Scientists track prized bluefin tuna

They can swim but they can't hide.

CSIRO scientists have tracked the highly-prized Southern Bluefin tuna to research their long-term feeding habits, for the first time.

The work has revealed where and when the tuna feed, at what depth and even the temperature of the water, despite the migratory habits of the fish which can have them covering 8,000 kilometres each year.

PhD student Sophie Bestley and her colleagues at the CSIRO's Wealth from Oceans national research group have surgically implanted miniaturised electronic data-storage tags into juvenile tuna off the coast of southern Australia.

Already the team has found that tuna feed almost anywhere they can and three days out of every four, even during their annual migrations.

The CSIRO believes this information will assist future management of what is currently an overfished and depleted species which supports a global commercial fishery worth more than $US1 billion ($A1.25 billion) a year.

"Direct and long-term information on feeding is extremely rare for migratory marine species," Ms Bestley said.

"It is available for Bluefin Tuna only as a result of their unique physiology and the long-term electronic tagging programs instituted by the CSIRO.

"When the tuna feed, the metabolic energy produced during digestion causes their tummies and surrounding tissues to heat up.

"This warming is recorded by internal temperature sensors on the tags."

The tagging has revealed the tuna to be supreme hunters, feeding on 76 per cent of all days in waters ranging from five degrees celsius to 23 degrees.

They can catch a meal at the surface or as deep as 672 metres.

But the sensors also recorded periods of fasting lasting up to 24 days, usually in waters warmer than 15 degrees.

The fish in the CSIRO study swam from the Great Australian Bight to winter in the south Indian Ocean, before returning often close to where they were originally tagged the previous summer.

Ms Bestley said studying the feeding and migratory habits of the tuna would help determine the future size of a sustainable catch.

"My work will help us build a better understanding of where they are at, what time of year and whether they are likely to be harder or easier to catch in different places and times," she said.

"This information can then be used in models to provide advice on the status of the stock and what levels of catch are sustainable and will allow the stock to rebuild."