How we reviewed available evidence

There’s a mountain of research on marine protected areas. So we sampled a small portion of the literature, and shortlisted 42 peer-reviewed scientific studies: 39 were selected from the first 1,000 relevant hits on Google Scholar and three were added following consultation with experts. (Read more about our methodology and selection criteria here; you can access all 42 studies we reviewed here. Additional studies may be added to the infographic after this story’s publication.) These studies look at several specific environmental, social and economic outcomes of marine protected areas, and evaluate whether those outcomes were better, the same as, or worse than for unprotected sites or for the same site before it was protected. Our review isn’t exhaustive and our search terms have focused specifically on certain outcomes, rather than on all the outcomes of marine protected areas that have been studied.

We also talked to seven marine experts to supplement our understanding of some of the broader trends and to fill in potential gaps.

Information gaps

The body of scientific literature on marine protected areas may be vast, but the quality of the studies within it is highly variable and riddled with information gaps.

One such gap, experts say, is that marine protected areas sometimes lack clear objectives when they are created. Was the marine protected area designed to protect a particular species? To improve fish catch in nearby fishing grounds? Or to attract tourists? “Not all marine protected areas can or should be expected to meet all possible objectives,” said Benjamin Halpern, a marine biologist at the University of California, Santa Barbara.

This lack of clear objectives can be problematic. Not only does it make the evaluation of a marine park’s effectiveness hard, but it also can lead to unrealistic expectations of potential benefits for biodiversity, fisheries or human welfare that aren’t always achieved, said Natalie Ban, a marine biologist at the University of Victoria, Canada.

Historical environmental and socioeconomic data from before marine protected areas were established is often unavailable, making it difficult to identify changes with certainty. Moreover, a large chunk of studies are snapshots that measure effects in a single season or year, or at a single location, making their results difficult to generalize.

“The most critical gap with respect to ecological outcomes, in my opinion, is sufficient data over time to determine how, how quickly and for how long potential ecological effects are occurring,” Halpern said. “Most studies look at two or just a single point in time. Monitoring change over time provides much richer understanding of how ecosystems within and outside marine protected areas are responding, which in turn helps us understand why they are changing.”

Many studies also do not compare marine protected areas with comparable control sites — that is, unprotected areas of the ocean that are otherwise equivalent to the protected areas.

This lack of baseline data, long-time-scale information and good control sites can be a major roadblock to teasing out the effects of a marine protected area from other rival factors: Were the changes in fish populations caused by the creation of the marine protected area or by other ocean conditions, fishing pressure or the park’s location? Within our literature sample, for instance, only three studies accounted for multiple alternative explanations.

“Not enough thought is given to the monitoring design when [a] marine park is being established,” David Gill, a marine researcher at George Mason University and visiting scholar at the NGO Conservation International, told Mongabay. “The gold standard would be to conduct surveys both inside the marine protected area before it is established and in comparable unprotected sites, and then continuously monitor both sets of conditions over time to see if the changes are just because of the marine protected area, or something else.”

But achieving this gold standard is tricky, Gabby Ahmadia, a marine biologist with the World Wide Fund for Nature (WWF), told Mongabay.

Take the ongoing monitoring program at the Bird’s Head seascape in West Papua province, Indonesia. Ahmadia, whose team has been surveying a network of 12 multiple-use marine protected areas in the seascape, told Mongabay that selecting and then monitoring control sites is uncharted territory. “Nobody had done this before, so it wasn’t something that I could look at in other marine protected areas,” Ahmadia said.

“You can use all your factors and environmental indicators and social criteria to say these would be good control areas, but you can only guess what’ll be underwater,” she said. “Sometimes you go underwater and you think no, this is not a good reef to survey. Or there have been some days when we were surveying sites and the reefs were dead. We spent two to three days searching for intact reefs but we couldn’t find any. It was definitely a learning experience. But we were able to get enough control sites that it did work.”

However, there are a growing number of meta-analyses and systematic reviews; our sample included 10. These distill and explain some of the broader trends associated with marine protected areas.

