HALIFAX - Scientists have discovered more about the migratory patterns of bluefin tuna species and say the revelations could help prevent the disappearance of the heavily harvested giants of the sea.
American and Canadian researchers tracked the movement of populations in the Mediterranean and the western Atlantic, finding that they intermingle as young fish in the Gulf of Mexico.
David Secor of the University of Maryland said the mixing of the two populations could give a false picture of the size of the western stock -- a highly prized catch off eastern Canada that supplies the hungry international sushi market.
"It could indicate that we've had too much of an optimistic perspective about the health of this western population," said Secor, whose paper comes out Friday in the journal Science.
"It could give us the false impression that things are fairly stable when in fact they're declining."
The researchers examined parts of the tunas' inner ears to identify their origins and found that the two groups venture to the Gulf of Mexico as juveniles, but then return to their places of origin on either side of the Atlantic to spawn.
Canadian and U.S. fisheries managers use stock assessments from the Gulf of Mexico and other areas to determine the health of the population.
That is then used to set quotas, something scientists say should be re-evaluated now that they know most of the Mediterranean fish return there rather than travel north to be caught off Canada.
"The stock assessment in both the eastern and western Atlantic needs to better reflect mixing between the two populations," said John Neilson, a marine biologist at the St. Andrews Biological Station in New Brunswick who co-authored the report.
"This is an important piece of the puzzle for the life history of this highly migratory species."
The researchers say the new insight will help understand what's happening to the species that has been under reduced quota for years in Canadian waters, but has yet to show signs of a major recovery.
It's estimated that the western stock is only at 10 per cent of its biomass before it began to plummet in the 1970s.
Nova Scotia tuna fisherman Robert Conrad says the fishery appears to be in better shape this year, but that the stock is much smaller than it was decades ago and the fish themselves aren't reaching the storied "giant" sizes that were common in northern waters.
The fish can reach four metres long, weigh more than 250 kilograms and are known for speeds that can exceed 70 km/h.
"Now we rarely see one of those great big ole fellas whereas once we were catching routine numbers of them," he said from his home on Nova Scotia's St. Margarets Bay.
"There's no question that it's not like it was. It used to be it was the bluefin we could count on. Those times are gone."
The findings could give governments new tools to jointly manage a fishery that has no regard for borders and is highly migratory. The issue has been a thorny one for countries that squabble over each others management practices.
The researchers hope the information will be used at the upcoming meeting of the International Commission for the Conservation of Atlantic Tunas next month.
The group has been sharply criticized for what some say is a failure to conserve the stock that has been aggressively fished in the Mediterranean by boats trying to pull in fish that can fetch tens of thousands of dollars.
