Sightings | Alumni Profile

Tagging Giants for Research
Barbara Block Takes on One of the World's Premier Conservation Challenges by Pursuing the 'Most Majestic Fish in the Sea' p.3

The scientists also have tried antenna-bearing “pop up” tags that attach to one of the animal’s fins. Then at a preprogrammed time, the tag pops off, floats to the surface and transmits data to Block’s lab via satellite. But they’ve found those are not as reliable as the archival tags. The latter get returned, along with their recorded information, by the fishermen who catch their bearers for the last time. The incentive to send them back is a $1,000 payment funded by Tag-A-Giant sponsors—other fishermen.

Block boasts that archival tag return rates have climbed to 25 percent, while the total number of electronic tags deployed in the western Atlantic have grown to about 800. “She’s been nothing short of prolific in getting the tags out,” says Richard Ruais, director of the East Coast Tuna Association. “It’s not an easy thing. It’s quite a feat to put a tag on a bluefin tuna.”

A tuna tagging comes after an electrifying, rod-bending moment: An excited angler hooks a bluefin from a sport boat’s “fighting chair,” and the reeler and boat skipper work in concert to pull in the fish without overtiring it. Then control of the prize is ceded, literally with the toss of a tennis ball. The tennis ball is attached to a line from Block’s boat that the sports fishermen attach below the hook before cutting their own line.

After taking over the catch, Block’s group quickly executes a well practiced ballet. They pull the heavy tuna onto their boat’s well-padded deck, slip on a blindfold in order to calm it, insert a seawater hose in its mouth so it can continue breathing through its gills, and quickly record its weight and measurements. Then they perform quickly healed minor surgery to insert the “archival tag” into the skin of the bluefin’s belly. The total capture time measures just minutes.

In addition to the vital information about their movements, the tags also reveal new clues about their heat-producing physiology, a trait unusual among normally “cold-blooded” fish. That was the subject Block investigated in a related fish group—billfishes —in her Duke doctoral dissertation.

In subsequent research at the University of Chicago, she learned that tuna use a combination of muscle action and heat transfer between warmer and cooler blood vessels to raise temperatures in most of their bodies. That makes them different than billfishes, which warm only their eyes and brains.

Recent research in her lab at the Monterey Bay Aquarium’s Tuna Research and Conservation Center used tag data about depth and body temperatures to help reveal and explain an unusual bluefin trait—diving down to cold, sea bottom environments. “Instead of staying down there, it kept coming up to the surface and then going back down again, as if it had lungs instead of gills,” she says.

Followup research revealed that “warm tunas have cold hearts.” Bluefin hearts, because they are positioned near the tuna’s seawater-bathed gills, cannot rely on warmed-up blood. The animals must repeatedly resurface to keep their heart temperatures elevated enough to function.

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