SPECIES DESCRIPTION

Taxonomy and distribution Phylum – Chordata Subphylum – Vertebrata Superclass – Gnathostomata Class – Osteichthyes Subclass – Actinopterygii Infraclass – Teleostei Superorder – Acanthopterygii Order – Perciformes Family – Scombroidae Genus – Thunnus Species – thynnus

The bluefin tuna was first described by Linnaeus in 1758 as Scomber thynnus. Many other denominations followed, such as Thunnus vulgaris and Thunnus thynnus. One capture-based aquaculture tuna species is considered in this paper – the northern bluefin tuna (Thunnus thynnus) (not including Thunnus orientalis) (Collette, Reeb and Block, 2001), with reference to the southern bluefin tuna (Thunnus maccoyii). The Thunnus thynnus (Figure 1) is found in Labrador, Canada and continues south to the Gulf of Mexico and the Caribbean Sea and also off the coast of Venezuela and Brazil in the Western Atlantic. In the Eastern Atlantic it occurs from the Lofoten Islands off the coast of northern Norway south to the Canary Islands and the Mediterranean Sea. There is also a population in South African waters.

Habitat and biology

Northern bluefin tuna are large pelagic marine fish. The juveniles are encountered in epipelagic waters whereas large tunas tend to be mesopelagic and are found also in deeper and cooler waters. The species has considerable thermal tolerances, as it can be found in waters as cold as 10 °C, as well as in tropical areas (Brill, 1994). Generally the most critical environmental parameters for these large pelagic fish are sea surface temperature and the levels of dissolved oxygen and salinity. The species has been observed both above and below the thermocline. Juvenile fish tend to live near the surface.
The following three growth stages can be distinguished: i) larvae – recently hatched individuals which are considerably different in appearance from juveniles or adults; ii) juveniles – similar in appearance to adults, but sexually immature; and adults – sexually mature fish (Figure 2). The maximum reported weight of an adult specimen has been 684 kilograms, with a total length of 458 cm. The species seems to have an average lifespan of around 15 years, while the longevity for both the Atlantic and the southern

Figure 2
Thunnus thynnus larvae (left), a school of juveniles (middle) and adults in a fattening cage (right)

bluefin tunas was estimated at around 20 years (Cort, 1990). For adults natural mortality rates range from 0.2 to 0.6, while natural rates for juveniles are higher.
Schooling and migration All bluefin tuna species move constantly in search for food and to maintain a constant water flow over their gills. The Atlantic bluefin (Thunnus thynnus), pacific bluefin (Thunnus orientalis), and southern bluefin (Thunnus maccoyii) tunas all migrate seasonally over long distances between temperate waters, where they feed, and tropical waters, where they spawn.

FIGURE 3
Thunnus thynnus juveniles schooling associated with the bullet tuna, Auxis rochei

Spawning of all three species is generally restricted to relatively restricted areas in temperate and tropical waters.
Thunnus thynnus may form giant schools spreading over several nautical miles when migrating into the Mediterranean Sea to spawn during the summer months. Most bluefin school according to their size, however it is not unusual for different size size-groups to school together. Juveniles are, therefore, often associated with smaller tuna species such as the skipjack or bonito (Figure 3). While schooling is believed to be sight-oriented, schools have been observed at night.
Bluefin tuna are excellent swimmers and can swim at high speed for long periods as they are able to absorb and utilize large amounts of oxygen. Their bodies are designed for high performance at both sustainable and burst swimming speeds (Dickson, 1995). Tuna must swim constantly to satisfy their oxygen requirements in order to stay alive. Their swimming pattern seems to be influenced by both the distribution of food and the need to return to their ancestral spawning grounds at the appropriate time. To efficiently transfer oxygen from the gills to the other body tissues, tunas have hearts that are approximately 10 times the size of those of other fish, relative to the body weight, and blood pressure and pumping rate about three times higher.
Tunas have two types of muscle, white and red. The white muscles function during short bursts of activity, while the red muscles, which have a relatively large mass, allow the fish to swim at high speeds for long periods without fatigue, as demonstrated by tagging studies with conventional and sonic tags (Joseph, Klawe and Murphy, 1988; Bushnell and Holland, 1997).

Feeding

Tuna larvae live in warm surface waters and feed primarily on zooplankton, including small crustaceans and the larvae of crustaceans, fishes, molluscs and jellyfish. Tuna larvae are preyed upon by zooplankton foragers, such as larger larvae and early juveniles of other pelagic fish. Juvenile and adult tuna generally prey on fish, squid and crustaceans. The larger specimens, which feed on pelagic fishes, are positioned at the top of the trophic web and locate their prey visually. To satisfy their nutritional requirements tunas have to swim long distances. Their type of locomotion is particularly well adapted to the search for prey in large water volumes with the least expenditure of energy. Tuna break up schools of prey, producing disorientation and straggling (Webb, 1984; Partridge, 1982). When prey is detected, the tuna changes their behaviour and have a general increase of activity, e.g. increase in swimming speed, change in swimming pattern and energetic pursuit to obtain smaller schooling fish such as anchovies.

Reproduction

The spawning of Thunnus thynnus has been so far detected in only two areas: the Mediterranean and the Gulf of Mexico. In the Gulf of Mexico, spawning occurs from April to June when the water temperature is 25–30 °C and in the Mediterranean from May/June to August. Karakulak et al. (2004a; 2004b) reported bluefin spawning in the Levantine Sea (Eastern Mediterranean basin) with a peak in the activity in May.
Sexual maturity of the Atlantic bluefin tuna is reached at the age of 5 to 8 years, while in the eastern Atlantic maturity is reached earlier, at 4–5 years. Scientists have found that in the Balearic Islands (Mediterranean) bluefin tuna are able to spawn from 3 years old (Abascal, Megina and Medina, 2003). Bluefin tunas may release from 5 to 30 million eggs and spawning occurs in open water close to the surface and in areas where the survival expectations of the larvae is highest.