Even though there are about 5,000 marine protected areas (MPAs) around the planet, less than two percent of the world’s oceans are fully protected. The number of MPAs is increasing rapidly, but the benefits they generate are still difficult to predict. It would be nice to know if the ones we've already created are effective.

With such a small percentage of our seas set aside for safekeeping, understanding what makes an MPA effective becomes critical for conservation. A team of researchers, led by Graham J. Edgar from the University of Tasmania, set out to uncover the factors that determine an MPA's success. As a byproduct of these efforts, they learned that successful MPAs better protected large fish, which were declining by 80 to 90 percent outside the areas. But they also found very few of the existing MPAs could be considered successes.

Protected areas suffer from similar issues to endangered species in that sometimes the protection exists solely on paper. These “paper parks” lack the funding and manpower to enforce rules against illegal harvesting. Other issues, such as regulations that allow harmful harvesting methods or the movement of animals between the reserve and waters that lie outside its boundaries, can also influence whether MPAs live up to their full potential.

These socioeconomic and biological factors make it difficult to predict the conservation benefits of new MPAs, as success is variable among even well-designed ones. Unlike previous studies, which only assessed one factor at a time, Edgar’s new survey looked at how five key features interact to influence success: degree of protection, level of enforcement, age, size, and proximity.

Using data collected from dive surveys at 87 MPAs around the world, the team looked at how biomass, abundance, and diversity of species varied among each reserve. When parks had a combination of five features, the conservation benefits of MPAs increased exponentially. These key features were: fully protected (no extractive activities allowed), well enforced, old (more than 10 years), large (over 100 square kilometers), and isolated by deep water or sand.

MPAs with four or five of these key features were regarded as effective, while MPAs with fewer than three of them appeared to have nearly no value for conservation. Of the 87 MPAs investigated, only four possessed all five key features while just another five had four of them. Together, these MPAs account for 10 percent of the total surveyed, which the authors conclude probably overstates the true proportion of effective MPAs worldwide.

While all five factors are hard to achieve—only a small subset of MPAs are likely to qualify as large—the authors say that most MPAs could achieve the other four. Yet in a related News and Views article, Benjamin S. Halpern of the University of California at Santa Barbara points out that most managers don’t have the option of placing protected areas in isolated locations. Unfortunately, that was the most significant factor in determining a reserve's effectiveness. Either way, the path forward is clear: stakeholders, managers, and politicians need to work together to hit as many of these five goals as possible in future MPAs.

The study was also telling in that it used effective MPAs as a standard for what unfished environments look like, allowing the first global assessment of the magnitude of fishing effects. Based on these numbers, the authors estimate that fish biomass has declined about two-thirds from historical baselines as a result of fishing.

For large fish and sharks, the reduction in biomass was even greater: 80 percent of large fish, 84 percent of groupers, 85 percent of jacks, and 93 percent of sharks have apparently been removed from reefs by fishing. The researchers note that a reduction of 80 percent coincides with the threshold value used to categorize species as critically endangered on the IUCN Red List.

Applying Red List thresholds to exploited fish stocks is controversial, largely due to a difference in beliefs as to whether fish should be considered wildlife or commodities. So the researchers simply state that the high number of large species included in this decline indicates that countless species of fish are likely threatened and that effective MPAs will need to have a major role in safeguarding many of these species.

Nature, 2014. DOI: 10.1038/nature13022 (About DOIs).