When speaking of "tuna", you could in fact be referring to any one of several species of fish (14 or more, depending on the language!). The word itself seems to have come into use in the second half of the last century, originating probably in Europe, either the Dalmatian coast of the former Yugoslavia or from the Iberian Peninsula. Generally referring to species, scientifically known as members of the Thunnini tribe, tunas are closely related to mackerels (they are members of family Scombridae, a group of marine fishes which includes mackerels and bonitos).

Tuna can come under at least 13 different names:

English name	French name	Spanish name	Scientific name
Tunas	Thons	Atún	Thunnini
Albacore	Germon	Atún blanco	Thunnus alalunga
Yellowfin tuna	Albacore	Rabil	Thunnus albacares
Blackfin tuna	Thon - nageoires noires	Atún aleta negra	Thunnus atlanticus
Bigeye tuna	Thon obese	Patudo	Thunnus obesus
Southern bluefin tuna	Thon rouge du Sud	Atún del Sur	Thunnus maccoyii
Atlantic bluefin tuna	Thon rouge	Atún	Thunnus thynnus
Pacific bluefin tuna	Thon bleu du Pacifique	Atún aleta azul del Pacífico	Thunnus orientalis
Longtail tuna	Thon mignon	Atún tongol	Thunnus tonggol
Skipjack	Listao	Listado	Katsuwonus pelamis
Kawakawa	Thonine orientale	Bacoreta oriental	Euthynnus affinis
Little tunny	Thonine commune	Bacoreta	Euthynnus alleteratus
Black skipjack	Thonine noire	Barrilete negro	Euthynnus lineatus
Bullet tuna	Bonitou	Melvera	Auxis rochei
Frigate tuna	Auxide	Melva	Auxis thazard

Tunas are pelagic marine fish, spending their entire lives relatively near the surface of tropical, subtropical and temperate oceans and seas.

Five species are targeted in the Atlantic tuna fishery: yellowfin (Thunnus albacares), skipjack (Katsuwonus pelamus), bigeye (Th. obesus), albacore (Th. alalunga) and Atlantic bluefin tuna (Th. thynnus).

Three of these species, yellowfin, skipjack and bigeye, are often referred to as the "tropical tunas", for they range throughout the tropical and sub-tropical waters of the Atlantic, Indian and Pacific Oceans. Albacore are more often found in sub-tropical to temperate waters, while bluefin can be found on both sides of the North Atlantic as far north as Greenland and northern Norway.

In addition to these tuna species, there are a few species of billfish that are often caught in the same fisheries, especially swordfish (Xiphias gladius), white marlin (Tetrapturus albidus) and blue marlin (Makaira nigricans). In general, the billfish are less important commercially.

Tuna - Beyond the Tin

Many people recognise tuna only by the tinned pink-ish chunks they find in oil or brine; or in the dark-fleshed steaks on supermarket shelves, or maybe even in a Japanese restaurant in the form of sushi or sashimi.

But tuna fish are impressive beasts, designed for speed, efficient predation, and capable of travelling vast distances though the open oceans. They can also attain a huge size - the Atlantic bluefin tuna has been measured at 4.5 metres in length, weighing 680kg (heavier than a horse!). Bigeye and yellowfin are more modest in size, about 2.0 to 2.5 m in length and 200 kg in weight. The smallest of the five species discussed here are albacore (140 cm, 60 kg) and skipjack (108 cm, 35 kg).

Tunas' life-spans vary from a few years for the smaller species to 12 to 15 years for the larger ones. The longevity record for tunas of 20 years is held by the Atlantic bluefin tuna and its close relative the Southern bluefin tuna.

Feeding

Tunas feed during daylight hours, locating their prey visually. In addition, tunas can detect minute traces of oils, proteins and other chemicals produced by their prey. They are normally opportunistic feeders, preying on other fish (including smaller and/or juvenile tunas), crustaceans and squid.

To satisfy their food requirements, tuna often have to swim long distances, and the type of locomotion they use is therefore particularly adapted to the search for prey in a large volume of water. Upon detection of their prey, some changes in tuna behaviour have been observed, including a change in swimming pattern, jaw snapping, and the display of dark stripes on their flanks.

Predation strategies include the breaking up of schools of prey, resulting in disorientation and straggling, and searching for prey in schools. Most tuna schools are formed according to size, such that juveniles of the bigger species are often associated with schools of smaller species such as skipjack or bonito. As well as aiding predation, schooling also offers protection, particularly for juveniles, from those animals, including sharks and toothed whales, which feed on tunas. Schooling helps to confuse predators and reduces the likelihood that any single fish will become a victim to a predator. Prior to the intense fishing pressure to which they are now subjected, Atlantic bluefin tuna used to form giant schools, spread over several nautical miles when migrating into the Mediterranean Sea to spawn during the summer.

Swimming and Migration

Tunas are excellent swimmers, and their bodies are designed for high performance at both sustained swimming and bursts of high speed. But, they must swim constantly - in fact their survival depends on it. This is because they obtain their oxygen supply from water flowing over their gills. Instead of pumping the water, they force water over their gills by swimming through it with their mouths open. This may be an efficient way to satisfy their oxygen requirements, but the drawback is that they will suffocate if they stop swimming.

Migrations are seasonal movements, often over long distances, for the purpose of feeding or reproduction. Temperate tunas, including albacore and Atlantic bluefin, migrate long distances between temperate waters, where they feed, and tropical waters, where they spawn. Tropical tunas are less migratory, however.

The net distances travelled by tunas exceed those of any other fish. Records obtained from tagging studies have shown distances travelled as:

- 10,790 km for a Pacific bluefin tuna (Thunnus orientalis) (from southeast of Japan to off Baja California)
- 9,500 km for a skipjack tuna (from off Baja California to the Marshall Islands)
- 8,500 km for an albacore tuna (from off California to off Japan)
- 7,700 km for an Atlantic bluefin tuna (across the Atlantic Ocean)
- Yellowfin and Bigeye have also been recorded travelling more than 5,000km

Fishermen are able to predict on the basis of oceanic conditions where tuna are likely to appear, and then transfer their operations to those areas.

Breeding

Tuna spawn in open water close to the surface. Most species spawn only in waters with surface temperatures of greater than 24˚C, in which their young stand the greatest chance of survival. Tropical tunas appear to spawn in equatorial areas all year around and at higher latitudes during the warm seasons. Albacore and bigeye, however, migrate annually from temperate feeding areas to tropical spawning areas. The bluefin tunas, including the Atlantic bluefin, exhibit a homing behaviour when they mature, and return to restricted areas in the Atlantic, Pacific and Indian Oceans to spawn.

Most species of tuna lay between 2 and 70 million eggs. Of the major commercial species, skipjack reach sexual maturity earliest and produce the greatest number of eggs. Consequently, this species has been able to withstand substantial fishing pressure, and has been described as "the rat of the sea" due to its rapid reproductive rate.

Special Characteristics - a warm blooded fish

Fish are cold-blooded animals and as a result their body temperatures are generally close to that of the surrounding water. Tuna, however, have evolved to be able to maintain their powerful swimming muscles at temperatures of around 30˚C in water temperatures as low as 7˚C, a trait unique among all other fishes except for their relatives the swordfish and certain species of shark.

By keeping their swimming muscles warm tuna are able to sustain high swimming speeds for long periods and to recover quickly after prolonged exertion. It also allows them to delve into colder deep waters that would be otherwise inaccessible. Bigeye tuna, for example, have been observed to dive to depths of 250 meters in just one minute, experiencing a rapid change in water temperature from 24˚C to 9˚C.