Marine protected areas can benefit marine life, but impacts are uneven

By closing off areas of the ocean to fishing and other extractive activities, marine protected areas, especially marine reserves that ban all or most such activities, are expected to help species recover. A great deal of research has looked into whether they do, although much of it is not of the highest quality: many studies do not have rigorous designs and cannot definitively attribute observed changes to the marine protected area itself.

“You just need a caveat that while some of what you’re seeing might be attributable to the marine protected areas, you can’t necessarily prove it beyond any doubt,” said the University of Victoria’s Ban.

Of the 29 studies that looked at environmental outcomes within our sample, 19 were either case reports or simple inside-versus-outside comparisons that did not account for alternative explanations for the patterns they recorded. Only three studies considered a range of alternative factors and were more rigorous in their approach. The remaining seven were meta-analyses that looked at dozens of studies.

When considered together, though, most studies suggest that marine protected areas can be good for marine life.

In a meta-analysis published in 2003, for example, Halpern of UC Santa Barbara combined survey data from 89 studies and found that population densities, biomass, diversity and size of fish were generally considerably higher inside marine reserves than outside. He further analyzed a subset of 17 studies that had information from before the reserves were established, and concluded that fish density and biomass increased within one to three years of the creation of the reserves.

But the reserves affect different species in different ways, Halpern wrote. Slow-growing, long-lived species like cod tend to respond to protection more gradually than fast-growing, short-lived species like scallops. Similarly, heavily exploited species are likely to respond to protection more quickly than those that are not fished, because fishing — the main activity impacting their survival — is suddenly removed from the reserve.

In a more recent study published last year, George Mason University’s Gill and his colleagues compared fish data from 218 marine protected areas with matching unprotected areas to weed out potential rival explanations. The team found that, on average, fish biomass inside the protected areas was 1.6 times higher than in similar unprotected areas. “We lost about a quarter of our dataset because we did not find appropriate protected and unprotected matches, but by doing this, we were also more confident about our results,” Gill said.

Overall, researchers say that there is quite a lot of evidence to show that marine protected areas can benefit species, particularly those that have been threatened historically within the protected area boundaries.

But some recent studies have found that merely creating a marine protected area does not mean that biodiversity conservation goals will be achieved. They have to be done right in order to work.

In a study published in 2014, for example, the University of Tasmania’s Edgar used data from 87 marine protected areas collected largely by trained recreational scuba divers, and found that the levels of fish biomass in more than half of the protected areas were not very different from adjacent fished locations. These marine protected areas were simply “paper parks,” he wrote in The Conversation, “lines on the map that fail to achieve desired conservation outcomes.”

“This is a very worrying statistic in terms of biodiversity conservation goals,” Edgar told Mongabay. “I think most of the public is misled in the sense that they consider [the] establishment of a marine protected area as being an important conservation goal but the reality is that there are very little biodiversity benefits from most marine protected areas. The public is getting quite a wrong impression in terms of the safeguards for marine biodiversity.”

He deemed only 10 percent of the parks he studied “effective” based on their having very high numbers of large fish like sharks. All these protected areas had five recurring features, he found: they had no-fishing zones, were well-enforced, more than 10 years old, relatively large in area, and isolated from fished areas.

Similarly, Gill’s study found that protected areas that had adequate staff performed nearly three times better than parks that did not.

“In short, if marine protected areas, or networks of marine protected areas are large enough relative to the range size of species of interest, away from sources of stress that marine protected areas can’t control, such as land-based pollution from rivers, and well-enforced, they provide dramatic benefit towards biodiversity conservation,” Halpern said.

Our understanding of the impacts of marine protected areas on marine ecology, however, is skewed towards certain areas and species, said Alex Caveen, author of the book, “The Controversy Over Marine Protected Areas: Science Meets Policy,” who has reviewed the scientific literature on marine protected areas from 1990 to 2010. Many studies tend to focus on fish, for example, especially the commercially important ones, he said. Studies have also looked more at impacts on reef-type habitats than on other coastal habitats like mangroves or kelp forests.

