One of the few bright spots in the struggle to protect the world’s fragile oceans has been the rapidly increasing number of “marine-protected areas,” places where fishing is limited or banned and where, presumably, depleted species can recover by simply being left to themselves. The benefits of hands-off environmental protection may seem self-evident. But creating a preserve and rebuilding a healthy ecosystem are not necessarily the same thing. A recent study published in Nature found that, more often than not, marine-protected areas don’t work as well as they could.

Researchers with the University of Tasmania studied 87 marine-protected areas in 40 countries worldwide, and found that 59 percent of the areas were no better off than areas where fishing was allowed. The reasons for failure varied, but they boiled down to this: Not all marine-protected areas are alike. Some allow fishing; others forbid it. Some are managed well; others are managed badly. Some are relatively intact; others have been left barren by generations of overfishing.

The researchers identified five essential characteristics of the most successful marine-protected areas: These areas were designated “no take” (allowing no fishing whatsoever), their rules were well enforced, they were more than 10 years old, they were bigger than 100 square kilometers, and they were isolated by deep water or sand. Compared with regular fished areas, the areas that had four or five of those attributes had a far richer variety of species, five times the biomass of large fish and 14 times the biomass of sharks, which are indicators of ecological health.

Most underachieving marine sanctuaries had only one or two of these magic factors, and thus “were not ecologically distinguishable from fished sites.” The four sanctuaries lucky enough to have all five characteristics were isolated areas in the oceans off Costa Rica, Colombia, New Zealand and Australia. The “coral triangle” of Southeast Asia also got high marks, but it did not have as great an array of large species as its more isolated counterparts.