The Inner Lives of Bluefin Tuna

In her cluttered office overlooking Monterey Bay, Barbara Block is poring over computer printouts that are revolutionizing what the world knows about one of the most sought-after, mysterious animals on Earth--the giant Atlantic bluefin tuna.

“This is my favorite biological activity--getting inside the heads of these fish,” said Block, a Stanford University marine biologist who directs the Tuna Research and Conservation Center at Monterey Bay Aquarium.

High-tech tracking devices that she attached to bluefin tuna off North Carolina’s coast more than a year ago are producing daily records of where the fish travel, when and how deep they dive and even how often they eat.

“This is silver platter stuff,” the wiry, intense Block said. “This is the life of an animal that’s been my focus all these years, served up in all this detail.”

Block and a team she co-directed with Eric Prince of the National Marine Fisheries Service tagged 200 medium and giant bluefin in the winter of 1997. She’s hoping to tag 1,000 of the fish by 2000.

The initial tagging has raised more questions about the bluefin than it has answered, Block said. But if she can tag 1,000 fish, the data they provide will go far toward resolving debates that have raged for years over the creatures’ migration patterns and spawning habits.

Changing Concept of Tuna Behavior

Fishermen, environmentalists and politicians on both sides of the Atlantic are watching Block’s tuna tagging closely. Those who want to catch more of the giants as well as those who want the catch rate slashed say that her work may determine the bluefins’ fate by changing international management of the fish.

“Her research is critical, crucial, fundamentally important,” said Carl Safina, an environmentalist who wrote a book about the bluefin and fears that the fish may be wiped out unless stiffer catch restrictions are imposed. “Not to mention that she is about to really revolutionize our whole concept of how animals use the oceans.”

Scientists have long been able to say things about the behavior of species in the ocean, Safina said, “but we know nothing about the behavior of individuals.” That is, until Block married satellite technology with bluefins.

Block attached satellite pop-up tags and archival tags with microprocessors to the bluefins she captured. Shot onto the animals’ dorsal fins with a dart, the pop-up tags detached and floated to the surface 90 days later. They then relayed their positions to a satellite that sent the information to Block’s laboratory computer. By mapping the signals, Block traced the tuna across the ocean.

“The fish spread throughout the entire western Atlantic,” Block said. “A couple had crossed the dividing line between the western Atlantic and the eastern Atlantic stocks, but just barely. They swam over 1,700 nautical miles.”

Fishermen who believe that there is only one stock of bluefin tuna ranging across the ocean are excited about the fact that some of the tagged fish swam into the eastern Atlantic. But Block said the sample was too small to draw any conclusions.

More answers will come as the archival tags are returned, she said. Surgically implanted in the tuna, the archival tags record a wealth of data that the pop-ups do not provide. The archival tags monitor a fish’s body temperature, its location and the depth and temperature of the surrounding water every two minutes for up to seven years.

Using techniques she developed by working with the United States’ only captive bluefin tuna population--managed by the Monterey Aquarium’s Charles Farwell--Block and her crew hauled 400- and 500-pound bluefins out of the water. They sewed 160 archival tags into them in operations that took three minutes or less.

The data from the archival tags can only be retrieved when the fish are caught and the tags are removed. So Block has offered a $1,000 reward to anyone who sends a tag back. So far, she has received 10 archival tags, a percentage that she said is right on track with what she expected.

One big surprise was that the data showed the fish diving at sunrise and sunset like clockwork. Another surprise was that fish were found far from the Gulf of Mexico spawning ground, even though they were of breeding age and it was spawning season.

Block said she does not have enough information to know why the fish weren’t where they were supposed to be. Nor can she yet answer the key question of whether there is one Atlantic tuna stock or two. That, Block said, will take much more tagging to determine.

She is working with a computer technology company to develop a tag that will combine the storage capacity of the archival tag with the communications ability of the pop-up.

There is a sense of urgency to her work because bluefin stocks are thought to be depleted by overfishing on both sides of the Atlantic.

