Posted in | News | Ecology | Ecosystems

Study Outlines Effect of Overfishing on Marine Protected Areas Around the World

Recent research by Tel Aviv University discloses major ecological damage to various marine protected areas (MPAs) across the globe.

Fish Population

Fish population. Image Credit: Tel Aviv University.

A strong “edge effect” was identified within MPAs—a sharp 60% reduction in the fish population living at the edges of the MPA (up to a distance of 1–1.5 km within the MPA) when compared to core areas. The “edge effect” substantially decreases the effective size of the MPA, and largely originates from human pressures, primarily overfishing at the borders of the MPA.

Published in the journal Nature Ecology & Evolution, the research was carried out by Sarah Ohayon, a doctoral student at the laboratory of Prof. Yoni Belmaker, School of Zoology, George S. Wise Faculty of Life Sciences, and the Steinhardt Museum of Natural History at Tel Aviv University. 

MPAs are designed to protect marine ecosystems, as well as to restore and conserve fish populations and marine invertebrates whose numbers are greatly decreasing due to overfishing. MPAs’ efficiency is proven by numerous studies conducted globally.

However, most studies sample only the “inside” and “outside” of the MPAs. A knowledge gap still exists about what happens within the space between the MPA's core and the surrounding areas open for fishing.

Ohayon elaborates that when an MPA functions properly, the assumption is that the recovery of marine populations in the MPAs will result in a spillover—a process where fish and marine invertebrates migrate outside the borders of the MPA. Thus, the MPA contributes to the conservation of marine nature along with the renewal of fish populations outside the MPA that has decreased due to overfishing.

To answer the question of the dominant spatial pattern of marine populations from within MPAs to surrounding open fishing areas, a meta-analysis was conducted involving spatial data of marine populations from dozens of MPAs situated in various parts of the oceans.

When I saw the results, I immediately understood that we are looking at a pattern of edge effect. The edge effect is a well-studied phenomenon in terrestrial protected areas, but surprisingly has not yet been studied empirically in MPAs.

Sarah Ohayon, Doctoral Student, School of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University

This phenomenon occurs when there are human disturbances and pressures around the MPA, such as hunting/fishing, noise or light pollution that reduce the size of natural populations within the MPAs near their borders,” adds Ohayon.

The scientists identified that 40% of the no-take MPAs worldwide (areas where fishing activity is fully prohibited) are less than 1 km2, which means that the entire area experiences an edge effect. Totally 64% of all no-take MPAs globally are smaller than 10 km2 and might account for only about half (45%–56%) of the expected population size in their area compared to a situation without an edge effect.

The results suggest that the global effectiveness of existing no-take MPAs is far less than previously believed.

It should be stressed that the edge effect pattern does not eradicate the chance of fish spillover, and it is quite possible that fishers still enjoy large fish coming from within the MPAs. The concentration of fishing activity at the borders of MPAs serves as proof of this. Meanwhile, the edge effect makes it clear that marine populations near the border of MPAs are decreasing at a faster rate than the recovery of the populations around the MPA.

The research outcome also highlights that in MPAs with buffer zones around them, no edge effect patterns were recorded; however, a pattern consistent with fish spillover outside the MPA was noticed. Moreover, a smaller edge effect was noticed in well-enforced MPAs than those where illegal fishing was recorded.

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

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

Sarah Ohayon, Doctoral Student, School of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University

Journal Reference:

Ohayon, S., et al. (2021) A meta-analysis reveals edge effects within marine protected areas. Nature Ecology & Evolution.

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