Tuna
Tuna is any of several species of ocean-dwelling fish comprising the Thunnini tribe in the mackerel family Scombridae, including Thunnus and related genera. Tuna is also the name of the commercial food from these species. According to Nelson (1994), the five genera of tunas are Thunnus, Euthynnus, Auxis, Katsuwonus, and Allothunnus, encompassing 14 species.
Tunas are fast-swimming fish which are important for food and for sport fishing. Some members are also able to raise their body temperature about the temperature of the surrounding water.
Some tuna species and fisheries are overfished and there are risks of some tuna fisheries collapsing.
Overview
Members of the Scombridae family have two dorsal fins with 5 to 12 finlets behind the second dorsal and the anal fins (Nelson 1994). The first dorsal fin has 9 to 27 rays, the pelvic fins have six rays and are place behind the pectoral fins, which are high on the body (Nelson 1994). .
Tunas are fast swimmers, having been clocked at 77 kilometers per hour (48 miles per hour). They also have some complex adaptations that allow fast swimming, such as the swimming muscles which are warmed through heat exchange.
Fish are poikilothermic in that they do not maintain constant internal temperatures and the temperature often mirrors the ambient temperature. However, tuna, as well as certain other fish species, maintain elevated body temperatures to varying degrees. Bluefin tuna (and porbeagle sharks) can elevate body temperatures in excess of 20 °C above ambient water temperatures. In many cases, this phenomena has been traced to heat exchange, as warmer blood being returned to the gills in small veins runs close to colder, oxygenated blood in narrow arteries leaving the gills. This ability to have elevated temperatures allows fish to be active in colder waters and to have enhanced swimming ability because of the warmer muscles. One species of "primitive" mackerel (Gasterochisma melampus), some billfishes, all sharks in the family Lamnidae (shortfin mako, long fin mako, white, porbeagle, and salmon shark) also are known to have this capacity. The degree of being able to have elevated temperatures varies, however, with the billfish only being able to warm their eyes and brain.
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.
Species
| Thunnus genus | ||||||||||||
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 Yellowfin tuna, Thunnus albacares
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See text. |
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 rochei
- Frigate tuna Auxis thazard thazard
- Kawakawa (little tunafish or mackerel tunafish) Euthynnus affinis (Cantor, 1849)
- Little tunny (little tunafish) Euthynnus alletteratus
Two other outside the tribe Thunnini are sometimes considered tunas:
- Butterfly kingfish (Butterfly mackerel) Gasterochisma melampus Richardson, 1845
- Dogtooth tuna Gymnosarda unicolor (Rüppell, 1836)
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 populations seem to support fairly healthy fisheries (for example, the central and western Pacific skipjack tuna, Katsuwonus pelamis), but there is mounting evidence that overcapitalization threatens tuna fisheries 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 international 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. Some say this is unfortunately in accord with the Japanese government's refusal to deal with sustainable environmental ideas and the sometime utter neglect of animal rights or preservation needs, such as fishing quotas.
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 [1]. 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[citation needed] 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.[2] Further, the closely related Eastern Little Tuna (Euthynnus affinis) has recently come to market as a low-mercury, less expensive substitute for canned albacore.
Canned tuna was first produced in 1903, and quickly became popular.[1]
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. [3] They met for the first time in Kobe in Japan in January 2007. Environmental organisations made submissions [4] 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. [5]
Association with dolphins
Many tuna species associate with dolphins, swimming along side them. These include yellowfin tuna. Species which do not include albacore and skipjack. Methods of fishing tuna have become more "dolphin friendly", becoming less prone to entangle, injure or kill dolphins. However, there are no universal independent inspection program or verification of "dolphin safeness" to show that dolphins are not harmed during tuna fishing. According to the Consumers Union, this gives the claims such as "dolphin safe" little credibility.[1]
Methods of capture
- Arabic method of Almadraba, in which nets are used, creating a maze in which the tuna are secured.
- big-game fishing
- longline fishing
ReferencesISBN links support NWE through referral fees
- 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
- "Thunnus". FishBase. Ed. Ranier Froese and Daniel Pauly. January 2006 version. N.p.: FishBase, 2006.
- Nelson.
External links
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