Tuna are a group of salt water fish from the family Scombridae, particularly of the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of 70 km/h (43 mph). Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an oxygen-binding molecule, which tuna express in quantities far higher than most other fish. Some larger tuna species, such as bluefin tuna, display some warm-blooded adaptations, and can raise their body temperatures above water temperatures by means of muscular activity. This enables them to survive in cooler waters and to inhabit a wider range of ocean environments than other types of fish.
Altogether, over fifty different species, all belonging to the family Scombridae, are commonly referred to as tuna. The term tuna derives from Latin thunnus / Greek θύννος, thynnos.
True tuna belong to the genus Thunnus, or "tuna". There are eight species in the genus. Until recently, it was thought that there were only seven species, and that Atlantic bluefin tuna and the Pacific bluefin tuna were subspecies of the same species. In 1999 Collette established, on molecular and morphological considerations, that these are separate species.[1][2]
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In addition, over forty related species with tuna-like characteristics, all in the family Scombridae, are also commonly referred to as tuna; genetically speaking, however, none are related to tuna of the genus Thunnus. In most cases they are more closely related to the mackerel family. Some prominent examples are:
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Bigeye tuna Thunnus obesus showing finlets and keels. Finlets are found between the last dorsal and/or anal fin and the caudal fin. They are rayless and non-retractable.
Drawing by Dr Tony Ayling
Thunnus are able to maintain body temperature above the temperature of ambient seawater. For example, bluefin can maintain a core body temperature of 75–95 °F (24–35 °C), in water as cold as 43 °F (6 °C). However, unlike typical endothermic creatures such as mammals and birds, tuna do not maintain temperature within a relatively narrow range.[35]
Tuna achieve endothermy by conserving the heat generated through normal metabolism. The rete mirabile ("wonderful net"), the intertwining of veins and arteries in the body's periphery, transfers heat from venous blood to arterial blood via a counter-current exchange system. This reduces surface cooling, maintaining warmer muscles. This supports higher swimming speed with reduced energy expenditure.[35]
For powerful swimming animals like dolphins and tuna, cavitation may be detrimental, because it limits their maximum swimming speed.[36] 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.
Maximum reported sizes of tuna species
Tuna is an important commercial fish. The International Seafood Sustainability Foundation compiled a detailed scientific report on the state of global tuna stocks in 2009, which includes regular updates. According to the report, Tunas are widely but sparsely distributed throughout the oceans of the world, generally in tropical and temperate waters between about 45 degrees north and south of the equator. They are grouped taxonomically in the family Scombridae, which includes about 50 species. The most important of these for commercial and recreational fisheries are yellowfin (Thunnus albacares), bigeye (T. obesus), bluefin (T. thynnus, T. orientalis, and T. macoyii), albacore (T. alalunga), and skipjack (Katsuwonus pelamis).[37]
The report further 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 3.
[37] 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 their agreed 6,000 tonnes; the value of such overfishing would be as much as USD $2 billion.
[38] Such overfishing has severely damaged bluefin stocks.
[39] According to the
WWF, "Japan's huge appetite for tuna will take the most sought-after stocks to the brink of commercial
extinction unless fisheries agree on more rigid quotas".
[40]
Japan's Fisheries Research Agency counters that Australian and New Zealand tuna fishing companies under-report their total catches of southern bluefin tuna and ignore internationally mandated total allowable catch totals.[41]
In 2010, a bluefin tuna weighing 232 kilograms (511.47 pounds) was sold at Tokyo's Tsukiji fish market for 16.28 million yen ($US 175,000).[42]
In early 2011, a new record was set at 32.49 million yen (nearly $400,000) for a bluefin tuna weighing 342 kilograms (754 pounds), during an auction in Tsukiji Market, Tokyo. This equates to 95,000 yen per kilogram.[43]
In November 2011, another record was set when a fisherman in Massachusetts caught an 881-pound 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, due to the trawl net, the fish sold for just under $5,000 (USD)[44]
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Tuna being weighed on Greek quay-side
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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.[46] 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[47] 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 8th meeting in March 2012.
