Prized species’ east-west populations intermingle more than thought, complicating management
BIG BLUE Atlantic bluefin tuna can grow to nearly 10 feet and weigh in at 1,400 pounds. Numbers of adult spawning bluefins have dropped to less than 20 percent of 1960s figures. Read more. Gilbert 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.
READING THE STONESAnalyses of the growth rings in otoliths, or ear stones, of Atlantic bluefins suggest that the western and eastern tuna populations are mixing more than previously thought. Dave Secor
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 Large
Pelagics Research
Center 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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