Over-fishing has made Atlantic bluefin tuna a prized delicacy a century after the fish were scorned in Europe as pet food, according to studies on Sunday that urged better international protection.
Tuna are now like floating goldmines out in the ocean, said Brian MacKenzie of the Technical University of Denmark.
Bluefin tuna teemed in summers in the North Sea and the Norwegian Sea a century ago as part of migrations that can take them 10,000 miles a year but are now rarely seen in the region after a burst of industrial fishing from 1910-1950.
A century ago people would sometimes use tuna as pet food, MacKenzie said of his work studying sales records, fisheries yearbooks and other sources in north Europe to be published in the journal Fisheries Research.
Tuna's sudden popularity in northern Europe might have been linked to a surge of consumption in Germany during World War One when other food was scarce. It then caught on ever more widely.
Bluefin tuna, which spawn in the Gulf of Mexico and the Mediterranean, can be worth $10,000-15,000 each in Japan, where they are eaten raw as sushi, said Andre Boustany, an expert at Duke University in the United States. One single fish sold for a record $178,000.
The Atlantic bluefin stock has plunged in recent decades, meaning that even a total halt to fishing might not help stocks revive, he said.
Denmark opened a first cannery in 1929 and Norwegian catches alone topped 10,000 tonnes a year around 1950 -- compared to total catches of Atlantic bluefins in 2005 of about 23,000 tonnes, from Canada to the Mediterranean.
People in northern Europe have largely forgotten the vast tuna catches that once filled market halls last century. The fish can weigh up to almost 700 kilos (1,500 lbs) each.
We hope that our work will inspire a more precautionary approach to the management of bluefin tuna in the Atlantic, with more concern about re-establishing and maintaining the historical range of the species, MacKenzie said.
Another study based on electronic tagging showed how the fish often cross the Atlantic. In a 25-year life, a tuna might cover the equivalent of the distance to the Moon -- about 240,000 miles.
We released two fish hooked at the same time off Ireland -- six months later they were almost 6,000 km apart, one off the Bahamas and the other off the Strait of Gibraltar, said Michael Stokesbury of Dalhousie University in Canada.
That means that management of the stock should be done by countries on both sides of the ocean.
A fisherman from the Scandinavian Tuna Club might have chased the same giant bluefin as a Cuban fisherman, said Ron O'Dor, senior scientist of the Census of Marine Life, a 10-year project to document life in the oceans.
Part of the lesson here is that restoring tuna populations to health requires us to consider and manage activities one-fifth of the way around the world, he said.