Marine protected areas (MPAs) are a popular tool for helping the recovery of devastated fisheries. But a vast expansion of reserves around Australia’s Great Barrier Reef shows that protected areas may not necessarily boost catches in well-managed fisheries. Instead, they slashed profits for years.

“The declines [the researchers] find are huge,” says Martin Smith, an economist at Duke University in Durham, North Carolina, who was not involved in the study. For the Great Barrier Reef, he says: “The real benefit of marine reserves is not the fishery, but the broader ecosystem.”

In 2004, the Australian government drastically increased protection of the Great Barrier Reef Marine Park, safeguarding a broader range of habitats, including trawling grounds for tasty crustaceans known as prawns, or shrimp. The percentage of the park closed to fishing increased from about 5% to 33%. At the same time, the state of Queensland designated an additional protected zone nearby. In total, 117,000 km2 were placed off-limits. Authorities assured the fishing industry that its losses would be minimal: They predicted catches would drop just 10% and then recover, in as little as 3 years for short-lived species such as prawns.

Was it too good to be true? Four fisheries scientists—Rick Fletcher at the Department of Fisheries Western Australia and colleagues at the University of Canberra and the Queensland Department of Agriculture, Fisheries and Forestry—decided to investigate what happened. They examined the catch data from commercial fishing vessels before and after the closure. For good measure, they also looked at comparable fisheries north and south of the Great Barrier Reef.

All told, vessels have been catching 39% less seafood—roughly 4500 metric tons—in the Great Barrier Reef than before the closure, Fletcher and colleagues report in a paper published online ahead of print in Ecological Applications. (There was no change in the areas to the north and south.) Annual revenue fell by AU$58 million, rather than the predicted AU$13 million loss. Although some studies have shown that fish populations increased inside the reserves, the fishing outside hasn’t seen a benefit—even 9 years later. Over the past decade, the Australian government has paid out about AU$214 million in compensation to the industry. “The results to date suggest that the predictions were, at least, overly optimistic,” the authors conclude.

Officials at the reef part are mulling the results. "The scientific paper raises a number of issues that we will consider and analyse further," a reserverepresentative wrote ScienceInsider in an e-mail, adding that there are no plans to further expand the no-take zones.

The finding dismays fisheries biologist Daniel Pauly of the University of British Columbia (UBC), Vancouver, in Canada, a longtime advocate of MPAs. Pauly calls the new study well done, but points out that many reserves elsewhere have benefited people who fish. When a fishery has been exploited only modestly, as in the Great Barrier Reef, he explains, it can take longer for the numbers to increase. (Fish in a dense population must compete harder with each other for resources, for example, than do those in a population with only a few scattered survivors of the hooks and nets.)

One factor, Fletcher says, is that many species on the Great Barrier Reef require very specific habitat and tend not to stray far once they mature. If the fishing restrictions span these patches of habitat, that could limit the “spillover” into the waters still plied by fishing boats.

Ray Hilborn, a fisheries biologist at the University of Washington, Seattle, agrees that MPAs will boost harvests when fisheries are seriously depleted, but probably not otherwise. “To me the bottom line is that marine conservation is best achieved by working with fishing groups to manage fisheries well, not by locking up large parts of the ocean,” he wrote in an e-mail. But Pauly counters that reserves are a more feasible strategy for the many countries that, unlike Australia, have overfished their resources and lack capacity for effective management.

As for the Great Barrier Reef, Dirk Zeller, a fisheries ecologist at UBC, Vancouver, says officials at the reserve erred by basing their predictions on research from highly depleted fisheries, thus creating a too-rosy picture of the impact on fisheries. “I think they did make a strategic mistake,” he says. But Zeller believes that the expanded reserves will benefit fisheries in the long run by creating a large network of habitat that will ultimately be more resilient to other stresses, such as pollution and climate change. “I think the [the reserve] is doing the right thing.”

If the closures help preserve the biodiversity of the reef, Smith adds, then society might judge the compensation paid to the fishing industry to be money well spent. “It could be a good deal,” he says. “The Great Barrier Reef—who hasn’t heard of that?”