Sydney fishers aboard Charter Fish Narooma were in for the long haul after a bluefin hookup late June, knowing it was no ordinary fish.

Charter Fish owner Benn Boulton said everyone knew it was something special.

It was a team effort for anglers. At one stage three were on the rod.

More than four hours later, the tagged fish was landed and there was "a lot of high five-ing, yelling and screaming".

Once bluefin reach close to 100kg, Boulton says "they're definitely a trophy fish", and this one was well over at 148kg.

The fish died during the fight after line wrapped around its gills. It was unable to be revived for release, but its meat was shared among those aboard. Boulton said bluefin was a delicacy smoked or best eaten as sashimi.

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"Everyone loves to eat bluefin - there's never any waste," he said.

"These were our regular customers who we take each year; we recommend tag and release but individuals on our charters are entitled to a fish."

Boulton sent the fish's tag to CSIRO and was shocked to learn of their discovery.

"We were shell-shocked - we estimated it would have been at least five-years old," he said.

In fact, the fish was more than 29-years old - the oldest tagged fish in CSIRO history.

In 1994, CSIRO scientists first tagged the fish in the Great Australian Bight. It was 97cm and was believed to be three-years old.

Senior principal research scientist Campbell Davies said the tuna could have travelled distances halfway across the globe.

"Since then it would have been travelling vast distances across multiple oceans between its annual spawning grounds in the north-east Indian Ocean and the Tasman Sea, in the east, and as far as the Atlantic Ocean in the west," he said.

After 26-years, the fish was recaptured at 148kg and 185cm.

Davies said fish were tagged to help understand their biology, movement, migration and behaviour. He said tagging technology was evolving.

"In the past, we used conventional tags like the one found on this fish, but now we use satellite tags to track movement and genetic tags to help determine the total number of fish of different ages," Davies said.

"The data we collect is fed into our database on southern bluefin tuna and international stock assessments which informs their future sustainable management."

Over many decades, Davies said CSIRO developed a database of over 160,000 conventionally tagged individual southern bluefin tuna.

Nowadays, he said gene-tagging was more commonly used instead of plastic tags.

"More recently, we have developed gene-tagging, which uses genetic information from a tissue sample to match individuals to themselves," he said.

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"This is used to estimate the number of young fish entering the population each year."

Another research approach uses tissue samples to determine close relatives of the fish.

"Close-kin mark-recapture involves taking samples of adults and juveniles and using powerful statistics and genetics to identify close relatives, such as parents, siblings and half-siblings," Davies said.

"This data is combined with other information to estimate the number breeding fish in the population, which is of primary interest to fisheries managers.

"These developments have substantially increased the accuracy of monitoring southern bluefin tuna, and has broad applications to other fish and animal populations generally.

"It's an extremely exciting development."

Narooma Charter Fish takes part in tagging programs. Boulton said research was important for a sustainable fishery.

Crew member Georgia Poyner, who has conducted research for spearfishing tournaments, was also passionate about fish studies.

"Fishing plays an important role in researching species - it's a vital thing to study," she said.

Poyner said many people were unaware the lifespan and growth of different fish species varied.

"A lot of people don't realise smaller fish, like bream and drummer, are a slower-growing species than bluefin," she said.

Poyner said the bluefin's age was most impressive.

"It was definitely impressive how old the fish is and the days at liberty, how long it has been tagged for and not recaptured," she said.