We've known for a long time that bluefin tuna, which are prized for sushi and are being overfished, have been dwindling. Now an analysis of tuna numbers around the world finds that many other species may also be headed for trouble if catches continue to increase. Most of the populations are still healthy, but managers need to lower quotas to ensure their viability, the researchers argue.
"This is an important paper that corroborates widespread concerns about the sustainability of tuna fisheries but also offers hope that it's not too late to act," says marine ecologist Boris Worm of Dalhousie University in Halifax, Canada, who was not involved in the research.
The good news is that the overall picture is not as bleak as many researchers expected. A previous global analysis  of populations of tuna and other predatory fish, published by Worm and Dalhousie marine biologist Ransom Myers in 2003, estimated that humans had hauled out 90% of the fish. But the study was controversial  in part because it relied on reports of the amount of fish caught with one particular type of fishing gear, which may have led to an overestimate of declines.
María José Juan-Jordá, a Ph.D. student in fisheries science at the Universidade da Coruña in Spain, decided to analyze data from scientific surveys designed to determine population sizes. She and her colleagues gathered stock assessments from the regional fishery-management organizations responsible for stocks of tuna and related species in the high seas. They ended up with data for 17 tuna populations, including the major commercial fisheries, and for nine related species such as mackerel.
Populations had declined by an average of 60% since 1954 , the team reports today in the Proceedings of the National Academy of Sciences. "Maybe there is some good news that the populations are not as bad as previously thought," Juan-Jordá says. But she cautions that some of these populations are very precarious. Consistent with other studies, the study found that bluefin tuna in the Atlantic and Southern oceans and albacore tuna in the western Atlantic Ocean have clearly been overexploited, with their numbers reduced by more than 90%.
The other 22 populations of tuna and mackerel, including most of those used for canned tuna, remain in much better shape. This is due in part to their biology—tropical tuna grow and reproduce faster than Atlantic tuna, for example. However, demand for tuna continues to rise, as does the number of vessels chasing the fish, so even the healthy populations are shrinking.
There is no room for complacency, the researchers say, because many populations are close to being overfished. Timothy Essington, a marine ecologist at the University of Washington, Seattle, agrees. "The population trends are downwards, so now is a prime time to act to constrain further growth in these fisheries to keep them at sustainable levels." Given the uncertainty in stock assessments, Juan-Jordá recommends that fishery managers set lower targets for catches. "It's very easy to go over or under," she says. "You have to be precautionary."