Influence of attachment techniques on coral seeding unit deployment cost and performance

Attachment of coral propagules to the reef surface is a bottleneck in the efficiency and efficacy of coral restoration techniques, with hypothesized tradeoffs between the relative investment in attachment (labor and materials costs) and propagule yield. Here we quantified the investment required and retention over 1 year for 11 coral seeding attachment treatments involving combinations of five attachment methods and four seeding unit (SU; substrates designed for settlement and outplanting of coral larvae) types compared to an unattached control (concrete tetrapods wedged into the reef structure by a diver). Only two relatively costly attachment methods (bolting SUs into a hole drilled in the reef and the Coralclip) provided significantly better retention than the unattached control. In a subsequent experiment, cemented tetrapods and bolted ceramic stars plus unattached controls were implemented with coral settlers from two species. Concrete tetrapods attached with cement had higher retention ( p < 0.001 for both species), but did not have higher yield ( p = 0.248 for Acropora palmata ; p = 0.542 for Diploria labyrinthiformis ) of live corals than unattached controls. Attaching ceramic stars via bolts, though requiring approximately 10× greater labor for initial attachment (i.e. approximately 120 seconds vs. approximately 10 seconds for wedging the unattached controls), approximately doubled (for D. labyrinthiformis , over 8 months) or quadrupled (for A. palmata , over 6 months) the yield of live corals (i.e. proportion of outplanted SU retaining at least one live coral) over unattached controls. These results can help practitioners judge appropriate levels of investment in assisted recruitment using artificial substrates and attachment for their local context.