The second smallest of all tunas (Thunnus spp.), the longtail tuna (Thunnus tonggol) is unique among the ‘true’ tunas, being the only species to lack a swim bladder (an air-filled sac which helps fish to maintain buoyancy) (3)(4). The slender body and forked tail of the longtail tuna are silvery white, with elongate oval spots arranged in horizontal rows along its length. The two dorsal fins on the back are blackish in colour, with the larger of the two, the second dorsal fin, tinged with yellow. The anal fin is silvery, with a yellow tint, the pectoral and pelvic fins are black, and the caudal fin is blackish with streaks of yellowish-green (4)(5).
Also known as
Blue-fin tuna, Indian long-tailed tuna, Northern bluefin tuna, oriental bonito.
The longtail tuna is an exceptionally voracious predator, easily consuming the equivalent of two percent of its body weight (around one kilogram) in prey every day. It feeds opportunistically on a wide variety of fish, crustaceans and cephalopods(2)(3)(4)(5). Tuna (Thunnus spp.) can be distinguished from other bony fish by their high metabolic rate, and are unique among fish in being able to maintain their body temperature several degrees higher than the surrounding water. Tuna are extremely powerful, agile and capable of extraordinary bursts of speed; however, this is energetically demanding, and to ensure that enough oxygen is provided to the muscles, the fish must swim steadily for remarkable distances in order to ensure sufficient gas exchange at the gills (where oxygen from the water is passed through the gills and absorbed into the blood) (4). The longtail tuna breeds during the summer, with a typical spawning period lasting five to seven months (2)(3).
The longtail tuna occurs mainly in the Indo-West Pacific Ocean, including the Red Sea, around the Arabian Peninsula, off the coast of East Africa across to the coast of New Guinea, north to the waters around Japan, and south to the coast of Australia (4)(5)(6).
Tuna are among the most commercially valuable fish in the world, and most of the eight species in the Thunnusgenus are extensively affected by overfishing (7). Although the longtail tuna is not as highly regarded for its meat in comparison to species such as bluefin (Thunnus maccoyii, Thunnus thynnus), yellowfin (Thunnus albacares) and albacore tuna (Thunnus alalunga), the population collapse of these more economically important species may, in future, increase the pressure on the longtail tuna as an attractive alternative to meet the exceedingly high consumer demand (4)(7). The longtail tuna is also considered to be an excellent sport fish, and recreational fishing may contribute a low level threat to this species (2).
Conservation organisations, such as WWF and the Marine Conservation Society, are working to promote sustainable fishing and to reduce illegal, unregulated and unreported fishing (7)(8). In Australia, the government has recognised the importance of the longtail tuna to the recreational sector, declaring it a ‘recreational only’ species, and Australian organisations are also working together to undertake a large-scale survey of longtail tuna catches in the region (3). Elsewhere, it is thought that current catch levels of the longtail tuna are grossly underestimated, and more detailed survey work is required in order to assess the level of threat to this species (4).
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In fish, an unpaired fin on the under surface of a fish, behind the anus.
The tail fin of a fish.
From the Greek for ‘head-foot’, a class of molluscs that occur only in marine habitats. All species have grasping tentacles, and either an internal or external shell. Includes nautiloids, cuttlefish, squids, octopuses, and extinct ammonites and belemnites.
A region of relatively shallow water, not usually deeper than 200 metres, surrounding each of the continents.
Diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, slaters, woodlice and barnacles.
The unpaired fin found on the back of the body of fish, or the raised structure on the back of most cetaceans.
A category used in taxonomy, which is below ‘family’ and above ‘species’. A genus tends to contain species that have characteristics in common. The genus forms the first part of a ‘binomial’ Latin species name; the second part is the specific name.
In fish, the pair of fins that are found one on each side of the body just behind the gills. They are generally used for balancing and braking.
In fish, the pair of fins found on the underside of the body.
The production or depositing of large quantities of eggs in water.
Collete, B.B. and Nauen, C.E. (1983) FAO Species Catalogue. Volume 2. Scombrids of the World. An Annotated and Illustrated Catalogue of Tunas, Mackerels, Bonitos and Related Species Known to Date. United Nations Development Programme, Food and Agriculture Organisation of the United Nations, Rome. Available at: ftp://ftp.fao.org/docrep/fao/009/ac478e/AC478E00.pdf
Yonemori, T., Yanagawa, H. and Pong, L.Y. (1995) Interactions of longtail tuna fisheries in the western South China Sea. In: Shomura, R.S., Majkowski, J. and Harman, R.F. (Eds.) Status of Interactions of Pacific Tuna Fisheries in 1995. Food and Agriculture Organisation of the United Nations, Rome. Available at: http://www.fao.org/docrep/003/w3628e/w3628e00.htm#Contents
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