Optimizing return-on-effort for coral nursery and outplanting practices to aid restoration of the Great Barrier Reef

Coral nursery and outplanting practices have grown in popularity worldwide for targeted restoration of degraded high value reef sites, and recovery of threatened taxa. Success of these practices is commonly gauged from coral propagule growth and survival, which fundamentally determines the return-on-effort (RRE) critical to the cost-effectiveness and viability of restoration programs. In many cases, RRE has been optimized from past successes and failures, which therefore presents a major challenge for locations such as the Great Barrier Reef (GBR) where no local history of restoration exists to guide best practice. In establishing the first multi-taxa coral nursery on the GBR (Opal Reef, February 2018), we constructed a novel scoring criterion from concurrent measurements of growth and survivorship to guide our relative RRE, including nursery propagule numbers (stock density). We initially retrieved RRE scores from a database of global restoration efforts to date (n = 246; 52 studies) to evaluate whether and how success commonly varied among coral taxa. We then retrieved RRE scores for Opal Reef using initial growth and survivorship data for six key coral taxa, to demonstrate that RRE scores were high for all taxa predominantly via high survivorship over winter. Repeated RRE scoring in summer is therefore needed to capture the full dynamic range of success where seasonal factors regulating growth versus survivorship differ. We discuss how RRE scoring can be easily adopted across restoration practices globally to standardize and benchmark success, but also as a tool to aid decision-making in optimizing future propagation (and outplanting) efforts.