Tuna

True species

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.^[10][11]

The genus Thunnus is further classified into two subgenera: Thunnus (Thunnus) (the bluefin group), and Thunnus (Neothunnus) (the yellowfin group).^[12]

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Thunnus, the true tunas
Image	Common name	Scientific name	Maximum length	Common length	Maximum weight	Maximum age	Trophic level	Source	IUCN status
Thunnus (Thunnus) – the bluefin group
	Albacore tuna	T. alalunga (Bonnaterre, 1788)	1.4 m (4.6 ft)	1.0 m (3.3 ft)	60.3 kg (133 lb)	9–13 yrs	4.31	[13][14]	Least Concern[14]
	Southern bluefin tuna	T. maccoyii (Castelnau, 1872)	2.45 m (8.0 ft)	1.6 m (5.2 ft)	260 kg (570 lb)	20–40 yrs	3.93	[15][5]	Endangered[5]
	Bigeye tuna	T. obesus (Lowe, 1839)	2.5 m (8.2 ft)	1.8 m (5.9 ft)	210 kg (460 lb)	5–16 yrs	4.49	[16][17]	Vulnerable[17]
	Pacific bluefin tuna	T. orientalis (Temminck & Schlegel, 1844)	3.0 m (9.8 ft)	2.0 m (6.6 ft)	450 kg (990 lb)	15–26 yrs	4.21	[18][19]	Near Threatened[19]
	Atlantic bluefin tuna	T. thynnus (Linnaeus, 1758)	4.6 m (15 ft)	2.0 m (6.6 ft)	684 kg (1,508 lb)	35–50 yrs	4.43	[20][21]	Least Concern[21]
Thunnus (Neothunnus) – the yellowfin group
	Blackfin tuna	T. atlanticus (Lesson, 1831)	1.1 m (3.6 ft)	0.7 m (2.3 ft)	22.4 kg (49 lb)		4.13	[22]	Least concern[23]
	Longtail tuna, northern bluefin tuna, tongol tuna	T. tonggol (Bleeker, 1851)	1.45 m (4.8 ft)	0.7 m (2.3 ft)	35.9 kg (79 lb)	18 years	4.50	[24][25]	Data deficient[25]
	Yellowfin tuna	T. albacares (Bonnaterre, 1788)	2.4 m (7.9 ft)	1.5 m (4.9 ft)	200 kg (440 lb)	5–9 yrs	4.34	[26][27]	Least Concern[27]

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Other species

The Thunnini tribe also includes seven additional species of tuna across four genera. They are:

More information Other tuna species, Common name ...

Other tuna species
Common name	Scientific name	Maximum length	Common length	Maximum weight	Maximum age	Trophic level	Source	IUCN status
Slender tuna	Allothunnus fallai (Serventy, 1948)	1.05 m (3.4 ft)	0.86 m (2.8 ft)	13.7 kg (30 lb)		3.74	[28]	Least concern[29]
Bullet tuna	Auxis rochei (Risso, 1810)	0.5 m (1.6 ft)	0.35 m (1.1 ft)	1.8 kg (4.0 lb)	5 years	4.13	[30][31]	Least concern[31]
Frigate tuna	Auxis thazard  (Lacépède, 1800)	0.65 m (2.1 ft)	0.35 m (1.1 ft)	1.7 kg (3.7 lb)	5 years	4.34	[32]	Least concern[33]
Mackerel tuna, Kawakawa	Euthynnus affinis (Cantor, 1849)	1.0 m (3.3 ft)	0.6 m (2.0 ft)	13.6 kg (30 lb)	6 years	4.50	[34][35]	Least concern[35]
Little tunny	Euthynnus alletteratus (Rafinesque, 1810)	1.2 m (3.9 ft)	0.8 m (2.6 ft)	16.5 kg (36 lb)	10 years	4.13	[36]	Least concern[37]
Black skipjack tuna	Euthynnus lineatus (Kishinouye, 1920)	0.84 m (2.8 ft)	0.6 m (2.0 ft)	11.8 kg (26 lb)		3.83	[38][39]	Least concern[39]
Skipjack tuna	Katsuwonus pelamis (Linnaeus, 1758)	1.1 m (3.6 ft)	0.8 m (2.6 ft)	34.5 kg (76 lb)	6–12 yrs	3.75	[40][41]	Least concern[41]