Prized by sushi lovers for their red, fatty meat and revered by sportfishermen for their size and strength, bluefins are vastly different fish from the smaller albacore tuna that is chopped up and stuffed into cans.

“They are magnificent fish that can go wherever they like in the Atlantic,” said Molly Lutcavage, a marine biologist with the New England Aquarium who also is tagging bluefin. “They can travel from the Bahamas to Norway in as little as three to five weeks. It is quite moving to have one of these animals up close to you, next to a boat, and think about where they have been, the hundreds of thousands of miles they have traveled.”

The bluefin is a warmblooded swimming machine that grows to as much as 1,500 pounds and reaches lengths of up to 10 feet. It can live more than 30 years and migrates hundreds of thousands of miles across the open ocean in its lifetime. A bluefin can dive to 2,400 feet, and maintain a body temperature of 80 degrees even in waters as cold as 45 degrees.

A Lucrative Catch for Fishing Boats

Almost all the bluefin commercially caught are sold to Japan. A single bluefin usually sold for more than $30,000 in Japanese fish markets before the Japanese economy slumped and the yen slid in value against the dollar. Until recently, a New England bluefin fisherman could send a child through college on the proceeds of the sale of one fish. Even now, the fish wholesales for $6 to $7 a pound, making a 500-pound fish worth about $3,500 on the wholesale market.

Because of overfishing, the National Marine Fisheries Service estimates that the breeding stock of western Atlantic bluefin is only about 15% of what it was in 1975. Some of the fiercest arguments over the bluefin are about what it will take to rebuild the fish to sustainable levels.

The bluefin is managed by the International Commission for Conservation of the Atlantic Tuna. The United States is a signatory to the treaty that established the commission and abides by its regulations.

In 1982, the tuna commission ruled that there are two separate stocks of Atlantic bluefin, with the western Atlantic tuna spawning in the Gulf of Mexico and the eastern Atlantic tuna spawning in the Mediterranean.

The commission drew a line down the Atlantic, and severely restricted the catch of western Atlantic tuna by the United States, Canada and Japan, on the theory that the western stock was dangerously depleted. European and Mediterranean nations, however, were allowed to continue heavily fishing eastern Atlantic tuna, which are far more abundant than western Atlantic bluefins. This year, fishermen in the western Atlantic were allowed to catch 2,500 metric tons of the fish, while eastern Atlantic fishermen caught 45,000 metric tons.

The two-stock theory enrages western Atlantic fishermen, who believe that there is a single stock roaming widely in the Atlantic, or at least that there is heavy mixing of the two stocks. If that were so, the only way to ensure the commercial viability of the fish would be to impose stiff quotas on all bluefin fishing nations.

Richard Ruais, a lobbyist for New England’s commercial fishermen, says he believes that the work of Block and Lutcavage, who do not collaborate, will soon prove the fishermen are right. Lutcavage also has put satellite tags on giant bluefin in the Gulf of Maine.

“Barbara is a dynamo,” Ruais said. Although results from her tagging project and Lutcavage’s are preliminary, he said, “they have really opened the eyes of some of the scientists who have been playing with a model to try to assess the stock of bluefin.”

Ruais said he is most intrigued by the fact that both scientists found bluefin of spawning age outside their presumed breeding grounds during spawning season, and found that some of their tagged fish had crossed the line dividing the western Atlantic from the eastern Atlantic.

Just as Safina is confident that Block’s work will eventually show that there are indeed two stocks of bluefin tuna that should be separately managed, Ruais is confident she will demonstrate that there is a single stock or two stocks that are highly mixed.

“My organization has been fighting the two-stock hypothesis since it came into being,” Ruais said.

Staring down at juvenile bluefin swimming in one her laboratory tanks, Block said she is acutely aware of how much is at stake in the outcome of her research. She insists that she has no interest beyond assembling facts for others to use to make management decisions.

“I’m in the golden age of fisheries science,” Block said. “Never before have scientists had the tools to discern the true status of stocks, to figure out where they go, where they breed. Our goal is to deliver the truth here.”