Research has also tended to focus on marine reserves, experts say. Marine protected areas, however, can be of different kinds. While marine reserves tend to be more strictly protected, they comprised only 1.23 percent of global ocean as of 2016. “A lot of marine protected areas that are coming up do allow some fishing and other activities,” Ban said. “So then there is the question as to how well they protect biodiversity.”

Use the drop down menus to select data from your country of choice or to view data for a particular type of evidence. Click on the name of an outcome (such as abundance or economic benefits) to display data specific to it. Click on a square to see what evidence the outcome is based on. Download data from the 42 original studies here. Additional studies may be added to the infographic after this story’s publication. You can find this and other infographics in our Conservation Effectiveness series at ConservationEffectiveness.org. Data visualization by GreenInfo Network. 

How marine protected areas affect local people is less clear

Proponents argue that marine protected areas can have a slew of positive effects on fishing and coastal communities, including improved livelihoods from fisheries, political empowerment and new governance and job opportunities. Critics counter that marine protected areas restrict local communities’ access to their ancestral fishing grounds, make them poorer and often increase conflict between them and people involved in park management or tourism.

But much of the research that exists on this consists of case reports that depend on interviews with local people. (Within our sample of 16 studies looking into socioeconomic outcomes, 15 were case reports.)

“Such studies aren’t experimental and their results are subjective, but they are important because it matters whether the people think the marine protected areas have been effective or not,” Ban said. However, these studies don’t tell us much about whether the establishment of marine protected areas, or some other sociopolitical factor, is really what’s driving these perceptions.

In a 2014 case report we looked at, for example, researchers interviewing communities near four Thai marine protected areas found that some people perceived a decline in their livelihood from fishing and collecting shells. Others said they hadn’t seen much change, mainly because either the authorities allowed small-scale fishing inside the protected areas or did not enforce the regulations. Another case report found that communities around a marine protected area in Zanzibar, Tanzania, were generally unhappy with how the park was being managed. Benefits from tourism and other jobs related to the protected area were inequitably distributed, and the communities felt marginalized.

A few studies have found some positive effects, too. A 2010 meta-analysis that combined data from 21 studies reported that some fishing communities perceived an increase in food security after the establishment of marine protected areas. Similarly, a 2013 case report found that local communities generally perceived an improvement in economic and environmental benefits from a protected area in the UK.

Our search did not return any rigorously designed studies comparing villages around marine protected areas with similar villages around unprotected waters.

Gill agreed that only a handful of such rigorous studies exist, and pointed to one published in 2014 that looked at the social impacts of marine protected areas in Sulawesi, Indonesia. The researchers behind that study found divergent effects: Between 1997 and 2002, the protected areas appeared to increase livelihood options for people living near them compared to those living near unprotected waters, for instance. But the people living around the protected areas also rated their household well-being as lower than those in the control villages. These impacts changed after 2002 when the agencies managing the protected areas withdrew their support, the study found.

More such rigorous studies can tell us how the impacts of protected areas vary over time and across different communities, experts say, which can then potentially help managers design better marine protected areas. “If we continue to use marine protected areas, we better be sure to do it in ways that are equitable and not severely affecting the poor,” Gill said.

Net benefits for fisheries are also not well understood

In theory, marine protected areas, particularly those that are closed off to all or most fishing, can allow overfished species to mature undisturbed and produce more offspring. The offspring can then swim over the invisible boundaries to adjacent fished areas and provide bigger catches and more money for fishers. Indeed, as mentioned, a good body of research suggests that species targeted by fishers often do respond well to protection.

But do more and larger fish inside a protected area really mean higher catches outside its boundaries? Our review of the scientific literature retrieved six studies looking at this so-called “spillover,” including a systematic review published in 2016 that summarized results from 85 studies.

The systematic review found evidence of spillover in 80 percent of the studies it considered. But the researchers added a caveat: “[Twenty] percent of remaining studies that failed to provide any evidence of spillover is likely to be underestimated … because of publication bias in ecology, and specifically in marine protected area science,” where positive results are favored, they wrote.

But even if there is some spillover, is it enough to compensate for reduced fishing areas with increased fishing pressure and thus provide a net benefit to fishers?

