A new study by Tel Aviv University reveals significant ecological damage to many marine protected areas (MPAs) around the world. A strong “edge effect” was observed, resulting in a 60% reduction in the fish population living on their outer edges (1-1.5 km), compared to the core areas. The “edge effect” significantly diminishes the effective size of those areas, and largely stems from human pressures, first and foremost overfishing at their borders.

Marine protected areas were designed to preserve marine ecosystems, and help to conserve and restore fish populations and marine invertebrates whose numbers are increasingly dwindling due to overfishing. The effectiveness of the protected areas has been proven in thousands of studies conducted worldwide. At the same time, most studies sample only their “inside” and “outside”, and there still is a knowledge gap about what happens in the space between their core and areas around them that are open for fishing.

The study was conducted by Sarah Ohayon, a doctoral student at the laboratory of Prof. Yoni Belmaker, School of Zoology, The George S. Wise Faculty of Life Sciences, and The Steinhardt Museum of Natural History at Tel Aviv University. The study was recently published in the Nature Ecology & Evolution Journal.

The “Edge Effect”

When a protected area functions properly, the expectation is that the recovery of the marine populations within it will result in a spillover, a process where fish and marine invertebrates migrate outside its borders. In this way, the protected area can contribute not only to the conservation of marine nature, but also to the renewal of fish populations surrounding it that have dwindled due to overfishing.

To identify the dominant spatial pattern of marine populations from within the protected areas to the surrounding areas (that are open for fishing), the researchers analyzed marine populations from dozens of protected areas located in different parts of the oceans. 

“When I saw the results, I immediately understood that we are looking at a pattern of edge effect”, says Ohayon. “The edge effect is a well-studied phenomenon in terrestrial protected areas, but surprisingly it has not yet been studied empirically in MPAs. “This phenomenon occurs when there are human disturbances and pressures around the protected area, such as hunting/fishing, noise or light pollution that reduce the size of natural populations within the protected areas, close to their borders”.

No-Take Marine Protected Areas Are Too Small

The researchers found that 40% of the no-take MPAs (areas where fishing activity is completed prohibited) around the world are less than 1 km², which means that entire area is likely to experience an edge effect. In total, 64% of all no-take MPAs in the world are smaller than 10 km² and may hold only about half (45-56%) of the expected population size in their area compared to a situation without an edge effect. These findings indicate that the global effectiveness of existing no-take areas is far less than previously thought.

It should be emphasized that the edge effect pattern does not eliminate the possibility of fish spillover, and it is quite plausible that fishers still enjoy large fish coming from within the protected areas. This is evidenced by the concentration of fishing activity at their borders. At the same time, the edge effect makes it clear to us that marine populations near the borders of the protected areas are declining at a faster rate than the recovery of the populations surrounding them.

Buffer, Enlarge and Enforce

The study findings also show that in protected areas with buffer zones around them, no edge effect patterns were recorded, but rather a pattern consistent with fish spillover outside their borders. Additionally, a smaller edge effect was observed in well-enforced protected areas than in those where illegal fishing was reported.

“These findings are encouraging, as they signify that by putting buffer zones in place, managing fishing activity around marine protected areas and improving enforcement, we can increase the effectiveness of the existing protected areas and most probably also increase the benefits they can provide through fish spillover”, adds Ohayon.

“When planning new marine protected areas, apart from the implementation of regulated buffer zones, we recommend that the no-take MPAs targeted for protection be at least 10 km² and that their shape be as round as possible. These measures will reduce the edge effect. Our research findings provide practical guidelines for improving the planning and management of marine protected areas, so that we can do a better job of protecting our oceans.”