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Southern Bluefin Tuna

Introduction


Thunnus maccoyii

The Southern Bluefin Tuna is one of the sea's most impressive creatures. A beautiful and powerful fish, it is well suited to a long life endlessly swimming the open seas.

The Tuna population is thought to be one family of fish with origins in warm waters south of Java in the tropical Indian Ocean, the only known spawning area for Tuna in the world.

The Southern Bluefin is one of 13 other species of tuna in the Scombridae family and is related to the billfishes, which include swordfish, marlins and spearfish. Its cousins are the butterfly mackerel.

An adult Bluefin grows to around 200 kg and over 200 cm long. Its close relative, the Northern Bluefin Tuna, Thunnus thynnus, can grow to a massive 700 kg.

Characteristics

They are true athletes of the ocean, one of the fastest ocean swimmers in the world, and often travel in speed bursts of up to 70 km/hr during their migrations over thousands of kilometres of ocean.

At these high speeds, the side fins retract into special grooves and the eyes form a smooth surface with the rest of the head in one of the most hydrodynamically advanced bodies in the sea.

They must keep moving so that water passes over their gills, carrying oxygen to the muscle system. The slowest a tuna can swim safely to maintain the oxygen flow is to move its own length every second, faster than the fastest human swimmer at top speed.

During migration, the tuna relies on the dark muscle blocks which run down either side of its body. These muscles are specifically adapted to long, continuous swimming and operate only when oxygen is available.

When energy is required quickly, the large red blocks of muscle that form the remainder of the fish are used. A large energy demand will quickly exceed the oxygen supply and the muscles produce acid. If fish are killed after high activity, or if they struggle, the acid remains. When combined with the enormous heat generated the flesh is 'burnt'.

Tuna recover from these exhaustive activity bursts around ten times faster than any other fish species, mainly because tuna hearts are several times bigger than the hearts of other fish and are more similar to a mammal's heart.

The secret may also be in the tunašs ability to rapidly 'refuel' from stores of glycogen, a biochemical which is metabolised into energy-giving lactate for hard working muscles.

Feeding

Bluefin gets its incredible physical stamina from a healthy diet of fish, squid, krill and salps. In offshore waters, they also eat small crustaceans and much larger fish.

A skilled ocean hunter, the tuna uses its highly developed senses, binocular or stereoscopic vision, extremely sensitive hearing and special chemical detectors, to hunt prey in areas where warm and cold waters meet where there is more food.

One of these is the meeting of the warm East Australia Current and cooler sub antarctic water from the south. Adults prefer these areas to the inshore feeding grounds of younger tuna.

Its highly advanced circulatory and respiratory systems work to keep the temperature of its warm-blooded body constant. The tuna can vary its heat dissipation depending on its activity, high activity requires heat dissipation and low activity requires heat conservation.

The system accounts for the size and weight of the fish as well as the ambient water temperature, keeping the fish's body warmer than the surrounding water.

This is why the tuna can tolerate the wide range of water temperatures during its long transoceanic journeys. It is also one reason why Bluefin move vertically through the water column many times a day.

Life Cycle

Southern Bluefin Tuna live for 20 years, but only reach sexual maturity after seven to nine years, when between September and March they return to the spawning ground in the tropical Indian Ocean to breed. It is not clear whether all mature Bluefin spawn each year, every few years or even only once in their lifetime, but a single female can release up to 15 million eggs during a spawning period.

The tuna begin life as an egg around 1 mm in diameter in the upper layer of water where wind and wave action keeps temperatures uniform. Once hatched, the tiny Tuna are around 2.5 mm and barely distinguishable from other types of tuna.

As they grow to 12 mm they develop small and often faint pigment patterns on the fins and tail, called melanophores, which help tell the species apart. Beyond 12 mm, secondary pigment patterns develop which again confuse the identity of the species until much later.

As they grow, the young fish move southwards towards major feeding grounds in the colder Southern Ocean. The warm Leeuwin Current which begins near the spawning ground and is strongest in April, right after the spawning period, helps sweep the tuna down from the West Australian coast to the Great Australian Bight and beyond.

Young tuna prefer shallow water closer to the shore and also tend to surface more, perhaps to gather certain types of food, to help with body temperature regulation or in response to light and wind conditions. The Great Australian Bight is the main Australian region where young tuna surface, mostly in the period from November to April.

Survival

Few fish from each female survive the perils of fishing and other hungry tuna, sharks, fish, birds, even killer whales, and return to the breeding grounds of the Indian Ocean to complete the life cycle.

Without fishing pressure, this survival rate would normally ensure a stable population overall, but fishing in the last 20 years has taken many fish before they reach breeding age, which, together with direct fishing of the adult population, has led to a continual decline in the parent stock.

Other species of tuna, such as skipjack or yellow fin, which are shorter lived and faster growing can recover from fishing pressure more quickly than the Bluefin. The Southern Bluefin Tuna's naturally slow growth, long life span and its single spawning ground leaves the population particularly vulnerable.

