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Bicoastal Atlantic bluefin tuna

Prized species’ east-west populations intermingle more than thought, complicating management

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12:56pm, October 2, 2008
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Bluefin tuna get around. The highly prized fish traverse the Atlantic with a disregard for international boundaries that has set nations quarrelling over who gets to fish and who sets the limits. Now new research on the whereabouts of Atlantic bluefins could provide the hard numbers needed for developing effective strategies to save the fisheries from collapse.

“This is a substantial step forward in providing a comprehensive data set for management,” says Michael Sissenwine, former director of scientific programs and chief science adviser for the U.S. National Marine Fisheries Service and now at Woods Hole Oceanographic Institution in Massachusetts.

Because Atlantic bluefins have spawning grounds on both sides of the Atlantic (and perhaps in the middle, some scientists say) management agencies have treated them as two distinct populations: western bluefins that spawn in and near the Gulf of Mexico, and eastern bluefins that spawn in the Mediterranean. While scientists have known for several years that these populations mix, their socializing hasn’t been incorporated into management strategies.

The new study, published online in Science October 2, reports that substantial numbers of juveniles from the Mediterranean spend time in waters off the U.S. eastern coast. The western Atlantic population is already thought to be considerably smaller than the Mediterranean-based stock.

“The mixing may have caused people to overestimate the abundance of western bluefins. They may be routinely setting catch limits too high — that’s a problem,” comments John Magnuson of the University of Wisconsin-Madison.

Waters off the eastern United States may serve as a refuge for Mediterranean-born fish, since fishing quotas are much lower on the west side of the Atlantic, says Jay Rooker of Texas A&M University at Galveston and lead author of the new study. But bluefins born in the Gulf of Mexico may also be spending time abroad, he says. “This trend of juveniles not keeping to their side of the pond — it could go the other way as well.”

Atlantic Bluefin tuna stocks plummeted in the 1970s and have never fully recovered. The fish are built for speed — they have a silvery, streamlined body, retractable fins and can swim faster than 70 kilometers per hour. But like other fine racing machines, they are coveted, expensive and rare. Demand for sushi-grade tuna hasn’t helped — a single bluefin can fetch tens of thousands of dollars.

Last year the National Marine Fisheries Service called for a multiyear moratorium on Atlantic bluefin tuna fishing on both sides of the pond, but the appeal was rejected by the International Commission for the Conservation of Atlantic Tunas. ICCAT is scheduled to review the bluefin tuna management plan this November.

The new study examined otoliths, or “ear stones,” from nearly 200 Atlantic bluefins collected over a six-year period from both eastern and western stocks. Found in the inner ear, these ear stones start out as tiny calcium carbonate grains, but grow in layers as the fish age. For fish, the stones act as sound receptors, but scientists use them as a sort of natural tracking device. Carbon and oxygen are incorporated into the ear stones as they grow, and depending on the geochemistry of the water the fish spends time in, the ratios of the forms of oxygen and carbon differ.

Otolith analysis revealed that more than 90 percent of the older, “giant” bluefins found in the Gulf of Maine and Gulf of St. Lawrence were born in the Gulf of Mexico. This suggests that the entire eastern seaboard should be considered a high priority for conservation, says Molly Lutcavage, director of the LargePelagicsResearchCenter at the University of New Hampshire in Durham.

Lutcavage, who was not involved in the new study, says the dispersal of bluefins is complex, and management needs to consider a broader view of reproductive schedules and patterns. In September at the 2008 International Council for the Exploration of the Sea conference in Halifax, Nova Scotia, Lutcavage presented data from 41 bluefins tagged off Nova Scotia. During spawning season, several of these fish spent time along the edge of the Gulf Stream in the northern Atlantic, suggesting an additional spawning ground.

If these things aren’t considered in plans to rebuild the populations, Lutcavage says, “We could be missing the boat.”

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