EVERY November, young southern bluefin tuna, aged 1-4 years, embark on an epic swim from their winter foraging grounds in the Indian Ocean and the Tasman Sea to the Great Australian Bight where they spend the summer months feeding, mostly on the plentiful sardines.
For the commercial fishing industry based in South Australia’s Port Lincoln, it’s a short season, extending from late December to February, but potentially lucrative - most of the catch is exported to Japan for high value sashimi.
The industry is concerned that the noise from seismic surveys used in oil and gas exploration could affect the tuna’s arrival and departure times, affecting their fishing operations.
Now, a study using data from electronic fish tags has laid the foundation on which scientists can monitor changes in the tuna’s movement and behaviour, and understand more about how they are influenced by human activities and the environment.
A swimming machine
The study was part of the 4-year, $20 million Great Australian Bight Research Program - a collaboration between BP, CSIRO, the South Australian Research and Development Institute (SARDI), the University of Adelaide and Flinders University. The program has given rise to a quantum leap in knowledge about the Bight and the discovery of many species new to science.
CSIRO marine scientist Dr Campbell Davies led the southern bluefin tuna study.
“In terms of the scale of its migration, the southern bluefin tuna is an amazing fish,” he says. “They live to over 30 and, by age two, when they are 60 to 70 cm in length, many of them are swimming from the Bight to South Africa in a matter of weeks, swimming on average around 100 km a day. They are a highly tuned swimming machine.”
Economically important to the region
The southern bluefin tuna has only one population, and one breeding area, in the north-eastern Indian Ocean, near Java.
From the 1970s through to the mid-2000s, the species was overfished and the stock was severely depleted. In the past decade, the decline has been arrested by improved catch monitoring and management action at the national and international level, Davies says.
Listed as ‘conservation dependent’ under Australian legislation, the species is required to have a formal rebuilding plan which is being achieved through an internationally agreed management procedure to set the global catch quota. “We are now seeing some initial signs of rebuilding of the spawning stock as a result of catch reductions in the mid-late 2000’s and the implementation of this management procedure,” says Davies.
Southern bluefin tuna is one of Australia’s most valuable fisheries. With most of the quota holders based in Port Lincoln, it’s an important component of the regional economy - about 95 per cent of the catch is taken from the Bight, with almost all of this being exported to Japan.
Juvenile fish are captured in purse seine nets on the edge of the shelf, transferred to huge tow cages, and slowly towed back to Port Lincoln where, in a process known as tuna ranching, they are fattened up for a few months before being exported. “The fattening in cool water makes for good sashimi,” says Davies.
For recreational fishers, it’s an iconic species. A strong fighting fish, adults can be 2 metres long and weigh up to 200 kg.
With the world’s biggest swell, the Bight is a harsh environment in which to do science. This, combined with the tuna’s ocean-scale migrations, makes observation a challenge, so tagging is the most efficient way to collect data.
Long-term data sets are required so that scientists can distinguish changes caused by the environment from changes caused by human activities, explains Davies:
“In the 1990s, for example, there was a shift in juveniles’ preference for going from the Bight to the Tasman Sea for winter foraging, relative to wintering in the Indian Ocean. Through the early 2000s, most of them went to the Indian Ocean. Since 2006, we have seen an increase in preference back to the Tasman, as in the 1990s. We can’t categorically say why, but the Tasman Sea has been warming and may even have got too warm for comfort during the early 2000s. The point is, with decadal environmental cycles and long-lived fish you need long-term datasets to interpret what might be driving the change.”
Fortunately, CSIRO has a long history of tagging tuna and an extensive set of data is already available. In the 1990s and 2000s, more than 500 juveniles were tagged and at least 150 of these tags have been returned by fishers. As part of this latest study, the team tagged an additional 125 juvenile tuna in the Bight.
At regular time intervals, the tag records the fish’s depth and body temperature, the ambient water temperature and the level of light. When the fish is caught and the tag is returned to CSIRO, this information is used to calculate the fish‘s location (longitude and latitude) at each recording point, using an approach known as geolocation.
Tracking every move
Modelling the data collected by the tags reveals the tuna’s preferred habitats, their arrival and departure times to/from the Bight, and how these vary across individuals and years. It also shows that, while most juveniles leave the Bight in the autumn and winter months for the Indian Ocean or the Tasman Sea, some remain in the vicinity of the Bight all year round.
As the summer season progresses, the juvenile fish move east from the shelf waters in the central Bight into shelf break and slope areas offshore of the Bonney coastline.
“Juvenile southern bluefin tuna form large surface schools when they are in the Bight, feeding on bait and basking, a behaviour that the purse seine fisheries take advantage of—they use spotter planes to find schools,” says Davies.
The study found that as the warmer surface water extends deeper over summer, so too do the tuna, spending less time overall at the surface.
Eating habits exposed
Compared with areas outside the Bight, the juveniles were found to feed more frequently in the Bight, but on smaller portions.
“Bluefins, unlike many other fish, are endothermic, which means their body temperature is warmer than the ambient water,” explains Davies. “During digestion, their body temperature will rise before returning back to normal. The pattern in body temperature change can tell us how often they’re feeding. From this, combined with the size of the temperature change, we can also determine, relatively, the amount eaten.”
During their time in the Bight, the juveniles put on about 50 per cent of their year’s growth, adds Davies. “It’s likely due to the abundance and quality of sardines and the warmer water, which allows them to digest their food faster and utilise the energy for growth. We think this is a major reason why they come back to the Bight each year. You can often see them on the surface, cruising along on their sides, sunning themselves. We hypothesise that they are full and are warming up, to digest faster, before going back for more.”
A noise profile
As a first step in examining the potential impacts of oil and gas exploration on juvenile tuna in the Bight, the team compiled a profile of seismic surveys in the Bight dating back to the 1960s, and combined it with the tag data to identify where and when seismic activity and the presence of tagged tuna coincided.
While there were clear spatial overlaps, they varied between years and, with the information currently available, it was not possible to determine cause-and-effect relationships.
“The challenge is to collect enough data on the distribution of tuna at the same time as seismic surveys are being conducted,” says Davies. “We also need to measure the noise generated by each survey to determine the noise footprint, and to do this over enough years to develop a representative picture of how the tuna behave when there is, and isn't, exploration activity occurring.”
With the tools to do that comparison now developed, it’s all within the realms of possibility. “We just need the ongoing support to extend this rare long-term monitoring data of these impressive ocean migrants into the future.”