The term "tuna" comes from Spanish atún at-tÅn, assimilated from al-tÅn ØÙتو٠[Modern Arabic ØÙتÙ] : 'tuna fish' thunnus.[6] Thunnus is derived from Ancient Greek: ÎÏÎÎÎÏ, romanized: thÃnnos used for the Atlantic bluefin tuna,[7] that name in turn is ultimately derived from ÎÏÎÏ thÃnÅ, meaning "to rush, dart along".[8][9]
The cladogram is a tool for visualizing and comparing the evolutionary relationships between taxa, and is read left-to-right as if on a timeline. The following cladogram illustrates the relationship between the tunas and other tribes of the family Scombridae. For example, the cladogram illustrates that the skipjack tunas are more closely related to the true tunas than are the slender tunas (the most primitive of the tunas), and that the next nearest relatives of the tunas are the bonitos of the tribe Sardini.[2]
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 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 or whitish.[44][43] The caudal peduncle, to which the tail is attached, is quite thin, with three stabilizing horizontal keels on each side.[43]
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]
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]
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 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]
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.
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]
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][verification needed] Farming its close relative, the Atlantic bluefin tuna, Thunnus thynnus, is beginning in the Mediterranean, North America and Japan. HawaiÊi 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]
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]
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