Image of southern bluefin tuna in a cage

Bluewater Magazine Issue 124

Southern bluefin tuna (SBT) are an important commercial and recreational species within Australia. The species is considered a single stock and is found throughout the southern hemisphere mainly in waters between 30 and 50 degrees South but only rarely in the eastern Pacific.

Although occasional catches of SBT were recorded off New South Wales (NSW) before the Second World War, the fishery did not become commercial until the mid-1950s when pole and live bait techniques were introduced from the United States. The fishery continued to grow throughout the 1960s with catch in NSW and South Australia approaching 6 700 tonnes annually. Pole and live bait techniques were the main catching methods.

Major changes took place in the fishery in the 1970s. Fishers began searching further offshore because fewer fish were found in the traditional fishing area. The move offshore was accelerated by the introduction of larger pole boats, large purse-seining vessels and the use of long range spotting aircraft. Increased offshore fishing and the move to purse seining led to an increase in the number of large fish caught.

Prior to 1970 the only country other than Australia fishing for SBT was Japan. Japanese longline effort expanded throughout the 1950s and 60s with their SBT catch peaking at 77,491 tonnes in 1961. By 1970 the Japanese catch had declined to 40,000 tonnes however other countries such as Taiwan and Korea had begun to target SBT.

By the early 1980s the Australian fishery had expanded into Western Australia and increased effort saw a total catch of 21,000 tonnes in the 1982/83 season. This was the highest ever catch of SBT in the Australian fishery. While the fishery off WA was expanding by 1983 the catch in the NSW fishery had declined to less than half of the average of the preceding five years.

In the mid-1980s it became apparent that the SBT stock was at a level where management and conservation was urgently required. The main nations fishing SBT at the time, Australia, Japan and New Zealand, began to apply quotas to their fishing fleets from 1985.

Until the late 1980s the Australian SBT Fisheries had been directed towards supplying the high volume, low value domestic canning market. With reduced quotas, Australian fishers began to target larger fish primarily for the Japanese sashimi market.

In 1989 Japan entered into a joint venture arrangement with Australia whereby Japanese longliners were able to fish within the Australian Fishing Zone. This arrangement saw almost 50 per cent of the Australian quota taken by Japanese vessels. This trend continued until the arrangement was ceased in late 1995.

Development of ranching methods

In the early 90s, in an effort to increase the value of the catch, a project was undertaken to capture live fish and fatten them in sea pens. At the time tuna ranching was in its infancy. Japan was the only country undertaking it on any significant scale. There, Pacific Bluefin were targeted at 200 grams and grown out to 6-8kg before being harvested.  High mortality rates around 70 per cent meant low returns on investment.

In 1990/91, 20 tonne of pole caught SBT were kept alive in specially modified tanks and then transferred live into grow out pens off Port Lincoln. Pioneering Australian operators realised that there were limitations on the number and size of fish that could be transferred alive. To overcome this issue they developed a technique were purse seined fish were transferred live to a tow cage. The tow cage was then towed back to ranch sites before being transferred to static grow out pens. This was the genesis of modern tuna ranching. Similar techniques are now been used to ranch tuna in a number of countries including Mexico, Croatia and Italy. 

The modern process

Surface schooling or near surface schools of southern bluefin tuna are located using spotter planes, natural cues like congregating seabirds or sonar. Fish in the 12-25kg range are preferred. When a school of fish is located, chum boats keep the school near the surface while a tender from the purse seine vessel runs the net around the school. Once the school is encircled the lead line that runs along the bottom of the net is pulled closed to trap the fish – this technique is called pursing because it is similar to pulling the draw string of an old-fashioned purse.

The encircled fish are then held in the net while the tow cage is brought along side. Once the two nets are secured the net is cut and the fish are transferred.

Tow cages are generally 50m in diameter and are designed to hold tuna securely even in the very rough conditions that are often encountered in the Southern Ocean. In the early years tow cages contained relatively small amounts of fish. Modern cages average 9000 fish and can contain up to 15,000. A tow takes around 8-10 days to get from the fishing grounds to the ranching zone. Average speeds are very slow, around one knot. In areas where strong tides occur the tow vessel may even go backwards for periods of time.

In recent years the majority of the catch has come from areas adjacent to Kangaroo Island, however traditionally operators caught fish in the Great Australian Bight south of Ceduna. The shift has been linked to better quality fish being available closer to the farm sites in Port Lincoln.

The advent of this method and its refinement over the intervening years has seen the ranching sector become the dominant user of SBT quota in the fishery. In the 2015/16 season ranching accounted for approximately 4900 tonnes of Australia’s 5665 tonne national quota allocation.

Monitoring in the ranching sector

As ranching expanded in the early 90s it became apparent that a method to count live fish against quota would be required. The process that is currently in use differs little from the process used in 1995.  It involves AFMA taking a weight sample of 100 fish from the tow pontoon as it nears Port Lincoln. Fish are caught on a hook and line and taken out of the water and weighed on a scale, fish are then returned alive to the pontoon. The average weight of the 100 fish sample is used to estimate the average weight of all of the fish in the tow pontoon.

Once the tow pontoon reaches the ranching zone, fish are transferred out of the tow pontoons into between 2-4 ranch pontoons. Each ranch pontoon holds around 3,000 fish. Fish are transferred through an underwater gate and the transfer is videotaped. Quota is deducted based on a count of the video of the transfer multiplied by the average weight of the fish in the cage. AFMA observes and verifies the count and weight estimate for all transfers from tow pontoons into the ranch.

Longlining for SBT

In recent years there has been an increase in the proportion of the SBT quota taken by longlining off southern NSW and eastern Tasmania. Longlining is a method where baited hooks are attached to a mainline by short lines called snoods.  Snoods hang off the mainline at predetermined distances. The longline can be many kilometres long and can carry thousands of hooks. Pelagic longlines used for catching tuna are not anchored and are set to drift near the surface of the ocean. Radio beacons are attached so that the vessel can track them to haul in the catch.

The vast majority of the SBT taken in this sector is caught off the south east coast of Australia during the period from May to October. In the 2010/11 season the sector accounted for 2 per cent  (85t) of the fishery. In 2015/16 this had risen to 12 per cent  (700t). The increase is primarily due to more quota being available to the sector as the stock recovers and quotas rise.

It is interesting to note that anecdotal reports suggests that the recreational catch of SBT off NSW for the same season (2015/16 May -October) was quite low. Analysing the tracks of commercial vessels indicate the fish were further offshore than in previous years (70-100nm) and the weather was particularly rough during the latter half of the year. Both issues may have contributed to the poor catches reported.   

Future management

The Commission for the Conservation of Southern Bluefin Tuna (the international body that manages SBT globally) will be meeting in Indonesia in October 2017. The Commission will assess the results of the stock assessment that is carried out every 3 years to ascertain the current status of the stock. The stock is one of the few Bluefin tuna species that is showing positive signs of recovery. It is hoped that the this recovery can be sustained so both recreational and commercial can benefit from the resource well into the future.

View the article printed in issue 124 of Bluewater Magazine.