The high demand for the fish has led to overfishing of the species. Some estimates indicate the population of bluefin in the Western Atlantic has declined from a quarter million to 22,000 fish since 1970. As a result, the ICCAT lowered quotas for the number of bluefin that commercial fishers are allowed to catch in the Western Atlantic.
North American fishers argue that they are at an unfair advantage because the Western Atlantic quotas don't take into account the bluefins' movement across the Western and Eastern Atlantic.
In an accompanying article in Science, John Magnuson, a zoologist at the University of Wisconsin in Madison, Wisconsin, and his co-authors noted that North Atlantic fishers also claim they are being penalized for overfishing of bluefin in the Eastern Atlantic and Mediterranean.
In a 1994 report on Western Atlantic bluefin tuna, the U.S. National Research Council concluded that conservation policies needed to recognize the mixing that occurs between between Western and Eastern bluefin. It is difficult to do that without more reliable data on bluefin behavior, the report noted.
The new research helps fill that gap. "Block's study has shown that the degree of mixing across the Atlantic feeding and fishing grounds is greater and more complex than known prior to her work," said Magnuson.
Complex Patterns of Movement
To track the bluefins' movements, Block and scientists from the Monterey Bay Aquarium and the National Marine Fisheries Service used a new electronic tagging technology.
"Bluefin tuna are hard to study because they are completely submerged, and this new technology allows you for the first time to study where fish go," said Block.
Beginning in 1996, the researchers placed two kinds of tags on Atlantic bluefin tuna that they found off the eastern coast of North America.
Some of the tags, known as archival tags, were surgically implanted. They recorded a wide range of data about the fishes' whereabouts deep beneath the sea. Other tags"pop-up" satellite tagswere attached to the outside skin of the fish. They are designed to automatically detach from the fish about six weeks after being applied and pop up at the ocean's surface, where their data is downloaded to a computer via satellite.
The researchers have recovered 18 percent of the archival tags and 90 percent of the pop-up satellite tags. The data from both sets of tags show that bluefins' migratory behavior is very complex, the researchers said.
Nearly a third of the recovered tags were returned from the Eastern Atlantic and Mediterranean Sea. The others came from the Western Atlantic, primarily from waters off Maine, Nova Scotia, and Cape Cod, Massachusetts.
In most cases, the data showed, bluefin tuna that had been tagged in the Western Atlantic stayed a year or more in Western North Atlantic feeding grounds. But some of the recaptured fish had migrated to the Eastern Atlantic or Mediterranean Sea. Others had moved east and returned to western waters all in the same year.
"They are individuals just like you and me," said Block. "Some of us go to a restaurant in one part of Washington, D.C., others prefer North Carolina, some prefer Massachusetts. That is, what one fish does is dependent upon its own individual preferences."
The researchers found that although the fish often mix in eastern and western feeding grounds, they move to distinct spawning grounds in the Gulf of Mexico and the Eastern Mediterranean.
"Some fish breed in the Mediterranean, others go to the Gulf," said Block. "What this suggests is mixing in the North Atlantic on feeding grounds, but the existence of two breeding populations."
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