Smart tags show unexpected tuna trips

Bluefin tuna power across the Atlantic much more often than scientists had expected, a find that could make life messy for fishing-rights negotiators.

A bluefin tuna can swim across the Atlantic in 40 days. Gilbert Ryckevorsel/Science

The greater mobility of bluefins is just one insight to come from a new generation of fish-tagging experiments, explains Barbara A. Block of Stanford University. Scientists used to rely on tags that merely identify the animal. Now, electronic tags can report much about what a fish does between tagging and recapture. In the Aug. 17 Science, Block and her colleagues release results for Atlantic bluefins wearing two new models.

One of 15 species of tuna, these fish can grow into 10-foot-long, 1,500-pound torpedoes selling for up to $45 a pound. By the early 1980s, bluefin numbers had dropped alarmingly. Recovery of the fish population depends in large part on country-by-country fishing quotas doled out by the International Commission for the Conservation of Atlantic Tunas (ICCAT), based in Madrid. “The arguments over these fish are legendary,” says Block.

To better understand the fish, Block and her colleagues worked with sports and commercial fishers in 1996 to tag 377 bluefins in the western Atlantic. So far, the team has recovered 49.

Nearly a third turned up in the eastern Atlantic or the Mediterranean. ICCAT regulates eastern and western fish separately, assuming that only about 2 percent switch sides. John J. Magnuson of the University of Wisconsin-

Madison, coauthor of a commentary in the same issue, says the discovery of greater mixing emphasizes the two fisheries’ interdependence and the need to stop overfishing in the eastern Atlantic.

The two new tag types together provide day-by-day or more frequent records of location, dive depths, and body and water temperatures. Eastern and western fish seem to mingle in food-rich areas but retire to separate spawning zones. Block estimates that travel to the spawning grounds may take 2 weeks or so. She identified shallow dives in warm water that appear to be the first recorded bluefin spawning dives. Also, the data suggest that the fish spawn near the Bahamas, as well as in previously recognized areas of the Gulf of Mexico and the Mediterranean.

Magnuson, who chaired a National Research Council panel that in 1994 pleaded for more tuna data, calls the new study “a major contribution.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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