Which artificial reef designs help planted corals survive while also bringing back fish and naturally settling corals?

Details

Monitoring reef restoration and artificial reefs (ARs) has typically focused only on coral fragment survival, making broader goals such as ecosystem recovery difficult to evaluate. This study examined how AR design influences restored reef recovery through a two-year field experiment monitoring outplanted coral fragments, benthic cover, coral recruitment, and fish and invertebrate communities.

In Mkwiro, Kenya, researchers established four AR designs (16 m²), unrestored controls and natural reef patches as references (n = 10). The ARs consisted of concrete disks with bottles, layered concrete disks, metal cages, or a combination of these. A mixture of 18 branching coral species, mainly Acropora spp., was outplanted at a density of 7 corals m⁻².

After two years, 60% of all outplanted fragments had survived. Coral cover on most ARs was already comparable to that on reference patches, although it was dominated by Acropora. Survival differed among AR designs and was highest on cages because crown-of-thorns sea star predation was absent on this design. A total of 32 coral genera recruited on ARs; recruit densities were highest on reference patches, moderate on concrete ARs and low on cages.

ARs and reference patches had nearly twice the fish species richness and around an order of magnitude higher fish abundance and biomass than control patches. Fish abundance and biomass were strongly correlated with coral cover on ARs. ARs, reference patches and control patches each had distinct fish species compositions, but ARs and reference patches had similar fish-community trophic structures. Motile invertebrates, including gastropods, sea urchins, sea cucumbers and sea stars, were present on ARs, but were generally more abundant and diverse on natural reference patches.

Taken together, all ecological parameters studied progressed toward reef ecosystem recovery, with AR design and material having varying influences. The researchers recommend combining metal cages and layered concrete ARs to promote both high fragment survival and natural coral recruitment. Longer monitoring is needed to fully determine the effectiveness of reef restoration as a conservation tool supporting coral reef ecosystem recovery.