Tuna

Tuna, sometimes called tunafish, are several species of ocean-dwelling fish in the family Scombridae, mostly in the genus Thunnus.
Tuna are fast swimmers (they have been clocked at 77 km/h (48 mph)) and include several species that are warm-blooded. 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, than the muscle tissue of most other fish species. Some of the larger tuna species such as the bluefin tuna can raise their blood temperature above the water temperature with muscular activity. This enables them to live in cooler waters and survive a wider range of circumstances.
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, Allothunnus fallai), but there is mounting evidence that overcapitalization threatens tuna fisheries world-wide. The Australian Government alleged in 2006 that Japan has illegally over-fished 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.
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 . However, most canned light tuna is skipjack tuna and is low in mercury. 
Tuna are a common target for big-game fishing.
==Species==
There are nine species:
* Albacore, Thunnus alalunga (Bonnaterre, 1788).
* Yellowfin tuna, Thunnus albacares (Bonnaterre, 1788).
* Blackfin tuna, Thunnus atlanticus (Lesson, 1831).
* Karasick tuna, Thunnus karasicus (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).
== See also ==
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
* Butterfly kingfish (Butterfly mackerel) Gasterochisma melampus Richardson, 1845
* Dogtooth tuna Gymnosarda unicolor (Rüppell, 1836)
* "Tuna" is also a name for the sweet fruit of prickly pear cactuses in the genus Opuntia.
==References==
* 
== External links ==
* Tuna Recipes A collection of recipes containing tuna
* Monterey Bay Aquarium's Seafood Watch program
* Bluefin tuna nutrition information
* Table of relative mercury levels in fish and shellfish (FDA)
*GotMercury.Org, a mercury-in-fish calculator based FDA data with the EPA's formula for safe exposure.
*Tuna facts from Greenpeace
*The Age, Japan used 'back door' to hide $2bn bluefin haul
*Tuna Nutritional Information
Category:Scombridae
Category:Edible fish