced dorsal fins on its back; The first is "depressible" it can be laid down, flush, in a groove that runs along its back. Seven to ten yellow finlets run from the dorsal fins to the tail, which is lunate curved like a crescent moon and tapered to pointy tips. The caudal peduncle, to which the tail is attached, is quite thin, with three stabilizing horizontal keels on each side. The tuna's dorsal side is generally a metallic dark blue, while the ventral side, or underside, is silvery or whitish, for camouflage.
Tunas achieve endothermy by conserving the heat generated through normal metabolism. In all tunas, the heart operates at ambient temperature, as it receives cooled blood, and coronary circulation is directly from the gills. The rete mirabile ("wonderful net"), the intertwining of veins and arteries in the body's periphery, allows nearly all of the metabolic heat from venous blood to be "re-claimed" and transferred to the arterial blood via a counter-current exchange system, thus mitigating the effects of surface cooling. This allows the tuna to elevate the temperatures of the highly-aerobic tissues of the skeletal muscles, eyes and brain, which supports faster swimming speeds and reduced energy expenditure, and which enables them to survive in cooler waters over a wider range of ocean environments than those of other fish.
In 2005, Nauru, defending its vote from Australian criticism at that year's meeting of the International Whaling Commission, argued that some whale species have the potential to devastate Nauru's tuna stocks, and that Nauru's food security and economy relies heavily on fishing. Despite this, Nauru does not permit whaling in its own waters and does not allow other fishing vessels to take or intentionally interact with marine mammals in its Exclusive Economic Zone. In 2010 and 2011 Nauru supported Australian proposals for a western Pacific-wide ban on tuna purse-seining in the vicinity of marine mammals a measure which was agreed by the Western and Central Pacific Fisheries Commission at its eighth meeting in March 2012.
Commercial fishing vessels used to exploit this association by searching for dolphin pods. Vessels would encircle the pod with nets to catch the tuna beneath, however the nets were prone to entangling dolphins, injuring or killing them. Public outcry and new government regulations, which are now monitored by NOAA have led to more "dolphin friendly" methods, now generally involving lines rather than nets. However, there are neither universal independent inspection programs nor verification of "dolphin safeness", so these protections are not absolute. According to Consumers Union, the resulting lack of accountability means claims of tuna that is "dolphin safe" should be given little credence.
Japan is the biggest tuna consuming nation and is also the leader in tuna farming research. Japan first successfully farm-hatched and raised bluefin tuna in 1979. In 2002, it succeeded in completing the reproduction cycle and in 2007, completed a third generation. The farm breed is known as Kindai tuna. Kindai is the contraction of Kinki University in Japanese (Kinki daigaku). In 2009, Clean Seas, an Australian company which has been receiving assistance from Kinki University managed to breed Southern Bluefin Tuna in captivity and was awarded the second place in World's Best Invention of 2009 by Time magazine.
Mercury content in tuna can vary widely. Among those calling for improved warnings about mercury in tuna is the American Medical Association, which adopted a policy that physicians should help make their patients more aware of the potential risks. A study published in 2008 found that mercury distribution in the meat of farmed tuna is inversely related to the lipid content, suggesting that higher lipid concentration within edible tissues of tuna raised in captivity might, other factors remaining equal, have a diluting effect on mercury content. Due to their high position in the food chain and the subsequent accumulation of heavy metals from their diet, mercury levels can be high in larger species such as bluefin and bigeye. Mackerel tuna is one species of tuna that is lower in mercury concentration than skipjack or yellowfin, but this species is known as "black meat" or "dark meat" tuna, which is a lower grade for canning because of the color, unfavorable flavor, and poor yield.