The environmental benefits of marine reserves located near human populations are limited. This was demonstrated by researchers from the University of Montpellier, IRD and the University of New Caledonia, in conjunction with CNRS, as they studied 1,800 coral reefs, 106 of which were located in 20 marine reserves. This work was published in PNAS on 18 June.
Coral reefs are among the marine ecosystems most impacted by Humans, resulting in a loss of biodiversity and biomass (total mass of organisms). To combat direct human pressures, including overfishing, more than 2,000 marine reserves, covering more than 6% of reefs on a global scale, were established. Some of them are located on reefs in very close proximity to Humans or coastal communities; others on isolated reefs free of any human presence. The latter conservation strategy often attracts criticism: why preserve isolated environmental systems rather than protect and therefore restore systems severely damaged through contact with Humans?
It is in this context that this scientific study has been conducted by an international consortium since 2016. Its objective is to estimate fish biomass and the presence of top predators - such as sharks or large piscivores - in marine reserves to assess their effectiveness. These species play a key role in the effective functioning of coral reefs: as an essential link in the food chain, they accelerate the recycling of nutrients and eliminate diseased fish.
Partial protection of ecosystems closely related to Man
The global survey was conducted in 1,800 coral reefs, including 106 located in marine reserves. It shows that the establishment of reserves in proximity to Humans reduces but does not completely eradicate human pressures, notably those linked to fishing.
“On the whole, top predators are not found in more than 70% of coral reefs. They are found in less than 1% of reefs near human populations. Conversely, they are found in 59% of isolated reefs”, explains David Mouillot, one of the researchers who coordinated the study. Restoring populations of top predators such as sharks via small reserves located near anthropised coasts therefore seems unrealistic. The best way to preserve them is to implement protective measures on isolated sites.
The situation is somewhat different for fish biomass. Protection seems optimal in reserves where human pressure is moderate. “Different conservation strategies can be beneficial to coral reefs” concludes David Mouillot. “Our study suggests maintaining a wide diversity of reserves with varied objectives, and not overlooking the protection of sites far away from Human populations”.
Joshua E. Cinner et al., Gravity of human impacts mediates coral reef conservation gains, PNAS, 18 juin 2018