Tuna (Thunnus)

08 Jun, 2014 - 00:06 0 Views
Tuna (Thunnus) Yellowfin tuna

The Sunday News

Yellowfin tuna

Yellowfin tuna

Know Your Wildlife
TUNA (genus Thunnus), also called tunny, are salt water fish of great commercial value as food. They are related to mackerels and are placed with them in the family Scombridae (order Perciformes). Tunas vary considerably, both within and among species.
Description
Tunas are elongated, robust, and streamlined fishes, adapted for speed. They have a rounded body that tapers to a slender tail base and a forked or crescent-shaped tail.

In colour, Tunas are generally dark above and silvery below, often with an iridescent shine. The caudal peduncle, to which the tail is attached, is quite thin, with conspicuous keels (tips) on either side of the tail base, a row of small finlets behind dorsal and anal fins, and a corselet of enlarged scales in the shoulder region. It has two closely spaced dorsal fins on its back; the first is “depressible” thus it can be laid down, flush, in a groove that runs along its back. Seven to 10 yellow finlets run from the dorsal fins to the tail, which is lunate (thus curved like a crescent moon) and tapered to pointy tips.

Another notable feature is a well-developed network of blood vessels below the skin that acts as a temperature-regulating device associated with long-term, slow swimming. Because of their circulatory and respiratory systems, tunas are unique among fishes in their ability to maintain their body temperature above that of the surrounding water, often between five and 12 degrees Celsius above ambient water temperature. Some muscles may even be up to 21 degrees Celsius higher than the surrounding water. Also unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red.

The seven Tuna species
The seven species of tunas in the genus Thunnus are the northern Bluefin tuna (T. thynnus), albacore (T. alalunga), yellowfin tuna (T. albacores), southern Bluefin tuna (T. thynnus maccoyii), bigeye tuna (T. obesus), blackfin tuna (T. atlanticus), and longtail tuna (T. tonggol). These different species range from moderate to very large in size. The giant of the group is the northern Bluefin tuna, which grows to a maximum length and weight of about 4.3 metres (14 feet) and 800 kilogrammes (1,800 pounds) and is believed to live for up to 50 years. The yellowfin tuna reaches a maximum weight of about 180 kilogrammes (397 pounds), and the albacore grows to about 36 kilogrammes (79 pounds).

The northern Bluefin tuna characteristically has yellow finlets and is often marked with silvery spots or bars. It is important in sport and commercial fishing, with anglers rating it among the greatest trophies obtainable. Populations of northern Bluefin tuna in the Atlantic Ocean have declined significantly since preindustrial times because of overfishing. As a result, many scientists and environmental organisations have called for a halt on the harvesting of this species. The other commercially important species are the albacore, marked with a shining blue stripe on each side; the yellowfin, with yellow fins and a golden stripe on each side; and the bigeye, a robust fish with relatively large eyes.

Tunas migrate long distances over all the world’s oceans and occupy tropical, temperate, and even some cooler waters. The only two species of relatively limited distribution are the blackfin tuna (western Atlantic) and the longtail tuna (Indo-Pacific region).

Physiology
Thunnus are widely but sparsely distributed throughout the oceans of the world, generally in tropical and temperate waters between about 45 degrees 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 degrees Celsius in water as cold as six degrees Celsius. However, unlike typical endothermic creatures (dependent on the internal generation of heat) such as mammals and birds, tuna do not maintain temperature within a relatively narrow range.

Tuna achieve endothermy by conserving the heat generated through normal metabolism. The intertwining of veins and arteries in the body’s periphery, allows much of the 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. 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. In all tunas, however, the heart operates at ambient (immediate) temperature as it receives cooled blood, and coronary circulation is directly from the gills.

Biology
Tunas feed on fishes, squid, and a variety of planktonic organisms . They spawn in the open sea over very large areas. For powerful swimming animals like dolphins and tuna, cavitation (the formation of bubbles or empty space in a liquid) may be detrimental, because it limits their maximum swimming speed. 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, unlike dolphins; these fish do not feel the bubbles, because they have bony fins without nerve endings.

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