The migration patterns of giant Atlantic bluefin tuna have been unravelled through satellite tagging — and the results suggest that policies for managing the overfished species require an urgent rethink.

Bluefin are huge marine predators that can weigh up to 650 kilograms and are prized for their flesh. They are the most valuable fish in the ocean — in Japan, single fish can command prices of up to US$100,000. In recent decades, fishing boats have scrambled to catch as many as possible by trap, net, harpoon or long line, and as a result bluefin numbers have fallen by 80% or more since 1970.

Size matters: unequal fishery quotas may be endangering the giant bluefin tuna. Credit: G. VAN RYCKEVORSL

Atlantic bluefin (Thunnus thynnus) have two spawning grounds — the Gulf of Mexico and the Mediterranean. The gulf is the more heavily fished, so it is given greater protection by the International Commission for the Conservation of Atlantic Tunas (ICCAT), based in Madrid, Spain, which manages tuna stocks in international waters. ICCAT has set bluefin fishing quotas based on an imaginary line down the middle of the Atlantic: fishermen are allowed to catch 32,000 tonnes of bluefin a year to the east of the line, but only 3,000 tonnes to the west.

Scientists and conservationists have long been concerned about this policy, because they suspect that fish cross the dividing line when feeding in the open ocean. But until recently they had no way to prove it.

Now, a tagging study published in Nature (see page 1121) provides detailed information about bluefin movements. Getting these data wasn't easy. Over a nine-year period, Barbara Block of Stanford University, California, and her colleagues caught and tagged nearly 800 bluefin tuna. Global positioning data doesn't work when the fish are on deep dives, so the tags also collected detailed information such as light level and water temperature, which enabled researchers to piece together the course of each fish. Data were beamed back to a satellite, or retrieved after the fish was caught (the team offered a $1,000 reward for each one returned).

Using the data from the tags, Block and her colleagues confirm that there are two separate populations of bluefin in the Atlantic, and each returns to the spawning ground from which it came. Crucially, the researchers also find that the two populations merge when foraging in the open ocean, feeding on both sides of ICCAT's line. This means that fish from more fragile western stocks are being caught as part of the larger quotas in the east.

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Fishing for complements: the International Commission for the Conservation of Atlantic Tunas divides bluefin stocks into east and west populations, but there is now evidence that they mix while foraging.

The researchers further showed that longline fishermen hunting the smaller and more abundant yellowfin tuna (Thunnus albacares) in the Gulf of Mexico are probably having a devastating effect on spawning bluefin (see ‘Bayou fleet brings bluefin blues’). Although there have been reports of bluefin being caught accidentally in the area, it has been hard for scientists to gauge the scale of the problem.

Block's team combined their location data on the bluefin in the gulf with information from the Maryland-based National Marine Fisheries Service (NMFS) on the movements of fishing boats. They show that the boats are dropping their lines in areas where bluefin gather.

Block hopes the tagging data will be used to set policies that will protect the fish better. “If we don't do something, bluefin stocks will collapse,” she says. Block suggests dividing the Atlantic into more than two quota zones, and she wants longline fishing banned in certain parts of the Gulf of Mexico during the bluefin spawning season.

In the past, ICCAT has seemed reluctant to alter its policy, citing the need for more accurate accounts of fish movements. Scientists and conservationists alike hope that this study will fit the bill. “This is a very powerful paper,” says John Magnuson, an ecologist at the University of Wisconsin, Madison, who has studied bluefin stocks. “New tagging methods mean that what was simply a matter of conjecture is becoming visible to us.”

“It means we need to rethink bluefin management for the entire Atlantic,” adds Paolo Guglielmi, a Rome-based biologist for the conservation group WWF.

The NMFS, which controls fishing practices for much of the bluefin's spawning grounds in the northern Gulf of Mexico, was a part-funder of Block's study and will use the research to revise quotas for bluefin and other migratory Atlantic fish. “We are taking a fresh look,” says NMFS economist Rebecca Lent. “We welcome this scientific information.”

It is less clear whether ICCAT will change its policies when it sets new bluefin quotas next year. ICCAT officials met in Fukuoka, Japan, from 20 to 23 April to discuss management strategies for tuna and other migratory species. The meeting was partly sparked by Block's preliminary data, which were presented last May in Marseilles, France.

Officials have declined to discuss the outcome of the meeting. But ICCAT's chairman, Masanori Miyahara, cautions that the tagging data should be combined with studies of bluefin ovaries to pin down precisely when and where the fish are spawning.