Bluefin tuna populations mix for feeding, separate to spawn

Although they roam far to feed together, for two prized Atlantic bluefin tuna populations home is home, and they stick to their respective turf on opposite sides of the Atlantic when it comes to reproduction, say scientists tracking the giant fish। Scientists with the US and Canada-based Census of Marine Life (COML) have found that although threatened northern bluefin (Thunnus thynnus), the world's largest bony fish, freely mix together in their north Atlantic feeding grounds, they appear to invariably remain faithful to where they came to life, either in the Gulf of Mexico or the Mediterranean.The research reveals the "remarkable migrations and life-cycle secrets of the declining species," COML said in a research summary released Sunday.Researchers at the COML-associated Tag-a-Giant Foundation of Stanford University tagged hundreds of the northern bluefin to map their extensive movements.Two fish tagged within minutes of each other in feeding waters off of western Ireland provided examples for the existence of two distinct populations of bluefin that mingle for feeding but separate for breeding.

The two ended up over 5,000 (3,100 miles) kilometers apart within eight months, one having swum 6,000 kilometers (3,700 miles) to waters off northeast of Cuba, while the other moved to the southeast offshore Portugal।"This tagging data potentially provides new evidence that mixing is occurring in the northern waters of the eastern Atlantic and complement prior data showing that the western and eastern stocks of north Atlantic bluefin mingle in rich foraging grounds of the central Atlantic," said Barbara Block of the tagging operation, in a summary of the research.The scientists are now working on genetic tests to provide more evidence for the distinct origins of the fish.In a separate study under COML auspices, a group of Canadian and Danish scientists have more firmly tied the precipitous decline in the northern bluefin population in the 1960s to overfishing over the previous five decades.Prior to World War I, the bluefin was rarely a target of commercial or sport fisheries.But they showed that from the 1920s blue fin fishing took off with increased use of harpoon rifles and hydraulic lifts, which made it easier to harvest the giants, that reach up to 700 kilograms (1,550 pounds).Canneries helped increase the demand in the 1930s and 1940s, and the catch shot up exponentially from Scandinavia to the Mediterranean."The booming catches helped strip the Atlantic bluefin population in a relative blink of time -- 1910 to 1950," said a research summary.As populations fell, there was more catching of juvenile tuna, further devastating the population, according to Brian MacKenzie of the Technical University of Denmark.

"We've shown bluefin tuna were here for a long time in high numbers. High fishing pressure preceded the specie's virtual disappearance from the area and apparently played a key role.""But other factors under study might have compounded the fishery's demise -- the catch of juvenile tuna in subsequent years, for example."