The effectiveness of marine reserves compromised by human proximity

The ecological benefits of marine reserves located close to man are limited. This is the finding of researchers from the University of Montpellier, IRD and the University of New Caledonia, in association with CNRS, who studied 1,800 coral reefs, including 106 located in 20 marine reserves. This work is published in PNAS.

A grey reef shark swims over a reef in a large protected area far from humans. Photo credit: Nick Graham

Marine reserves: a tool to combat overfishing

Coral reefs are among the marine ecosystems most severely impacted by human activity, resulting in a loss of biodiversity and biomass (total mass of organisms). To combat direct human pressures, notably overfishing, more than 2,000 marine reserves have been set up, covering over 6% of the world's reefs. Some are established on reefs very close to man or coastal settlements, others on reefs isolated from any human presence. This second conservation strategy is often criticized: why preserve isolated ecological systems rather than protect and restore systems that have been severely degraded by human contact?

It is against this backdrop that this scientific study has been conducted by an international consortium since 2016. Its aim: to estimate fish biomass and the presence of top predators - such as sharks or large piscivores - in marine reserves, in order to assess their effectiveness. These species play an essential role in the proper functioning of coral reefs: an indispensable link in the food chain, they accelerate the recycling of nutrients and eliminate diseased individuals.

Partial protection of ecosystems close to man

The worldwide study was carried out on 1,800 coral reefs, including 106 located in marine reserves. It shows that setting up reserves close to man mitigates human pressures, but does not completely eliminate them, particularly those linked to fishing.
" Overall, top predators are absent from over 70% of coral reefs. They are present on less than 1% of reefs close to man. In contrast, they are present on 59% of isolated reefs," explains David Mouillot, one of the researchers coordinating the study. Restoring populations of top predators such as sharks via small-scale reserves located close to anthropized coasts therefore seems illusory. The best way to preserve them remains the implementation of protection measures at isolated sites.

For fish biomass, the situation is somewhat different. Protection seems optimal in reserves where human pressure is at an intermediate level.
" Different conservation strategies can benefit coral reefs," concludes David Mouillot. " Our study suggests maintaining a wide diversity of reserves with very varied objectives, and not neglecting the protection of sites far from man ".