Our review captured only one empirical study that looked at this (possibly because our review and search terms were not exhaustive). This study, published in 2001, found that the biomass of five commercially important fish families increased both inside and outside marine reserves in the Caribbean island of St. Lucia within three years of establishment. Moreover, the fish catch increased substantially — by between 46 and 90 percent, depending on the type of traps the fishers used.

The results of this single study cannot be generalized, of course. So we turned to our experts, who told us that while there are some empirical studies looking at how marine protected areas affect fisheries, most of them are not rigorous and their conclusions are mixed.

For example, a 2013 study found that fishers who worked near the Goukamma marine protected area in South Africa saw a nearly steady increase in the catch of commercially important Roman seabream (Chrysoblephus laticeps) over the 10 years since the park’s establishment in 1990. Some studies from the Mediterranean Sea have also found an increase of fish catch near marine reserves over the years.

By contrast, in a 2000 study from Kenya, researchers found that while the creation of a no-take marine protected area had led to some spillover, the fish catch was lower than it was before the park’s creation halved the available fishing area. Similarly, some researchers found that the increase in no-take zones in Australia’s Great Barrier Reef did not lead to the long-term improvements in fish catch that authorities had promised.

Most positive examples come from marine protected areas that are either very small (so the fishers lose only a small amount of their previous fishing area), or from parks located in areas where fish stocks are severely overexploited, said the University of Washington’s Hilborn.

Halpern agreed that the benefits of marine protected areas on fisheries are very context-dependent and traditional management techniques, such as seasonal fishing closures or restrictions on certain kinds of fishing equipment, sometimes come out ahead.

“In developed countries and large stocks, traditional fisheries management has been very effective, especially more recently,” he said. “In developing nations and highly diverse fisheries — as is the case with many coral reef fisheries in tropical countries — traditional fisheries management has not been as effective and marine protected areas are often a much more effective and viable strategy. But there are many counterexamples and other issues in play — in other words, context matters.”

So, wherever marine protected areas are being promoted as tools for sustainable fisheries, alternative fisheries management options must be considered, Hilborn said. “My bottom line is to first identify what our objective is, what the alternative methods to improve that is, and then getting a science-based process to evaluate the alternatives,” he said. “In some cases, the better alternative is going to be marine protected areas, in others not.”

What we’ve learned so far

The research in our sample of literature suggests — and experts generally agree — that marine protected areas do help many marine animals recover within their boundaries, especially species that have been heavily exploited by fishing. There is still room for improvement in study designs, though, and a need to fill research gaps, especially when it comes to studying certain marine habitats and non-commercial species.

Our understanding of how marine protected areas affect the well-being of local people is currently poor. Rigorous studies are rare, according to experts, and indeed our literature sample turned up mostly case reports showing varying outcomes that cannot necessarily be attributed to the marine protected areas.

Good-quality research on how marine protected areas impact fisheries is also lacking. What evidence exists is mixed, according to experts, and depends on the local context. Some marine biologists have, in fact, cautioned against making dubious promises about increasing fisheries yield when the current research cannot possibly predict it.

“Overall, I am satisfied with the research that is being done to see the ecological effectiveness of marine protected areas. I think it’s useful to keep studying that — how things change with time and protection,” Ban said. “But I do think we need a lot of emphasis on the human element: to what extent marine protected areas affect human well-being for coastal communities that rely on the oceans for their livelihoods.”

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Writer: Shreya Dasgupta, Researcher: Amy Fensome
Editors: Rebecca Kessler, Mike Gaworecki, Willie Shubert
Copyeditor: Hayat Indriyatno
Infographic: Zuzana Burivalova, GreenInfo Network

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This is part six in the Mongabay series Conservation Effectiveness. Read the other stories in the series here.

Banner image: Pennant coralfish via Max Pixel.

Editor’s note: Reviews conducted for this series are not exhaustive, and we encourage authors of research that is relevant to post links to their study in the comments section under each article.

Editor’s note 5/13/21: We have updated this story to reflect the fact that additional studies may be added to the infographic after this story’s original publication.