By Chris Gerbing - this article was first published in CSIROScope
A record-breaking tuna first tagged in the Great Australian Bight as a three-year old in 1994 has been caught off the southern NSW coast! Some 26 years later, this giant Southern Bluefin tuna (SBT) is giving scientists a rare glimpse into the life of these ocean ultra-marathon runners.
The marine ecosystem continually surprises us. We’ve previously shared the story of finding ‘Bluey’ after 22 years. We love making discoveries that propel our research towards sustainable ocean futures. And sometimes a discovery leaves us completely gobsmacked!
Our offishal record
Recreational fishers off the south coast of New South Wales have recaptured a tagged SBT. Our scientists first tagged this fish in the Great Australian Bight back in 1994. The SBT weighed in at 148 kilograms, measured an impressive 185 centimetres, and clocked up more than 29 years in age.
This fish is a CSIRO record! The oldest SBT ever recaptured after tagging and longest ‘days at liberty’ for a tagged fish. Not just a boon for the lucky anglers – who, by the way, had the battle of a lifetime landing it – but also for science and future management of this valuable fishery.
Angler Jason Garling (centre) captured this 29-year-old Southern Bluefin tuna using 24 kg stand up gear. They were aboard Charter Fish Narooma with Benn Boulton who reported the tag recapture to us. Source: Facebook, Charter Fish Narooma.
Tagged at first catch
We first met this tuna fish as a young three-year-old. It was during a research trip in the Great Australian Bight. Fish tagger Wade Whitelaw tagged the 97 cm fish with a conventional ‘spaghetti tag while aboard the vessel FINA K. This was part of a large-scale CSIRO tagging program, which has improved international stock assessments.
We recorded the tag number and information about the fish and registered it in our tag database. The database now holds records of more than 160,000 individual SBT our researchers have tagged and released over the years. We do this to understand their movement and population biology. This includes understanding their rate of mortality due to natural causes, and due to fishing, at different sizes and ages.
Many of these fish were tagged in the Great Australian Bight, where 95 per cent of Australia’s commercial SBT catch occurs. The Bight is an important summer feeding ground for juvenile SBT (aged one to five years). They return there on a regular basis.
TAG Database stats for this SBT recapture RECORD oldest SBT for a CSIRO tag recapture: 29.39 years RECORD days at liberty for a CSIRO tag: 9639 days Possible RECORD or at least second all-time CSIRO SBT recapture by weight: 148 kg (heaviest reported was a 155 kg 20-year old fish, which has some associated uncertainty) Top 20 all-time CSIRO SBT recaptures by length: 185 cm (longest was 195 cm but most records >185 cm are deemed ‘uncertain’ or inaccurate)
Tuna in the Tasman Sea
Research using satellite and archival tags allows us to track the movement of individual SBT. We’ve discovered as they mature, they inhabit more offshore waters and can extend their winter migrations into colder, more productive areas of the Southern Ocean.
As recreational and commercial fishers appreciate, the Tasman Sea is a regular autumn and winter feeding ‘stop-over’ on their annual migration. Afterwards they make the long sprint around the southern extreme of the Australian continent to the spawning grounds between Broome and Indonesia. Tagging has shown some young adult SBT do spend a full year in the Tasman, without taking this journey.
It’s not possible for us to tell how far this individual mature aged tuna has travelled. Or how long it spent on the east coast. But we can be confident this fish has made the journey between the spawning grounds in the north-east Indian Ocean and the Tasman Sea many times before. By retrieving the tag, we can add to our body of knowledge about this majestic ocean ultra-marathon runner.
How does this fish measure up?
We’ve already been able to compare the basic stats of this fish with known information about the species. According to past tuna research (PDF, 1.7MB), a fish this age would measure about 183 to 185 centimetres. This big fish is exactly the length range we’d normally expect to find.
By using an SBT length-weight relationship that we developed, we’ve found this individual is also right in the expected weight range. From our understanding of the reproductive biology of the species, it is likely that it is getting towards the end of its useful reproductive life. So, fisher Jason seems to have recaptured one fine jumbo SBT specimen!
Genetic tags – the future of counting fish
Returned conventional tags, like the one found in this fish, complement recent advances in technology like electronic and genetic tagging. These technologies combined give us a more holistic picture of the movement, migration and behaviour of Southern Bluefin tuna. Long ‘time-at-liberty’ returns, such as this one, improve our understanding of the longevity of the species. It gives us confidence that the age estimates we derive from otoliths (ear bones) are in the ballpark. While 29 years is a respectable age for an SBT, age estimates from otoliths indicate they regularly reach 35 years and can exceed 40 years of age.
We’ve developed genetic tagging technologies to improve fish tagging. Conventional tags can be dislodged from the fish. Or when a tagged fish is recaptured, we might not be contacted with the information to plug into our database. Genetic tagging of fish is like creating a life-long tag, or fingerprint, so the information can never be lost at sea!
We’ve used this genetic tagging approach to estimate the abundance of juvenile SBT since 2016. We feed our data into the science-based management approaches for setting the global catch levels. Our tagging work helps to manage the catches of SBT. New genetic-powered methods help to estimate the absolute abundance of the adult population – known as close-kin mark recapture. This robust scientific approach provides confidence that SBT can be fished sustainably into the future.
Found a fish with a tag? Email us at tags@csiro.au
