For the first time, ocean historians have been able to create a detailed picture of the "burst of fishing" from 1900 to 1950 that led to the collapse of bluefin tuna stocks off the coast of northern Europe.

The research, released Sunday afternoon, was completed by Brian R. MacKenzie of the Technical University of Denmark and the late Ransom Myers of Canada's Dalhousie University.

Myers, who died earlier this year, had worked on some of the most important surveys of global fish stocks.

Some of the research was completed with the Tag-A-Giant (TAG) program, which uses electronic tags to track migrations of giant bluefins off Ireland and in the Gulf of Mexico.

Barbara Block, a marine sciences professor at Stanford University and chief scientist of the CoML Tagging of Pacific Predators program and TAG, told the tagging technology has been active for 14 years and has led to a greater understanding of the North Atlantic fisheries.

"We are using some of the coolest technology on the planet," she said. "We are putting something the equivalent in price of a laptop computer inside the tuna ... and like a submarine, we are asking 'where is it going?'"

There are three bluefin species of tuna: Atlantic, Pacific and Southern Bluefin, and they are one of the most lucrative commercial fish in the sea. Their quickly declining numbers has been a great concern to scientists and sushi lovers around the world.

The Atlantic bluefin have two populations -- one in the East Atlantic, and one in the West Atlantic. The 45th meridian divides the two populations for purposes of the study.

The study began with a bit of history -- looking at the almost mythical bluefins that once teemed in northern European waters (North Sea, Norwegian Sea, Skaggerak, Kattegat, and Oresund) for a few months each summer.

The smallest weighed 40 kilograms, but there were also giants of up to 700 kilograms. They began disappearing when an industrialized fishery geared up in the 1920s, using harpoon rifles and hydraulic lift nets.

By 1960, the species had virtually disappeared from the region.

"We've shown bluefin tuna were here for a long time in high numbers," MacKenzie said in a news release. "High fishing pressure preceded the species' 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.

Tagging shows travel patterns

The research team hopes their work will inspire "a more precautionary approach to the management of bluefin tuna in the Atlantic."

Today, Atlantic bluefin stocks are down 80 to 90 per cent compared to historical breeding populations.

CoML-associated scientists working with the Tag-A-Giant (TAG) program tracked migration patterns, and learned that fish tagged within minutes of each other wound up more than 5,000 km apart eight months later.:

  • One swam 6,000 km, past Bermuda to waters about 300 kilometres northeast of Cuba
  • The other remained in the eastern Atlantic and moved off the coasts of Portugal
  • A third tagged bluefin swam into the Mediterranean Sea and was caught by fishers southeast of Malta in 2005

Tuna have generally been harder to track than other fish stocks. The species keeps body temperature independent of the ambient environment. This allows it to go into very cold, northern waters -- and travel much longer migration routes than other fish.

The electronic tags allow scientists to determine exactly where the tuna breed, and the oceanographic conditions they are selecting.

"That's the first time for a large tuna species that we've had this kind of data to say this," Block said. "This information can help scientists and policy-makers best protect the remaining tuna while they are spawning."