Use of Artificial Reef Structures to Rehabilitate Reef Flats Degraded by Coral Mining in the Maldives

Reef flats of low-lying atoll coral islands are not only barriers against wave action but also the source of sediments of which the islands are built. Degradation of the reef flats by human activities may seriously impair these functions and lead to erosion and inundation. In the Maldives, large areas of reef flats have been degraded by mining of coral for the construction industry. This study was set up to investigate the feasibility of using artificial reef structures to promote rehabilitation of a severely degraded reef flat. Three hundred and sixty tons of concrete reef structures have been deployed over a 4-ha experimental site on a 1–2 m deep reef flat in the Maldives which was mined for coral 20 years ago and still has less than 2.5% live coral cover. Colonization of four sets of three, approximately 50 m², artificial reef structures of varying topographic complexity and stabilizing effect, and one set of three replicate 50 m² mined control areas has been monitored. All structures were rapidly colonized by fish. Mined 50 m² control areas contained on average about 7 fish species and 20 individuals, while undegraded reef-flat areas of the same size on a nearby reef supported on average 35 fish species and 150 individuals. Within 12 months of deployment, three sets of the artificial reef structures were supporting on average 35 fish species and 150–300 individuals but the least topographically complex set only contained on average 20 species of fish and 100 individuals. Although the abundance and diversity of fish colonizing the structures seemed to indicate successful rehabilitation of the fish community, multivariate analysis showed that the fish community structure was still markedly dissimilar to that of undegraded reef-flat sites. Colonization of the structures by algae and invertebrates was also rapid. Structures were initially colonized by filamentous green algae (7 days), then barnacles (14 days), and subsequently by a diverse invertebrate community (6–8 weeks). First coral recruits (Pocillopora damicornis) were observed 6.5 months after emplacement, and within 11 months P. verrucosa, Acropora spp. and Porites spp. had recruited to the largest and most complex artificial structures. Corals were transplanted on one set of artificial structures to see whether this would accelerate recovery. Survival and initial growth rates of the transplanted colonies are reported.