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Description

The tuna is a sleek, elongated and streamlined fish, adapted for speed. It has two closely spaced but separated dorsal fins on its back; The first fin is "depressible" – it can be laid down, flush, in a groove that runs along its back; it is supported by spines.^[42] 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.^[43] A tuna's pelvic fins are located below the base of the pectoral fins. Both dorsal and pelvic fins retract when the fish is swimming fast.^[42]

The tuna's body is countershaded to camouflage itself in deeper water when seen from above, its dorsal side is generally a metallic dark blue while the ventral or under side is silvery, often with an iridescent shine.^[44][43] The caudal peduncle, to which the tail is attached, is quite thin, with three stabilizing horizontal keels on each side.^[43]

Physiology

Thunnus are widely but sparsely distributed throughout the oceans of the world, generally in tropical and temperate waters at latitudes ranging between about 45° north and south of the equator.^[45] All tunas are able to maintain the temperature of certain parts of their body above the temperature of ambient seawater. For example, bluefin can maintain a core body temperature of 25–33 °C (77–91 °F), in water as cold as 6 °C (43 °F). Unlike other endothermic creatures such as mammals and birds, tuna do not maintain temperature within a relatively narrow range.^[46]

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.^[47] 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.^[48] This allows the tuna to elevate the temperatures of the highly-aerobic tissues of the skeletal muscles, eyes and brain,^[46][47] 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.^{[citation needed]}

Also unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red myotomal muscles derive their color from myoglobin, an oxygen-binding molecule, which tuna express in quantities far higher than most other fish. The oxygen-rich blood further enables energy delivery to their muscles.^[46]

For powerful swimming animals like dolphins and tuna, cavitation may be detrimental, because it limits their maximum swimming speed.^[49] Even if they have the power to swim faster, dolphins may have to restrict their speed, because collapsing cavitation bubbles on their tail are too painful. Cavitation also slows tuna, but for a different reason. Unlike dolphins, these fish do not feel the bubbles, because they have bony fins without nerve endings. Nevertheless, they cannot swim faster because the cavitation bubbles create a vapor film around their fins that limits their speed. Lesions have been found on tuna that are consistent with cavitation damage.^[49]

Commerce

Tuna is an important commercial fish. The International Seafood Sustainability Foundation (ISSF) compiled a detailed scientific report on the state of global tuna stocks in 2009, which includes regular updates. According to the ISSF, the most important species for commercial and recreational tuna fisheries are yellowfin (Thunnus albacares), bigeye (T. obesus), bluefin (T. thynnus, T. orientalis, and T. macoyii), albacore (T. alalunga), and skipjack (Katsuwonus pelamis).^[45]

Based on catches from 2007, the report states:

Between 1940 and the mid-1960s, the annual world catch of the five principal market species of tunas rose from about 300 thousand tons to about 1 million tons, most of it taken by hook and line. With the development of purse-seine nets, now the predominant gear, catches have risen to more than 4 million tons annually during the last few years. Of these catches, about 68 percent are from the Pacific Ocean, 22 percent from the Indian Ocean, and the remaining 10 percent from the Atlantic Ocean and the Mediterranean Sea. Skipjack makes up about 60 percent of the catch, followed by yellowfin (24 percent), bigeye (10 percent), albacore (5 percent), and bluefin the remainder. Purse-seines take about 62 percent of the world production, longline about 14 percent, pole and line about 11 percent, and a variety of other gears the remainder.^[45]

The Australian government alleged in 2006 that Japan had illegally overfished southern bluefin by taking 12,000 to 20,000 tonnes per year instead of the agreed upon 6,000 tonnes; the value of such overfishing would be as much as US$2 billion.^[50] Such overfishing has severely damaged bluefin stocks.^[51] According to the WWF, "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".^[52] Japan's Fisheries Research Agency counters that Australian and New Zealand tuna fishing companies under-report their total catches of southern bluefin tuna and ignore internationally mandated total allowable catch totals.^[53]

