Evaluating the impacts of site preparation and fragment size on coral outplanting success at a highly degraded reef site

Coral reefs are deteriorating worldwide from the combined pressures of global climate change and local stressors. As such, reef restoration interventions are being applied to boost natural recovery and reef health. Outplanting success reported for restoration activities to date has been highly variable. This study evaluated factors that influence outplanting success at an impacted reef, including site preparation (cleaning substrate vs uncleaned) and outplant size for two coral species— Pocillopora acuta (branching) and Porites lutea (massive). To assess overall success, survivorship, health and growth of the two coral species were evaluated over a 170-day period. Survivorship differed between coral species, with P. lutea exhibiting higher survival probabilities than P. acuta (48.4 ± 7% and 13.6 ± 4% respectively). Low survivorship was largely driven by high dislodgement of coral outplants over the study period (39.58% to 70.83% of outplants dislodged for P. acuta and P. lutea respectively). Site preparation did not improve coral survival or health and all outplants exhibited algal growth at the end of the study. P. acuta outplants exhibited paling prior to P. lutea outplants, however, at the end of the study, all outplants except for medium (5–10 cm) P. acuta outplants exhibited similar levels of paling. Outplant size did not impact growth or survivorship. Overall, P. lutea showed the greatest suitability to outplanting at a degraded site with the highest survival and lowest dislodgement rates. In this study, high dislodgement rates limited outplanting success, indicating that alternative attachment methods and predator deterrents should be evaluated as a priority when using coral outplanting at heavily impacted reef sites. Results from this study demonstrate that not all species are equally suited to outplanting in impacted systems, and highlights the importance of matching species selection to site conditions to optimise restoration outcomes.