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|Tunas (from top): albacore, Atlantic bluefin, skipjack, yellowfin, bigeye|
Tuna are fast swimmers. People have seen tuna swim at up to 77 km/h. Several species of tuna are warm-blooded. Most species of fish have white flesh. The flesh of the tuna is different. It is pink to dark red. This is because tuna muscle tissue contains greater quantities of myoglobin. Myoglobin is a molecule that binds oxygen. Some of the larger tuna species such as the Northern 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 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), but there is mounting evidence that overfishing threatens tuna fisheries worldwide.
Some fishermen in Australia now grow Southern bluefin tuna in fish farms. Fish farms are areas of water with nets around them, where farmers raise (keep and grow animals) fish. Some tuna farms are circular nets 40 meters in diameter and 20 meters deep. The fishermen catch the tuna at sea, and then they keep the tuna in these farms. The tuna grow and get fatter for three to six months. There are also tuna farms in the Mediterranean, North America, and Japan that raise Northern bluefin tuna. Some environmental groups say that tuna farms use too much fish as food for the tuna, and that the farms pollute the water (make the water dirty).
Tuna are high up in the food chain. They eat animals that ate animals. Their diet leads to the accumulation of heavy metals in their flesh. 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 to limit their intake of tuna and other types of predatory fish . However, most canned light tuna is skipjack tuna and is very low in mercury.
There are eight species:
- 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
- Slender tuna Allothunnus fallai
- Bullet tuna Auxis rochei rochei
- Frigate tuna Auxis thazard thazard
- Kawakawa (little tunafish or mackerel tunafish) Euthynnus affinis
- Little tunny (little tunafish) Euthynnus alletteratus
- Butterfly kingfish (Butterfly mackerel) Gasterochisma melampus
- Dogtooth tuna Gymnosarda unicolor
- "Thunnus". FishBase. Ed. Ranier Froese and Daniel Pauly. January 2006 version. N.p.: FishBase, 2006.