SOUTHERN BLUEFIN TUNA ranching

Flying into Port Lincoln across the Spencer Gulf, round fish pens can be seen from the air, flowing in threads from the harbour to open water. The Stehr Group operates over a dozen pens during the winter season and more in the summer. Each tuna pen spans 40 or 50 metres in diameter and can be as deep as 15 metres. Insuring the fish in these pens alone costs around A$250,000 annually, which is perhaps an indication of the value of the tuna fishing industry in Australia.

Rearing tuna is a far more complicated process than raising any other fish from a hatchery. Because tuna are very sensitive to their environment, and spawn only in deep water, the conditions under which they might spawn in captivity have not yet been perfected in Australia. Hagen Stehr actively promotes research in this area through spawning programs, with assistance from the government.

Spotter planes fly over the expanse of the Bight between December and March on the lookout for the discolouration in the water, which marks the presence of a large school of young tuna. It is while these tuna are migrating that they are captured, using a method called ‘purse seining’. The planes direct boats, loaded with hauls of live pilchards in their holds, towards the tuna. The pilchard bait is released and a large net encircles the school and traps as many fish as possible – in the thousands.

Beneath the water, the tuna are transferred to a pen which is then towed at a speed of one knot all the way back to the outer reaches of Port Lincoln, where the tuna are positioned to grow in water rich in oxygen thanks to strong currents circulating. Here the fish are fed Australian pilchards and imported fatty fish until they’ve grown sufficiently to sell on the open market. The process of fattening tuna for market takes about four months.

Preparing the tuna for market is a unique practice. Early in the morning, a diving team of four enters the pen and encircles the fish with a net that lies below them. As the sun rises, the net is tightened and raised toward the surface, making the area of water shallow enough for divers to chase each fish individually. They slide a hand into its gills (which appears to pacify the giant fish) and lead it to the edge of the boat.

The Japanese method of killing tuna is used in Australia. It is known as iki jime. A corer is pushed into the brain of the fish and a slim wire rod is slid in and along the fish’s spinal column, disconnecting the nerves from the spinal cord and preventing rigor mortis, which can adversely affect the flavour of the fish. The gills are removed, the fish is weighed and within a couple of minutes, the tuna is bathed in salted ice for its journey to the factory. This keeps the flesh fresh without freezing it, so it will taste its best hours or days later.

There is at least one tuna expert on board the fishing vessels at all time, judging the quality of the fish for prospective buyers. This person is in constant contact with the markets, informing them on the quality of the fish coming out of the southern seas.

Being a member of a tuna dive team is not without its dangers; great white sharks, breeding at the nearby and aptly named Dangerous Reef, are frequent visitors to the pens, to try and feed on the fattening tuna. They have been known to tear through the netting and eat the trapped fish, before becoming trapped in the net themselves. Divers are faced with the unenviable reality of not knowing what else is in the pen with them when they enter the cold water before sunrise. The work is physically taxing; there have even been cases of tuna having broken human arms with a flick of their tail.