Prized species’ east-west populations intermingle more than thought, complicating management
“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
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
The new study, published online in Science October 2, reports that substantial numbers of juveniles
from the Mediterranean spend time in waters off the
“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
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
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
If these things aren’t considered in plans to rebuild the populations, Lutcavage says, “We could be missing the boat.”
Jay R. Rooker, David H. Secor, Gregorio DeMetrio, Ryan Schloesser, Barbara A.
Block, John D. Neilson. 2008. Natal Homing and Connectivity in Atlantic Bluefin Tuna Populations. Science. Online Oct 2. when embargo lifts: www.sciencexpress.org.
Benjamin Galuardi, François Royer, Walt Golet, John Neilson, and Molly Lutcavage. 2008.
Complex migration tracks from Atlantic Bluefin Tuna Tagged off Nova Scotia,
Canada. Abstract from ICES CM 2008/P:16.