by KELLY SLIVKA, Source: NYTimes

Atlantic bluefin tuna aren’t much of a mystery when they’re dead. Sushi chefs roaming the Tokyo fish market can appraise a bluefin steak with a cursory glance. Multitudes of people are intimate with the distinct texture and flavor of bluefin meat.

But much about the lifetimes of these high-profile fish is unknown, and what is known is often misunderstood. “The bluefin tuna is an extremely complicated animal,” said Molly Lutcavage, the director of the Large Pelagics Research Center of the University of Massachusetts at Amherst.

Dr. Lutcavage has been tagging bluefin for 15 years in an effort to shed light on the travels of these long-lived oceanic wanderers. A study published this week in PLoS One that she co-authored sums up her most recent project, in which juvenile bluefin were tagged in the North Atlantic and tracked for a year.

“The tags essentially look like a short cigar” with an oblong spherical float and radio antenna on one end, she said. They are tethered to an anchor that lodges in a bluefin’s cartilage behind its rear dorsal fin.

As they travel along with the tuna, the tags record the vertical and horizontal movements of the animals through the ocean. Then they pop off the animals at a scheduled time, float to the surface and transmit the data via satellite to Dr. Lutcavage. “These electronic tags give us completely new insights into what the fish are doing,” she said. “It’s extremely exciting.”

Bluefin tuna research over the past seven years has taught scientists that many of the young ones found on the eastern seaboard of the United States were born in the eastern Atlantic or in the Mediterranean Sea, Dr. Lutcavage said. There’s a lot of evidence that bluefin return to the places where they were born to spawn, so she expected to see the juvenile fish that she tagged eventually head back east to reach their spawning grounds.

But when she examined the new data, she was surprised to see that instead of traveling back east, the tuna stayed in the western Atlantic near the U.S.coast.

Dr. Lutcavage is not sure that any generalities about bluefin behavior can be deduced from the data –- except that scientists need to learn more about the animals. “One of the most important things it tells us is what we don’t know about the fish, and in the case of bluefin tuna, that’s pretty significant,” she said.

The main reason that bluefin tuna are poorly understood is that most of the data collected from the fish is biased. “The vast majority of assessments of fish populations are based on what we call ‘catch and effort’ data,” she said, meaning that the information comes from fishermen catching tuna.

Fishermen tend to focus on areas where plenty of fish are to be found. Places where smaller, younger tuna go, or where tuna wander alone, are not well known.

Dr. Lutcavage’s data is important because it’s “fishery-independent.” The tags track tuna wherever they go — and bluefin tuna go far.

It takes an internationally collaborative effort to track the bluefin. Dr. Lutcavage said she communicates with the research group Azti Tecnalia in the Bay of Biscay, which also tags bluefins, so she can better understand the fish. Fish tagged in the Bay of Biscay have shown up off the east coast of the United States within a year, and vice versa, she said.

She has also received a lot of research help from fishermen. “Fishermen have been our partners on this research since day one,” she said, both by helping to finance her work and helping her catch and tag tuna in the field.

“Working together is a great way to close gaps,” Dr. Lutcavage said.