Once one of the great mysteries of the natural world, the bluefin tuna's migration pattern has finally been elucidated by the workings of an international team of scientists as part of the global 10-year Census of Marine Life. To reach their conclusions, they tagged almost a thousand organisms and studied historical records that showed how the once vibrant tuna numbers collapsed following the emergence of industrial fishing. Two types of tags were used: an external one fitted to the back of the fish and an internal one that required a minor surgical procedure; both tracked their movement and recorded depth and water temperature.
They now believe that two separate populations share foraging sites in the Atlantic — waters off the coasts of Spain, Portugal and Ireland and off the eastern shores of North America — before splitting up and heading to opposite sides of the ocean to breed. "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 Stanford University's Tuna Research and Conservation Center. "But when it is time for these fish to go back to their spawning grounds, they separate out."The scientists' findings also revealed "significant genetic differences" between the two populations — which they believe helps account for their geographically disparate breeding grounds.
"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," said Boustany.
Ron O'Dor, a scientist working on the Census of Marine Life, believes these studies should serve as a guide for how to study the movement and historical population patterns of different oceanic species. He said they could also impart some information invaluable in helping structure more effective management and enforcement regimes to return the oceans to a more pristine condition. "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 concluded.
Via ::BBC News: Tags reveal tuna migration routes (news website)