Southern bluefin tuna are found in southern Hemisphere waters, mainly between 30°S and 50°S. Their only known breeding area is in the Indian Ocean, south-east of Java, Indonesia. Mature females (over the age of 8) can produce several million eggs in a single spawning season. Breeding takes place from September to April. Juveniles (aged between 1 – 4 years) then migrate south down the west coast of Australia and spend the winter in deeper, cooler oceanic waters.
SBT can be found pretty much anywhere oceanic, where their favourite schooling prey items congregate – generally redbait, mackerel, or squid. This is usually inshore over deep reef structures from as shallow as 10m to 150m deep.
SBT can also be caught over the continental shelf in depths from 150 to 600m. SBT usually use the shelf as more of a highway, and travel with the currents and associated bait abundances. As a result, they may not hold in particular areas for long periods of time.
In Tasmania, SBT are now almost a year round proposition, however traditionally the season is from March to June. Once again traditionally they were mainly targeted off the east and south coasts of Tasmania, both inside the continental shelf inshore as well as over. But these days SBT are also found on the west and north coasts of Tasmania. Big jumbo or barrel sized SBT are generally caught by trolling lures around offshore islands or reef structures. For some reason the larger fish are not often caught over the continental shelf by trolling, but instead are caught regularly as bycatch by anglers chasing swordfish dropping down baits to the seas floor. The SBT hot spots in Tasmania are the ports of St Helens and Eaglehawk Neck on the east coast, and the offshore islands Pedra Branca and the Maatsuyker Island group off the south coast.
The SBT fishery in Victoria and South Australia has also seen similar changes to their tuna season, where they can now be caught in almost every month of the year.
The peak period for bluefin in Victoria is generally from February to July, however this can extend through until October and November.
The tuna season in South Australia generally kicks off early in December and can also extend through until October/November. In South Australia, SBT can be caught all along the ocean coastline, from Ceduna in the west all the way through to the Victorian border in the east. The main ports where SBT fishing is undertaken are Port MacDonnell, Victor Harbour, and the Eyre Peninsula.
The SBT fishery around the Victorian border is very closely linked, where gamefishers will regularly travel from Victoria to fish in South Australia if the bite is on, and at times boats will launch from Victoria but travel across the border by water to reach the fish. Traditionally in Victoria, the fishery has mainly been focused on the western coastline, with the main ports and access points being Portland, Apollo Bay, and Port Fairy. However, in the last few years (as seen in Tasmania), big numbers of bluefin have continued to push further east and are now caught regularly around the entrances of Port Phillip Bay and Western Port Bay, in Bass Strait.