Longtail tuna  (Thunnus tonggol)

The longtail tuna is classified as Data Deficient on the IUCN Red List (1).

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).

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).  

The longtail tuna is a predominantly coastal species, often associated with continental shelves in temperate and tropical waters, where it stays close to the surface (2) (4).

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).

Tuna are among the most commercially valuable fish in the world, and most of the eight species in the Thunnus genus 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).

Environment Agency - Abu Dhabi is a principal sponsor of ARKive. EAD is working to protect and conserve the environment as well as promoting sustainable development in the Emirate of Abu Dhabi.

View information on this species at the UNEP World Conservation Monitoring Centre.

Find out more about the longtail tuna and tuna conservation:

• National Longtail Tuna Survey:
  http://www.longtailtuna.com.au/
• WWF:
  http://wwf.panda.org/what_we_do/endangered_species/tuna/
• Marine Conservation Society:
  http://www.mcsuk.org/