In situ enhancement of coral larval settlement using acoustic enrichment, engineered habitats and living materials.

Worldwide reef ecosystem degradation is inspiring efforts to enhance in-situ coral larvae settlement onto degraded or artificial reefs. Here we simultaneously test three approaches for enhancing settlement over two weeks in Kāne'ohe Bay, O'ahu, Hawai'i. Nineteen artificial structures, mimicking natural reef mesoscale roughness, were deployed along two transect lines at distances between 1 and 42 m from an "acoustic enrichment" underwater speaker broadcasting healthy reef sound. Twelve additional cinder blocks supporting micro-habitat structures designed to attenuate flow were also deployed at various distances. Some micro-habitats were coated with a living bacteria reef ink (Brink) to further induce larval settlement. Statistical regressions identify Brink presence, structure type, and speaker distance to 20 m as significant factors in predicting coral larval settlement density, but conclusions about acoustic enrichment efficacy are tentative. The cumulative effects of acoustic enrichment, living microbial materials, and custom-designed habitats on enhancing in-situ larval settlement justify further scaled-up reef restoration trials.