The piece of tuna on your plate was not simply caught off a boat, filleted and presented. The workings of the tuna industry has become an art form, intricately orchestrated and powerfully driven. It relies on an awesome demonstration of teamwork, with each step relying on the successful completion of the one prior. Years of experience, research and attention to detail feeds into the practice, yielding world class, premium quality product.
Typically it all starts with the spotter plane. Flying above the Great Australian Bight, a pilot searches for schools of tuna. Using an expert eye they estimate the size and general age of each patch, attempting to identify which will be the most promising. The size of the fish has to be consistent with market requirements. The information is relayed to the skippers of the tuna boats which race to the patch of fish, hoping to catch them before another company.
Tuna are caught by purse seine, which in simple terms involves surrounding a school of fish with a net. However, this process is far from simple. One boat, called the Chum boat, releases masses of baitfish (typically pilchards) into the surrounding ocean. Tuna are predatory fish and swarm the boat, feeding on the bait. From the deck, the crew visually double-check the size estimates of the school. If the sizes are agreeable, the next step can begin.
A second boat, known as the seiner, releases the purse seine net into the ocean and circles the Chum boat. The net hangs vertically in the water with its bottom edge held down by weights and is top edge buoyed by floats. Once a ring of net successfully surrounds the tuna and Chum boat, the Chum boat exits via a pre-exisinting gap.
Now a few more boats enter the mix. A number of tinnies attach a rope to the net, holding it in place. The seiner boat pulls in the net to make the circle smaller whilst the tinnies work at maintaining the structure. The aim is to encourage the school to bunch, the image of a hurricane of tuna is not too far from the truth. Next, a certified diver joins the school. The diver triple-checks the size estimates and reports to the skipper on the seiner.
If all is going according to plan, a third boat, known as a tow boat, is called upon. The role of the two boat is self-explantory, it carries a tow cage used to transport tuna. Skilled skippers and crew align the tow cage alongside the purse seine net such that the gates on each net match. The seiner boat pulls in their net slowly, encouraging fish to naturally transfer from the fishing net to the tow pontoon with low stress. A diver is stationed at the gate, counting the fish as they move through.
Once the tow cage has approximately 10-15,000 fish the gates are dropped. With the tow cage attached, the tow boat travels back to Port Lincoln at a maximum speed of one knot. Such a slow speed minimises the damage done to tuna within the cage. In the waters behind Boston Bay awaits static tuna cages into which the tuna must be transferred. From the tow cage to the static cage the tuna are moved, in a process similar to that described above.
The prized Southern Bluefin Tuna are fed a diet of baitfish over a period of 4 to 5 months, during which time the fish grows, maximising its colour, texture and flavour. The aim is to double the weight of the fish. Additionally, as winter approaches, the waters cool and tuna begin to build up fat. It is these fatty reserves that are celebrated in the world of sashimi.
Harvesting of the tuna is swift with unparralleled attention to detail, ensuring minimal stress is felt by the fish. Upon stress lactic acid is released into the tuna’s system, which influences the flavour of the fish. By reducing the stress felt by the fish, the quality and the integrity of this product is maintained to the highest standard.
First around 100 fish in the static cage are sectioned off using a net. Next, divers manually get hold of the fish and guid them to platforms which lead to the deck of the boat.
Within 10 seconds of being out of water the fish are euthanised humanely, by a spike into their brain. This method, known as “iki jime” in Japanese, has been perfected over time to minimise stress. After being euthanised, the tuna is gilled, gutted and wired. The wire is fed down the nervous system, disabling it and preventing any post-mortem twitching that would affect the tenderness of the fish. Tuna are immediately transferred into an ice slurry or refrigerated sea water in order to keep fish fresh. From start to finish, only 40 seconds has passed. The speed of the process maintains premium quality.
On board the tuna harvest boats are representatives of Japanese companies who scrutinise the product before export. They look for good shaped fish that have suffered no damage in the harvesting process. A segment is taken from the tail to determine quality. A strong, deep red colour and good fat deposits are considered extremely favourable. Quality and colour of SBT muscle flesh is assessed 3 times prior to the sale of a fish. Tuna may be purchased frozen or fresh.
Southern Bluefin Tuna will be in the markets in Japan within 48 hours of harvesting and processing.