In recent years, opening day fish auctions at Tokyo's Tsukiji fish market and Toyosu Market have seen record-setting prices for bluefin tuna, reflecting market demand. In each of 2010, 2011, 2012, 2013 and 2019, new record prices have been set for a single fish – the current record is 333.6 million japanese yen (US$3.1 million) for a 278 kg (613 lb) bluefin, or a unit price of JP¥ 1,200,000/kg (US$5,057/lb). The opening auction price for 2014 plummeted to less than 5% of the previous year's price, which had drawn complaints for climbing "way out of line".^[54] A summary of record-setting auctions are shown in the following table (highlighted values indicate new world records):

In November 2011, a different record was set when a fisherman in Massachusetts caught an 881 lb (400 kg) tuna. It was captured inadvertently using a dragnet. Due to the laws and restrictions on tuna fishing in the United States, federal authorities impounded the fish because it was not caught with a rod and reel. Because of the tuna's deteriorated condition as a result of the trawl net, the fish sold for just under $5,000.^[60]

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  Tuna being weighed on Greek quay-side
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  Tuna at Tsukiji fish market, Tokyo
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  Tuna cut in half for processing at Tsukuji fish market

Methods

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Tuna pole and line fishing BBC Two

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Besides for edible purposes, many tuna species are caught frequently as game, often for recreation or for contests in which money is awarded based on weight. Larger specimens are notorious for putting up a fight while hooked, and have been known to injure people who try to catch them, as well as damage their equipment.

• Phoenician technique for trapping and catching Atlantic bluefin tuna called Almadraba, still used today in Portugal, Spain, Morocco and Italy which uses a maze of nets. In Sicily, the same method is called Tonnara.
• Fish farming (cage system)^[61]
• Tuna ranching
• Longline fishing
• Purse seines
• Pole and line
• Harpoon gun
• Big game fishing
• Fish aggregating device

Association with whaling

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.^[62] 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^[63] 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.

Association with dolphins

Dolphins swim beside several tuna species. These include yellowfin tuna in the eastern Pacific Ocean, but not albacore. Tuna schools are believed to associate themselves with dolphins for protection against sharks, which are tuna predators.^[64]

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.^[65] 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. There are neither universal independent inspection programs nor verification of dolphin safety, 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.

Fishery practices have changed to be dolphin friendly, which has caused greater bycatch including sharks, turtles and other oceanic fish. Fishermen no longer follow dolphins, but concentrate their fisheries around floating objects such as fish aggregation devices, also known as FADs, which attract large populations of other organisms. Measures taken thus far to satisfy the public demand to protect dolphins can be potentially damaging to other species as well.^[66]

Aquaculture

Increasing quantities of high-grade tuna caught at sea are reared in net pens and fed bait fish. In Australia, former fishermen raise southern bluefin tuna (Thunnus maccoyii) and another bluefin species.^[61] Farming its close relative, the Atlantic bluefin tuna, Thunnus thynnus, is beginning in the Mediterranean, North America and Japan. Hawaiʻi approved permits for the first U.S. offshore farming of bigeye tuna in water 1,300 feet (400 m) deep in 2009.^[67]

Japan is the biggest tuna consuming nation and is also the leader in tuna farming research.^[68] 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.^[69][70][71] The farm breed is known as Kindai tuna. Kindai is the contraction of Kinki University in Japanese (Kinki daigaku).^[72] In 2009, Clean Seas, an Australian company which has been receiving assistance from Kinki University^[73][74][75] managed to breed southern bluefin tuna in captivity and was awarded the second place in World's Best Invention of 2009 by Time magazine.^[76][77]

Fresh and frozen

The fresh or frozen flesh of tuna is widely regarded as a delicacy in most areas where it is shipped, being prepared in a variety of ways. When served as a steak, the meat of most species is known for its thickness and tough texture. In the U.K., supermarkets began flying in fresh tuna steaks in the late 1990s, which helped to increase the popularity of using fresh tuna in cooking; by 2009, celebrity chefs regularly featured fresh tuna in salads, wraps, and char-grilled dishes.^[78]

Canned

Tuna is canned in edible oils, in brine, in water, and in various sauces. Tuna may be processed and labeled as "solid", "chunked" ("chunk") or "flaked". When tuna is canned and packaged for sale, the product is sometimes called tuna fish (U.S.), a calque (loan translation) from the German Thunfisch. Canned tuna is sometimes used as food for pets, especially cats.