Commercial Status

Southern Bluefin is considered the ultimate delicatessen of the tuna family in Japan. For Bluefin sashimi (raw tuna fillets) the Japanese are willing to pay extremely high prices, due its size, color, high fat content , texture and taste.

The high price is caused by the fact that this tuna specie is very hard to get. Very few markets can compete with the prices that Japanese buyers are willing pay.

Future Supply

The Southern Bluefin tuna is the most overexploited tuna species. The stocks are heavily depleted. Japan, Australia and New Zealand have imposed restrictions on the catching of this tuna species. This situation has triggered actions by several environmental groups such as WWF and GreenPeace.

Many initiatives around the world have been taken to ranch wild caught bluefin tuna in captivity, in Croatia, Spain, Morocco, Australia and Japan. This industry will continue to grow, but due to the slow growth of the big-eye and the high costs involved, it cannot fulfill the demand for Bluefin in any way.

Legal Minimum Length

No minimum legal length currently set.

Bag Limit

Current Bag Limit: 2 per person  
Current Boat Limit: 6 per boat  

Type of Tackle

A Silstar 1280 10JIG rod fitted with an AX2660 sidecast reel. This rod gives the whip and strength to hold the biggest fish, and reel is geared just right for the big fish. The line should be no less than 10kg breaking strain.

Hook Selection

The Yo-Zuri Hydro Magnum Series (140mm/52g) is an ideal, strong, well constructed lure, able to withstand the pounding attack of a Tuna.

The Hydro Magnum can be trolled between 2 and 20 knots. It features heavy-duty through wire construction, super strong hooks and tough, durable finish. Performs best at speeds 6-12 knots. The 140mm lure costs around AUS $20.

My personal colour preference is Blue Mackerel, as I have had a lot of success with this colour.

Baits Used

Slimy mackerel (blue), Strip Baits, Lures, Skirted Lures, Slices. Tuna are normally taken while trolling at a fairly fast troll speed (6 to 8 knots).

Live bait always offers the best results. There are two options for live bait. One, is to get it close in shore at the bait grounds, usually found at the first shallow reefs outside of most ports. The second and my preferred option is to get your bait at the particular spot you intend to fish.

It makes sense, that if you are going to fish a special spot for major predators, there should be a ready supply of live bait, close at hand. If not, then why would the predators bother stopping there ? What comes up on a bait jig may not be the bait you think will work, but is usually what your quarry is feeding on.

To catch live bait, one of the simplest methods is the use of a live bait "jig", a pre-formed rig available from tackle outlets - cheap and very effective. Anchoring over bait grounds and berrying is the old and proven method. Use a couple of hand lines, with and without lead, so as to find the depth at which the bait is feeding.

Slimy mackerel would have to be my first preference. Yellowtail, pike, Taylor, Nannygai and small Trevally are all good live baits.

Berley Mix

For bait fish, berley is a must and a fine surface berley is better than a thick slick. Chook pellets, tuna oil and bread, and diced or cubed fish flesh in a hessian bag or pot does wonders. Tuna are extremely partial to Mackerel. Pilchards work well, as well as Garfish.

This berley works well for catching both your bait and the big fish. For the bait fish, I add a little bran. It spreads quickly, attracts more pelagic bait and less of the unwanted varieties.

Rigs Used

Rig No. 1
This rig is best suited off a boat trolling at around 6 to 8 knots. Another option is to lay down a berley trail while drifting, and continuously casting into the trail. Trolling however is the preferred method. When the tuna are blasting bait fish on the surface, casting lures at them provides one of the real fun ways to hook onto them. Anything which will run straight at high-crank speeds is the answer when tossing lures at feeding tuna.

For casting, the best casts are those which go right across the school and land in the water away from the feeding zone. The lure is then whisked back through the school of feeding fish as fast as the reel and angler can wind. Strikes are often visible as competing fish crash the lure as it is across the surface.

Rig No. 2
Rig 2 is perfect for fishing with live bait such as mackerel. When fishing any spot, you can always put out a live bait rig with a float, suspending the bait about two or three metres below the surface.

Virtually any small fish can be used as live bait, including things you drag up while fishing the bottom. The best baits though are slimy mackerel and garfish.

Live bait traces can be made up before you go fishing and consist of a black swivel at one end and a 6/0 to 8/0 hook at the other, connected by two metres of 20 to 30kg breaking strain nylon trace, Bimini Twist.

Do not use wire as the fish will often avoid it and you will only get strikes from mackerel and sharks.

The bait is usually hooked through the shoulder area being careful not to set the hook too deeply in the flesh. Ensure the hook passes above the lateral line along the fishes body. The lateral line is the fish's central nervous system and putting a hook through it will quickly kill the bait.

Set the baits about 50 metres from the boat and you can get on with whatever fishing you normally do.


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