High-tech determines tuna's migrations

Aug 14, 2007 07:08 GMT  ·  By

I do not know who invented canned tuna, but I guess he/she did it for a workmate coming to annoy you with all those scents at the meal time. Anyway, tuna is one of the most prized commercial fishing species and in Japan they pay fabulous prices for this fish, highly appreciated for making sushi.

The species of tuna are so sought after, that most of them experience a worldwide decline.

Now, new studies employing high-tech electronic tags are shedding light on the migration and spawning secrets of Atlantic bluefin tuna, the largest tuna species: up to 4,58 m (15 ft) long and 684 kg (1,520 pounds) heavy.

One international team tracked two giant bluefins tagged off the Irish coast. The two individuals swam to opposite sides of the Atlantic Ocean, at over 3,000 mi (5,000 km) apart. One tuna made 3,730 mi (6,000 km) southwest, reaching waters about 186 mi (300 km) northeast of Cuba, while the other moved off the coasts of Portugal.

These separate routes confirm the hypotheses that two distinct tuna populations forage in common grounds but breed thousands of km apart in the Gulf of Mexico and the Mediterranean.

An American team tagged 28 bluefin tunas that breed in the Gulf of Mexico. The fish preferred breeding areas with a steep continental shelf and temperature between 75 and 81 ?F (24 and 27 ?C). This means that the smallest changes in ocean temperatures will affect the bluefin's reproduction.

This tuna is "data deficient" on the list of the World Conservation Union's Red List of Threatened Species, meaning that a lot of information is missing in what concerns the species' population and distribution, crucial to determine its risk of extinction. Anyhow, the populations are going down due to overfishing and some historic tuna populations have been wiped out as recently as the 1960s.

A research made at the Census of Marine Life (CoML) tracked down the rise and fall of a bluefin fishery founded in northern Europe in 1910. Commercial and sport fisheries in the same area failed by the early 1960s forever.

"The revelation that bluefin tuna were present in such large numbers in northern European waters is remarkable. It is shocking that they are no longer present [in that region] in anywhere near as large numbers" said co-author Brian MacKenzie of the Technical University of Denmark.

The same phenomenon can be currently observed in other tuna populations.

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

"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.

"We tagged two 600-pound [272-kilogram] bluefin tuna (off Nova Scotia, Canada). 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 mi [40 km] of the spot we originally tagged them. 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." said Molly Lutcavage, a marine biologist from the University of New Hampshire.

This is the goal of the Tag-A-Giant Foundation (TGF), a private research group dedicated to the investigation of the bluefins for the development of sustainable fishing practices. By now, over 1,300 tunas have been tagged. The tags collected various information, from body and water temperatures, to depth and light levels.

"We rely on the fishing community to turn in the internal tags when fish are caught. We make it easier to turn them in by giving out a thousand dollars [U.S.] for our tag." said Barbara Block, a marine physiologist from Stanford University and TGF's scientific advisor.

External tags detach of the tuna at a certain time and float to the surface, where they automatically send back data to the satellites, which re-transmit them to the scientists. Newer tags could monitor the tuna's heart rate.

"Tuna are great athletes. They are the Lance Armstrongs of the sea. Tracking heart rate would help us understand how they make such marvelous migrations." said Block.

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Breeding migrations of the tuna
Tagging a giant tuna
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