There is really only one ocean. But over time, it's been cordoned off into various regions, with the most fluid of boundaries. Today, geographers recognize more than 50 seas within five major oceans. There are also more than 150 Exclusive Economic Zones where individual coastal nations exercise sovereignty up to 200 nautical miles from their shores. 

Now, thanks to the rise of marine protected areas (MPAs), the global ocean is becoming increasingly partitioned. The term is a catchall for sites like ocean sanctuaries, marine parks, and no-fishing zones—scattered havens where marine life is supposed to thrive, free of human interference (or, at least, subject to limited human interference). The world's 5,000-plus MPAs include national treasures like the Galápagos and the Great Barrier Reef, but they also include small "fishery-management zones" that are undistinguished except for fine-print prohibitions on certain types of fishing gear. Even the Great Barrier Reef is open to extractive activities like trawl fishing and deep-sea dredging.

Only 2 percent of the ocean is currently covered by some sort of MPA. (In contrast, 12 percent of the world's land is protected in national-park systems and wildlife preserves.) And only half of that 2 percent—a mere 1 percent of the ocean—is classified as "no-take," or completely closed to fishing and other extractive activity.

The international conservation community has long heralded the role of MPAs in protecting ocean resources. But amid growing concern over how to save the seas from overfishingacidification, and "dead zones," ecologists and economists are beginning to ask a fundamental question: Are these special conservation zones actually achieving anything?

A hypothetical visualization of overlapping MPAs, courtesy of the Pacific Fishery Management Council.

Such queries are especially important in light of news that Palau, a small island nation in Micronesia, intends to turn its entire territory into one giant marine reserve. Commercial fishing would be banned from Palau's coasts to the outer reaches of its Exclusive Economic Zone—in sum, an area of about 230,000 square miles. Palau, it seems, has decided that attracting more tourists and scuba divers is worth shunning the commercial fishing industry.

Palau's tiny islands are not visible on this map, which shows, in the central highlighted section, Palau's Exclusive Economic Zone and proposed marine sanctuary. The sanctuary would cover an area as large as France. (

In March, two economists at Columbia University put some numbers to the upside of MPAs, arguing in a paper for the National Bureau of Economic Research that these zones are only worthwhile if they make up at least 8.5 percent of a country's Exclusive Economic Zone. That's the tipping point at which the costs of implementing MPAs are outweighed by the benefits they confer through boosts to the country's tourism and fishing industries.

But thinking about conservation only in terms of percentage of area covered may not be the best strategy. In February, the marine geographer Rodolphe Devillers and other researchers published a study asserting that the majority of MPAs are "residual"—established in locations of convenience rather than in places where they could actually do good. Creating a "no-take" marine reserve sounds like a significant action, unless you choose to locate it in a corner of the ocean where no one wants to fish or drill, and where there are few species in need of protection. 

According to Devillers and his co-authors, this is a consequence of politics, industry influence, and the widely accepted use of "area size" as a measure of success. When governments like Australia's try to enact MPAs in a way that pleases everyone, their goals are are to minimize costs, avoid conflict with commercial interests, and still be able to boast about protecting "x" square miles of habitat. A Case in point: In early April, the Australian government "created Australia’s largest fully protected marine reserve near two far-flung islands, in a move which environmental groups say will help safeguard rare whale species." As The Guardian reported:

The reserve now spans 71,200 sq km of ocean. Heard and McDonald Islands, an Australian external territory located 4,100 km south-west of Perth, are barren, uninhabited outposts considered among the most remote places on Earth.

All this isn't to say that the Heard and McDonald Islands are devoid of natural resources. In fact, according to the MPAtlas database, the territory serves as an important breeding habitat for seabirds and seals, and the surrounding waters act as a "nursery" for commercially harvested fish species. It's just that the biggest threat these barren, uninhabited Antarctic islands appear to be facing isn't exploitation by commercial fishermen or oil companies, but rather climate change. And that's not something a marine reserve can protect against.

Whether an MPA advances conservation or is mere "window dressing," as the Devillers paper puts it, depends on a combination of variables. In February an article in Nature identified five factors as "essential" to the success of any MPA, with "success" measured by the biomass of all fish and the diversity of species in an area.

1. No-take: Fewer than half the world's MPAs have this feature, which prohibits all harvesting of natural resources.

2. Enforced: Combating illegal harvesting costs money. It means making sure that people don't dive, fish, or dump in areas where those activities are restricted. And that means hiring people to monitor the areas—on foot, by boat, or with drones

3. Old: The benefits of MPAs take time to materialize. The most successful ones are at least 10 years old, according to the study. 

4. Large: The best MPAs are more than 100 square kilometers in area.

5. Isolated: According to the study, an MPA is more likely to be successful if it is separated from similar habitat by deep water or sand. This makes protected fish more likely to stay within the MPA's boundaries. But it undermines one of the primary reasons policymakers give for instituting MPAs in the first place: the "spillover effect," which assumes that as an MPA fills up with healthy fish, some of these fish will swim into neighboring (and unprotected) waters, where fishermen can scoop them up and benefit from the MPA as well. 

Among the 87 MPAs in the Nature study, fewer than 10 had four or more of these features. Fifty-nine percent "had only one or two" of these characteristics "and were not ecologically distinguishable from fished sites," wrote the authors. In contrast, the "effective MPAs" had "twice as many large fish species, five times more large fish biomass, and fourteen times more shark biomass than fished areas." 

Of course, this discussion of the effectiveness of MPAs leaves out a huge portion of the world's water. Only 42 percent of the global ocean is captured by Exclusive Economic Zones and thus under some form of national jurisdiction. The other 58 percent is no-man's-land: the high seas.

A map of the ocean's Exclusive Economic Zones (green) and high seas (blue). (PLoS Biology)

The practice of nations implementing MPAs in their respective seazones is widely established. But the question of whether the high seas should be similarly protected is just beginning to be debated. 

In late March, an article in PLOS Biology asserted that the best way to recover stocks of highly migratory—and very valuable—species like bluefin tuna is to simply close the high seas to fishing. "No-take" zones would go from covering 1 percent of the ocean to 59 percent. 

The proposal is getting serious attention. Earlier this month, the UN Ad Hoc Open-ended Informal Working Group began discussing the feasibility and desirability of such a radical measure. But as the name of the working group suggests, action on this front probably isn't coming soon.