Southern Bluefin Tuna, Thunnus maccoyii (Castelnau 1872)

Other Names: Bluefin, Japanese Central Pacific Bluefin Tuna, SBT, Southern Tuna, Southern Tunny

Summary:

Large, powerful fast-swimming oceanic predators with an advanced circulatory system that keeps the body temperature above that of the surrounding seawater.

A blackish-blue tuna becoming silvery-white below, with alternating rows of dots and lines, a yellow anal fin and yellow finlets with black margins, and a yellowish to bluish first dorsal fin.

Southern Bluefin Tuna (SBT) are highly valued in global markets, being the most expensive fresh seafood in the world. About 95% of the SBT catch is consumed in high-end Japanese sashimi market.

Video of SBT in pens at Port Lincoln, South Australia

Video of SBT being harvested from tuna pens at Port Lincoln, South Australia

Cite this page as:

Bray, D.J. & Gomon, M.F. 2024, Thunnus maccoyii in Fishes of Australia, accessed 20 Apr 2024, https://fishesofaustralia.net.au/home/species/732

Southern Bluefin Tuna, Thunnus maccoyii (Castelnau 1872)

More Info

Distribution	Recorded from every Australian state, but absent from the coasts of the Northern Territory and northern Queensland, and very rare in central and western Bass Strait along the south coast. Elsewhere the species is circumglobal in temperate and cold temperate waters of the southern hemisphere, ranging across the Pacific, Indian, Southern and south-eastern Atlantic oceans, mostly between 30°S and 50°S, to almost 60°S (rare in the Eastern Pacific). Southern Bluefin Tuna breed between October and March in an area off Java, Indonesia and migrate down the Western Australian coast during their first year. Some fish then head west into the Indian Ocean, while others head eastwards into the Great Australian Bight.
Features	Dorsal-fin XII-XIV + 13-16 + 8-10 finlets; Anal-fin 12-15 + 7-10 finlets; pectoral fin 33-36; caudal-fin 17; Pelvic fin I, 5; Lateral line about 270; Gill rakers 26-34. Body fusiform, of moderate depth (25- 28% FL), greatest depth near middle of first dorsal fin, robust, tapering abruptly to very slender caudal peduncle; each side of caudal peduncle with prominent lateral keel and smaller keels above and below it. Head of moderate size (approx. 29% FL); eyes moderately small (13-18% HL); mouth moderately small, terminal, angled obliquely, gape extending to below middle of eyes; teeth small, conical, single row in each jaw, smaller teeth on palatines and vomer; preopercles rounded. Two dorsal fins situated close together, first dorsal highest anteriorly, length of base slightly less than head length, posterior edge of fin concave; second dorsal triangular, of moderate height (8-15% FL), base shorter than that of first dorsal; anal fin about same size and shape as second dorsal-fin, located below and slightly behind it; caudal fin lunate. Pectoral fins rather short (not more than 80% HL in specimens 65-145 cm). Pelvic fins thoracic, originating under pectoral fins. Scales cycloid, small over main part of body, slightly larger in pectoral regions; lateral line moderately arched.
Size	The species can reach a length of more than 2 metres, and weigh more than 200 kg.
Colour	Dark blue above, silvery below; caudal keels yellow, though colour may be missing in larger adults; second dorsal fin, caudal fin and finlets also tinged with yellow.
Feeding	Juveniles feed on planktonic crustaceans. Adults are opportunistic predators feeding mainly on pelagic fishes, but also on crustaceans and squid.
Biology	SBT live up to 40 years, and migrate long distances from their southern feeding areas to their only spawning ground near Java, Indonesia each year. Breeding occurs from September to April each year, and mature females may spawn many times, releasing millions of eggs, before returning to colder waters. The eggs hatch after a few days and the larvae metamorphose after about 20 days. The eggs and larvae move south from the spawning grounds off Java, and juveniles are often seen south of Perth, Western Australia, during their first year. By two to three years of age, juveniles are seen in surface waters off southern Australia, and in the Tasman Sea.
Fisheries	Southern Bluefin Tuna are an extremely valuable and highly prized commercial species. The flesh is medium-flavoured and most fish are sold in the Japanese Sashimi market. The worldwide fishery is estimated to be worth more than 1 billion dollars. Historically the species was heavily fished, with catches reaching 80,000 tonnes per year during the 1960’s. By the 1980's the catch had halved resulting in the imposition of voluntary quotas by the main fishing nations (Australia, Japan and New Zealand). Despite this, Southern Bluefin Tuna stocks continued to decline and the Commission for the Conservation of Southern Bluefin Tuna was established in 1994 to manage the global fishery. Except in Australia, most Southern Bluefin Tuna are caught on longlines. The majority of Australia’s SBT quota is farmed in Spencer Gulf near Port Lincoln, South Australia where fish are fattened up over several months before being harvested at 30-40 kg. From September to March, schools of mostly immature fish (aged 2-4 years) are enclosed in purse seines in the Great Australian Bight. The schools are then slowly towed to Port Lincoln in South Australia and transferred to floating sea cages anchored to the sea floor. SBT are also targeted by game fishers off southwestern Victoria and Tasmania.
Conservation	EPBC Act 1999 : Conservation Dependent IUCN Red List : Endangered NSW Fisheries Management Act 1994 : Endangered Victorian Flora and Fauna Guarantee Act 1988 : Threatened SBT are highly vulnerable to overexploitation, especially as genetics have shown there is a single population worldwide, and they all spawn in one place in Indonesia between Australia and Java. The species has been under considerable fishing pressure, with a recent paper suggesting that the fish stocks have essentially crashed. The current adult biomass is considered to be about 5% of the estimated original biomass (Collette et al 2011).
Remarks	Although Southern Bluefin Tuna spend part of their life cycle in cool waters, they are ‘warm-blooded’, and their core body temperature may be 4 degrees higher than the temperature of the surrounding water.
Similar Species	Similar in appearance to Longtail tuna (Thunnus tonggol), differing in having shorter pectoral fins and darker caudal keels.
Etymology	The species is named for Professor Frederic McCoy, the first director of the National Museum of Victoria, Melbourne, Australia.
Species Citation	Thynnus maccoyii Castelnau 1872, Proc. Zool. Acclim. Soc. Vic. 1: 104, Melbourne market, Victoria, Australia.
Author	Bray, D.J. & Gomon, M.F. 2024
Resources	Atlas of Living Australia Australian Faunal Directory Catalog of Fishes

Southern Bluefin Tuna, Thunnus maccoyii (Castelnau 1872)

References

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