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A tuna is a saltwater fin fish that belongs to the family Thunnini, a sub-grouping of the mackerel family (Scombridae) – which together with the tunas, also includes the bonitos, mackerels, and Spanish mackerels. Thunnini comprises fifteen species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max. length: 50 cm (1.6 ft), weight: 1.8 kg (4 lb) up to the Atlantic blue fin tuna (max. length: 4.6 m (15 ft), weight: 684 kg (1,508 lb). The blue fin averages 2 m (6.6 ft), and is believed to live up to 40 years.

Their circulatory and respiratory systems are unique among fish, enabling them to maintain a body temperature higher than that of the surrounding water. An active and agile predator, the tuna has a sleek, streamlined body, and is among the fastest-swimming pelagic fish.  Found in warm seas, it is extensively fished commercially, and is popular as a game fish. As a result of over-fishing, stocks of some tuna species such as the Southern Bluefin tuna have been reduced dangerously close to the point of extinction. 

The tuna is a sleek and streamlined fish, adapted for speed: built like torpedoes, have retractable fins and their eyes are set flush to their body.  It has two closely spaced dorsal fins on its back. Seven to 10 yellow fin lets run from the dorsal fins to the tail, which is lunate – curved like a crescent moon – and tapered to pointy tips. The caudal peduncle, to which the tail is attached, is quite thin, with three stabilizing horizontal keels on each side. The tuna's dorsal side is generally a metallic dark blue and the underside is silvery or whitish, for camouflage. 

Thunnus are widely but sparsely distributed throughout the oceans of the world, generally in tropical and temperate waters at latitudes ranging between about 45° north and south of the equator. All tunas are able to maintain the temperature of certain parts of their body above the temperature of ambient seawater. For example, Bluefin can maintain a core body temperature of 25–33 °C (77–91 °F), in water as cold as 6 °C (43 °F). However, unlike "typical" endothermic creatures such as mammals and birds, tunas do not maintain temperature within a relatively narrow range.  Tunas achieve endothermy by conserving the heat generated through normal metabolism. In all tunas, the heart operates at ambient temperature.  This allows the tuna to elevate the temperatures of the highly-aerobic tissues of the skeletal muscles, eyes and brain, 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. 

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. 

Dolphins swim beside several tuna species. Tuna schools are believed to associate themselves with dolphins for protection against sharks.

Commercial fishing vessels tend to exploit this association by searching for dolphin pods. Vessels would encircle the pod with nets to catch the tuna beneath; however the nets were prone to entangling dolphins, injuring or killing them. Public outcry and new regulations 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. 

Bluefin tuna migrate across oceans and can dive more than 4,000 feet. They are tremendous predators from the moment they hatch, seeking out schools of fish like herring, mackerel and even eels. They hunt by sight and have the sharpest vision of any bony fish. There are three species of Bluefin:  Atlantic (the largest and most endangered), Pacific, and Southern. Most catches of the Atlantic blue fin tuna are taken from the Mediterranean Sea, which is the most important Bluefin tuna fishery in the world. The Atlantic Bluefin is a highly sought-after delicacy for sushi and sashimi in Asia! Driven by high prices, fishermen use even more refined techniques to catch tuna. As a result, the fish are disappearing. Tuna are a top predator in the marine food chain, maintaining a balance in the ocean environment.

Yellowfin tuna 

Yellowfin tuna are torpedo-shaped with dark metallic blue backs, yellow sides, and a silver belly, which has about 20 vertical lines. The second dorsal fin and the anal fin, as well as the fin lets between those fins and the tail, are bright yellow, giving this fish its common name. The second dorsal and anal fins can be very long in mature specimens, reaching almost as far back as the tail and giving the appearance of sickles or scimitars. The pectoral fins are also longer than the related Bluefin tuna. Yellowfin tuna can live up to six or seven years. 

The yellowfin tuna is among the larger tuna species, reaching weights over 180 kg (400 lb), but is significantly smaller than the Atlantic and Pacific Bluefin tunas, which can reach over 450 kg (990 lb), and slightly smaller than the Big eye tuna and the southern Bluefin tuna.

Yellowfin tuna are epi-pelagic fish that inhabit the mixed surface layer of the ocean above the thermocline. Sonic tracking has found that although Yellowfin tuna, mostly range in the top 100 m (330 ft) of the water column and penetrate the thermocline relatively infrequently. They are capable of diving to considerable depths. 

Yellowfin tuna often travel in schools with similarly sized companions, like skipjack and Big eye, and are also known to associate with dolphins and with drifting flotsam such as logs and pallets, and sonic tagging indicates some follow moving vessels. 

Yellowfin tuna prey includes other fish, pelagic crustaceans, and squid. Like all tunas, their body shape is particularly adapted for speed, enabling them to pursue and capture fast-moving baitfish such as flying fish, sauries, and mackerel. Schooling species such as myctophids or lanternfish and similar pelagic driftfish, anchovies, and sardines are frequently taken. Large yellowfins prey on smaller members of the tuna family such as frigate mackerel and skipjack tuna.

Yellowfins are able to escape most predators, because unlike most fish, tuna are warm-blooded, and their warm muscles make them extremely strong swimmers, yellowfin tuna reaching "speeds of up to 50 miles per hour". They are highly migratory, can navigate enormous distances, sometimes crossing entire oceans. They are found throughout the Pacific, Atlantic and Indian Oceans. 



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