Tagged Tuna Reveal Migration Secrets

Helen Scales
for National Geographic News
August 13, 2007
High-tech electronic tags are unveiling new insights into the migration and spawning secrets of Atlantic bluefin tuna, according to a pair of new studies.

The work could lead to better protections for the commercial species, which many experts say is declining worldwide (see related photos of the demand on tuna fisheries).

In one report, appearing in the journal Hydrobiologia, a team of international scientists documented two giant bluefins tagged within minutes of each other off the coast of Ireland.

The two fish swam to opposite sides of the Atlantic Ocean—ending up more than 3,000 miles (5,000 kilometers) apart.

One of the fish traveled 3,730 miles (6,000 kilometers) southwest to waters about 186 miles (300 kilometers) northeast of Cuba. The other remained in the eastern Atlantic and moved off the coasts of Portugal.

The fishes' dramatic separation supports theories that two distinct tuna stocks forage in common grounds but swim thousands of miles apart to breed in the Gulf of Mexico and the Mediterranean (see graphic).

For another unrelated study, a U.S. team used the high-tech tags on 28 tuna to identify the particular habitats of the bluefins that spawn in the Gulf of Mexico.

The work—to appear in an upcoming issue of the journal Marine Biology—found that the fish seem to prefer breeding grounds where the continental shelf is steep and the temperature is between 75 and 81 degrees Fahrenheit (24 and 27 degrees Celsius).

This suggests that even small changes in ocean temperatures could affect when and where bluefins breed.

Rise and Fall

The Atlantic bluefin tuna is listed as "data deficient" on the World Conservation Union's Red List of Threatened Species.

This means that researchers simply don't have enough data on the species' population and distribution to assess its risk of extinction.

But the data that are available suggest that the huge tuna—especially prized by sushi connoisseurs—are declining due to overfishing.

(Read "Bluefin Tuna in Atlantic Nearing Extinction, Conservation Group Says" [July 24, 2006].)

And scientists know that certain historic tuna populations have been decimated as recently as the 1960s.

For example, a third study released by researchers from the Census of Marine Life (CoML) documented the rise and fall of a bluefin fishery that developed in northern Europe in 1910.

Both commercial and sport fisheries in that area collapsed by the early 1960s without recovering.

"The revelation that bluefin tuna were present in such large numbers in northern European waters is remarkable," said Brian MacKenzie of the Technical University of Denmark, a co-author of the historical analysis.

"It is shocking that they are no longer present [in that region] in anywhere near as large numbers," he said.

Such drops already appear to be underway in modern tuna populations, experts say (see a photo of bluefins in the Mediterranean).

"The Gulf of Mexico stock of mature bluefin tuna went through a dramatic decline, and it now could be only around 10 percent of 1970 levels," said study co-author Andre Boustany, a marine biologist from Duke University in North Carolina.

But right now "the mixing of tunas in foraging grounds makes it really hard to set proper quotas, because a particular fishery might take fish from both east and west stocks," Boustany said.

Molly Lutcavage is a marine biologist from the University of New Hampshire who was not involved in the latest tagging studies.

Lutcavage had previously observed diverging migration paths of tuna tagged off Nova Scotia, Canada—a case that turned out to be very similar to the Irish study.

"We tagged two 600-pound [272-kilogram] bluefin tuna in the same location," Lutcavage said. "One traveled to the Gulf of Mexico, one went way off the Spanish coast."

"Eleven months later their pop-up tags reported from Nova Scotia within 25 miles [40 kilometers] of the spot we originally tagged them," she said.

"An outstanding question is whether bluefin tuna spawning takes place in a broader area outside the well-known spawning sites in the Gulf of Mexico and Mediterranean."

Tagging Technology

Answering such questions is one of the goals of the Tag-A-Giant Foundation (TGF), which helped produce the study of tuna off Ireland.

The private research group is dedicated to studying bluefins and developing sustainable fishing practices.

So far the program—a sister effort to the ten-year CoML survey—has tagged more than 1,300 tuna.

Each tuna is individually caught and 6-inch (15-centimeter) tags are either implanted in the body cavity or attached to the surface of the skin.

The fish is kept breathing on deck by inserting a hose of salt water into its mouth.

Both internal and external tags collect various data, including body and water temperatures, depth, and light levels.

"We rely on the fishing community to turn in the internal tags when fish are caught," said Barbara Block, a marine physiologist from Stanford University and TGF's scientific advisor.

"We make it easier to turn them in by giving out a thousand dollars [U.S.] for our tag."

The external tags pop off the tuna at a preprogrammed time and float to the surface, where they automatically report back data to the researchers via satellites.

The tags are accurate to within 10 to 60 nautical miles (18.5 to 111 kilometers).

"That's pretty good considering the fish are crossing huge ocean basins," Block said.

(Related news photo: "Seals, Sea Lions, Satellites Help Map Ocean" [February 8, 2007].)

The next step in tagging technology is miniaturization, which would allow smaller fish to be tracked. Researchers also want to be able to monitor the tuna's heart rate.

"Tuna are great athletes. They are the Lance Armstrongs of the sea," Block said.

"Tracking heart rate would help us understand how they make such marvelous migrations."

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