Australia

Canned tuna was first produced in Australia in 1903 and quickly became popular.^[80]

In the early 1980s canned tuna in Australia was most likely southern bluefin, as of 2003^[update] it was usually yellowfin, skipjack, or tongol (labelled "northern bluefin" or "longtail").^[80]

Australian standards once required cans of tuna to contain at least 51% tuna, but those regulations were dropped in 2003.^[81][82] The remaining weight is usually oil or water.

United States

The product became more plentiful in the United States in the late 1940s. In 1950, 8,500,000 pounds of canned tuna were produced, and the U.S. Department of Agriculture classified it as a "plentiful food".^[83]

In the United States, 52% of canned tuna is used for sandwiches; 22% for tuna salads; and 15% for tuna casseroles and dried, prepackaged meal kits, such as General Mills's Tuna Helper line.^[84] Other canned tuna dishes include tuna melts (a type of sandwich where the tuna is mixed with mayonnaise and served on bread with cheese melted on top); salade niçoise (a salad made of tuna, olives, green beans, potatoes, hard-boiled eggs and anchovy dressing); and tuna burgers (served on buns).

In the United States, the Food and Drug Administration (FDA) regulates canned tuna (see part c).^[85]

Precooked

As tunas are often caught far from where they are processed, poor interim conservation can lead to spoilage. Tuna is typically gutted by hand, and later precooked for prescribed times of 45 minutes to three hours. The fish are then cleaned and filleted, canned (and sealed), with the dark lateral blood meat often separately canned for pet food (cat or dog). The sealed can is then heated under pressure (called "retort cooking") for 2–4 hours.^[86] This process kills any bacteria, but retains the histamine that may have been produced by those bacteria, and so may still taste spoiled. The international standard sets the maximum histamine level at 200 milligrams per kilogram. An Australian study of 53 varieties of unflavored canned tuna found none to exceed the safe histamine level, although some had "off" flavors.^[80]

Light and white

In some markets, depending upon the color of the flesh of the tuna species, the can is marked as "light" or "white" meat, with "light" meaning a greyish pink color and "white" meaning a light pink color. In the United States, only albacore can legally be sold in canned form as "white meat tuna";^[87] in other countries, yellowfin is also acceptable.

Ventresca tuna

Ventresca tuna (from ventre, the Italian word for belly),^[88] is a luxury canned tuna,^[89] from the fatty bluefin tuna belly, also used in sushi as toro.^[90][91]

Mercury and health

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.^[94] 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.^[95] Mackerel tuna is one species of tuna that is lower in mercury concentration than skipjack or yellowfin,^[96] 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.^[97]

In March 2004, the United States FDA issued guidelines recommending that pregnant women, nursing mothers, and children limit their intake of tuna and other predatory fish.^[98] The Environmental Protection Agency provides guidelines on how much canned tuna is safe to eat. Roughly speaking, the guidelines recommend one 6-ounce (170 g) can of light tuna per week for individuals weighing less than 110 pounds (50 kg), and two cans per week for those who weigh more.^[99] In 2007, it was reported that some canned light tuna such as yellowfin tuna^[100] is significantly higher in mercury than skipjack, and caused Consumers Union and other activist groups to advise pregnant women to refrain from consuming canned tuna.^[101] In 2009, a California appeals court upheld a ruling that canned tuna does not need warning labels as the methylmercury is naturally occurring.^[102]

A January 2008 report revealed potentially dangerous levels of mercury in certain varieties of sushi tuna, reporting levels "so high that the Food and Drug Administration could take legal action to remove the fish from the market."^[103]