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.[48]
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,[49] however the nets were prone to entangling dolphins, injuring or killing them. Public outcry and new government regulations, which are now monitored by the 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 that 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.[50]
Increasing quantities of high-grade tuna are reared in net pens and fed bait fish. In Australia, former fishermen raise southern bluefin tuna, Thunnus maccoyii, and another bluefin species.[45] 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.[51]
Japan is the biggest tuna consuming nation and is also the leader in tuna farming research.[52] 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.[53][54][55] The farm breed is known as Kindai tuna. Kindai is the contraction of Kinki University in Japanese (Kinki daigaku).[56] In 2009, Clean Seas, an Australian company which has been receiving assistance from Kinki University[57][58][59] managed to breed Southern Bluefin Tuna in captivity and was awarded the second place in World's Best Invention of 2009 by Time magazine.[60]
Canned tuna on sale at a supermarket
Canned tuna was first produced in 1903, quickly becoming popular.[61] Tuna is canned in edible oils, in brine, in water, and in various sauces. In the United States, 52% of canned tuna is used for sandwiches; 22% for salads; and 15% for casseroles and dried, packaged meal mixes.[62]
In the United States, only Albacore can legally be sold in canned form as "white meat tuna";[63] in other countries, yellowfin is also acceptable. While 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").[61]
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 pre-cooked for prescribed times of perhaps 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. The sealed can itself is then heated (called retort cooking) for 2 to 4 hours.[64] This process kills any bacteria, but retains the histamine that can produce rancid flavors. 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.[61]
Australian standards once required cans of tuna to contain at least 51% tuna, but these regulations were dropped in 2003.[65][66] The remaining weight is usually oil or water. In the US, the FDA regulates canned tuna (see part c).[67] In 2008, some tuna cans changed from 6 ounces (170 g) to 5 ounces (140 g) due to "higher tuna costs".[68]
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Tuna steak served in a French bistro
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Canned tuna can be a good source of omega-3 fatty acids. It sometimes contains over 300 milligrams (0.011 oz) per serving.[69]
Mercury content in tuna can vary widely. For instance, testing by Rutgers University reportedly found that a can of StarKist had 10 times more mercury than another can of similarly-identified tuna. This has prompted a Rutgers University scientist whose staff conducted the mercury analysis to say, "That's one of the reasons pregnant women have to be really careful ... If you happen to get a couple or three cans in the high range at a critical period when you are pregnant, it would not be good." 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.[70]
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.[71] These findings suggest that choosing to consume a type of tuna that has a relatively higher natural fat content might help reduce the amount of mercury intake, compared to consuming tuna with a low fat content.
The industry-sponsored group Center for Consumer Freedom, which doesn't release the names of its contributors, claims the health risks of methylmercury in tuna might be dampened by the selenium found in tuna,[72] although the mechanism and effect of this still is largely unknown.[73]
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 albacore.[citation needed]
In 2009 a California appeals court upheld a ruling that canned tuna does not need warning labels as the methylmercury is naturally occurring.[74]
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.[75] The Environmental Protection Agency provides guidelines on how much canned tuna it is safe to eat. Roughly speaking, the guidelines recommend one 6 oz. can of light tuna a week for those weighing less than 110 pounds and two cans a week for those who weigh more.[76]
In 2007 it was reported that some canned light tuna such as yellowfin tuna[77] 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.[78]
The Eastern little tuna (Euthynnus affinis) has been available for decades as a low-mercury, less expensive canned tuna. However, of the five major species of canned tuna imported by the United States it is the least commercially attractive, primarily due to its dark color and more pronounced 'fishy' flavor. Its use has traditionally been restricted to institutional (non-retail) commerce.
A January 2008 investigation conducted by the New York Times found 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."[79]
A book by Jane Hightower, Diagnosis Mercury: Money, Politics and Poison, published in 2008, discusses human exposure to mercury through eating large predatory fish such as large tuna.[80][81][82]
There are five main tuna fishery management bodies: 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.[83] The five gathered for the first time in Kobe, Japan in January 2007. Environmental organizations made submissions[84] 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 are scheduled to meet at another joint meeting in January or February 2009 in Europe.[85]
In 2010, Greenpeace International has added the albacore, bigeye tuna, blackfin tuna, Pacific bluefin tuna, Atlantic bluefin tuna, southern bluefin tuna and the yellowfin tuna 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 have a very high risk of being sourced from unsustainable fisheries."[86][87]
It is widely accepted that bluefin tuna have been severely overfished, with some stocks at risk of collapse.[88][89] According to the International Seafood Sustainability Foundation (a global, non-profit partnership between the tuna industry, scientists, and the World Wide Fund for Nature), Indian Ocean yellowfin tuna, Pacific Ocean (eastern & western) bigeye tuna, and North Atlantic albacore tuna are all overfished. In April 2009, no stock of skipjack tuna (which makes up roughly 60 percent of all tuna fished worldwide) was considered to be overfished.[90] However, the BBC documentary, South Pacific, which first aired in May 2009 stated that, should fishing in the Pacific continue at its current rate, populations of all tuna species could collapse within 5 years.[91] It highlighted huge Japanese and European tuna fishing vessels, sent to the South Pacific international waters after overfishing their own fish stocks to the point of collapse.
A 2010 tuna fishery assessment report released in Jan 2012 by the Secretariat of the Pacific Community (SPC), supporting this finding, recommending that all tuna fishing should significantly reduce or increase no further and that limits on skipjack fishing be considered.[92]
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