It weighs half a ton and tastes great in sushi, but can the bluefin tuna swim across the Atlantic? A new large-scale study shows that many of the enormous fish, once thought to be provincial, do indeed swim all the way from the U.S. East Coast to the Mediterranean--and back, in some cases. The results raise questions about decades-old policies of fisheries management.
Bluefin tuna that spawn in the Gulf of Mexico are larger and mature later than bluefin that spawn in the Mediterranean Sea. That's why the International Commission for the Conservation of Atlantic Tunas (ICCAT), which sets fishing rules, treats the two as separate populations: Quotas for western tuna populations, which have suffered more from overfishing, are an order of magnitude times lower. But researchers have suspected for decades that the two tuna populations may be linked, because European fishermen often catch fish tagged off the U.S. coast.
In the 17 August issue of Science, Barbara Block and her colleagues at Stanford University report results from the first 5 years of a long-term project to track tuna migration patterns. Beginning in 1996, they marked 377 fish off the North Carolia coast with tags that record location, depth, and water temperature every 2 minutes. (Initially, they implanted tags that fishermen return for a reward, but gradually, they switched to tags that are strapped to the fish for up to a year before they automatically detach, bob to the surface and transmit their stored data.) Data from 147 fish showed that migration across the ocean is common: About one-third of fish with the implanted tags were caught by fishermen in the eastern Atlantic and Mediterranean. Some fish even swam to European waters and back.
Researchers say that the study provides the strongest evidence yet that the mingling of tuna populations is common. New England Aquarium biological oceanographer Molly Lutcavage hopes the results will convince ICCAT, which has doubted the importance of transatlantic mixing, to take this process into account. "This should put the skepticism to rest," she says.