By Mark Kinver
Science and nature reporter, BBC News
The secrets of the bluefin tuna's migration have been unlocked by one of the most comprehensive studies of the giant fish, say scientists.
Researchers believe two separate populations of the fish share feeding sites in the Atlantic before heading to opposite sides of the ocean to breed.
To help reveal their migratory pattern, an international team has tagged almost a thousand specimens.
The findings form part of the global 10-year Census of Marine Life.
The team also studied historical records that showed how bluefin numbers, once abundant in the North Atlantic, collapsed after the emergence of industrial fishing.
"What the tagging has shown is that the tagged fish all occur in the same area of the North Atlantic to feed," explained Andre Boustany, from the Tuna Research and Conservation Center at Stanford University, California.
These foraging areas included waters off the eastern shores of Canada and the US, and off the coasts of Spain, Portugal and Ireland. "But when it is time for these fish to go back to their spawning grounds, they separate out," he added.
Dr Boustany said the data also revealed that northern bluefin tuna (Thunnus thynnus) not only returned to a specific spawning site year after year, but also chose the location in which they were born.
He said this behaviour suggested the fish were genetically hard-wired to do this.
"There has to be some kind of genetic component because the fish may be in the western Atlantic for three or four years, hanging out in feeding areas, but when it is time to spawn they make a bee-line straight through the Straits of Gibraltar to their spawning grounds in the Mediterranean," he told the BBC News website.
The project has fitted almost 1,000 tags to bluefin tuna
Laboratory tests showed there were "significant genetic differences" between the two populations, Dr Boustany revealed.
"This would not happen unless the fish were natally homing and the two stocks were not interbreeding at all.
"This has been hypothesised for a while, but it was very difficult to show conclusively without carrying out genetic and tagging studies."
The tagging project, called Tag-A-Giant (TAG), has deployed almost 1,000 tags.
Two types of device were used - external tags fitted to the back of the fish, and internal ones, which require a minor surgical procedure.
Both tracked the movement of the fish, and also recorded depth and water temperature.
Mike Storesbury from Dalhousie University, Canada, said that accessing the data from the internal tags required fishermen to return the devices after the fish had been caught.
But the external tags, he added, were programmed to be automatically released after a predetermined time.
"The really good thing about pop-up satellite tags is that you do not have to get the tag back," Dr Storesbury explained.
The tags floated to the surface of the water before uploading the collected data via the Argos satellite system.
Fitting the tag to the tuna, on average, took two-and-half minutes, the researcher revealed.
"It is a very quick procedure," said Dr Storesburg. "When we bring the fish on board we stick a saltwater hose in its mouth so you are hydrating the gills.
"This allows the fish to breathe all the time it is on board and reduces its stress levels."
Lessons from history
Another part of the tuna study for the Census of Marine Life, a decade-long study involving marine scientists from more than 80 nations, was a detailed historical study of the fish in the first half of the 20th Century.
Industrial-scale fishing saw a vast number of tuna being caught
It showed how the once abundant species virtually disappeared from the waters around northern Europe following an explosion in commercial fishing.
Brian McKenzie from the Technical University of Denmark said it was like fitting together pieces of a jigsaw to get the full picture.
"We used a variety of data sources," he explained. "One of the places was the International Council for the Exploration of the Seas [Ices].
"However, they have no data on tuna landings until 1927. So, from 1927 until 1950, we primarily used Ices' data, but before 1927 we looked in national fishery year books."
He added that papers from peer-reviewed journals also provided useful data for understanding the fate of the fish in the region.
"We are learning an awful lot from the mistakes we made in the past," Professor McKenzie added.
'Ignorance is no excuse'
Ron O'Dor, one of the marine census's senior scientists, said the combination of the two studies provided a unique view of the state of the bluefin tuna populations.
"This is an interesting time for us because we are seeing a pattern from history, we are also starting to experience similar patterns now," he said.
Professor O'Dor compared the study to the popular TV police drama CSI: Crime Scene Investigation.
"We are now thinking of ourselves as CSI: Ocean," he joked.
"We have these incredible technologies that allow us to go out into the ocean and determine what exactly is going on.
"But it is not cheap - we have to be able to deliver this message to someone who can turn it into a legal or regulatory framework for managing fisheries.
"We can't really tell people what to do, we can only tell them what is happening."
He added that oceans could be returned to the condition they were in the past, before the introduction of industrial fishing, if effective management and enforcement schemes were put in place.
"Studies like this within the Census of Marine Life show that we can know a tremendous amount about what is going on in the oceans. Ignorance is no excuse anymore," he said.