Tuna Range Wide, Dive Deep
Bluefin tuna is shown grilling at the Maricultura company's harvest dinner in Rancho Santa Fe, Calif., Jan. 27, 2007. Pacific bluefin tuna leave Japan's coastal waters and swim east to school off California and Mexico.
Credit: AP Photo/Chris Park

A 10-foot-long Atlantic bluefin tuna was fitted last week with the 1,000th electronic tracking tag ever placed on this fast, large and powerful yet threatened species.

The fish was then released in the Gulf of St. Lawrence off Port Hood, Nova Scotia.

Sushi connoisseurs revere bluefin tuna above all other fish species, but soaring commercial values have contributed to a near-collapse of bluefin populations in both the West and East Atlantic.

The fish was tagged by a scientific team from the Tag-A-Giant (TAG) campaign of Stanford University, Dalhousie University and the Monterey Bay Aquarium, working in collaboration with Canadian fishermen from Prince Edward Island and Nova Scotia.

TAG strives to set sustainable limits for commercial and recreational fisheries so they can recover from a vast population decline resulting from overfishing.

Since TAG first started capturing and marking bluefin tuna in 1996, half of the tags have been recovered by fisherman (who are rewarded about $500 for turning in tags to scientists) or reported back via satellite, yielding population numbers and other data.

“Occasionally we have someone who will pack a fish on ice and send it to us, but most of the time we just get the tag back,” said TAG Foundation spokesman Randy Kochevar.

The remaining tags are still at large on giants swimming in the North Atlantic, Mediterranean and Gulf of Mexico, he said.

Each tag transmits electronic data that specifically records pressure (or depth), sunrise and sunset data (used to pinpoint the fish’s location), water temperature and the fish’s body temperature. In turn, researchers can identify how populations of bluefin tuna use the North Atlantic for their migratory patterns, the structure of the fish’s population and physiology details.

The tags that transmit data via satellite allow researchers to track a tuna’s individual position as it migrates throughout the North Atlantic Ocean to visit spawning grounds in the Mediterranean and in the Gulf of Mexico.

The tagging data thus far is detailed in the Oct. 31 issue of Science, showing that the large fish that visit the northern waters of Canada are derived primarily from the Gulf of Mexico breeding stock, and that there is a mixture of fish from both Gulf of Mexico and Mediterranean stocks in waters along the U.S. continental shelf.

The electronic tags have also revealed these giant tuna dive to depths of nearly a mile and range through waters from the tropics to frigid polar seas. Bluefins have the rare capacity to maintain a warm, stable body temperature throughout their wide thermal niche — much like a mammal or bird.

TAG data have also helped to uncover where, when and how bluefin tuna spawn, as well as at what age they mature. The data also are helping to increase the accuracy of population estimates for Atlantic bluefin tuna.

The research was funded by the Tag-A-Giant Foundation, Monterey Bay Aquarium Foundation, Department of Fisheries and Oceans Canada, and NOAA.