Positive feedbacks driving habitat-forming species recovery and population growth are often lost as ecosystems degrade. For such systems, identifying mechanisms that limit the re-establishment of critical positive feedbacks is key to facilitating recovery. Theory predicts the primary drivers limiting system recovery shift from biological to physical as abiotic stress increases, but recent work has demonstrated that this seldom happens. We combined field and laboratory experiments to identify variation in limitations to coral recovery along an environmental stress gradient at Ningaloo Reef and Exmouth Gulf in northwest Australia. Many reefs in the region are coral depauperate due to recent cyclones and thermal stress. In general, recovery trajectories are prolonged due to limited coral recruitment. Consistent with theory, clearer water reefs under low thermal stress appear limited by biological interactions: competition with turf algae caused high mortality of newly settled corals and upright macroalgal stands drove mortality in transplanted juvenile corals. Laboratory experiments showed a positive relationship between crustose coralline algae cover and coral settlement, but only in the absence of sedimentation. Contrary to expectation, coral recovery does not appear limited by the survival or growth of recruits on turbid reefs under higher thermal stress, but to exceptionally low larval supply. Laboratory experiments showed that larval survival and settlement are unaffected by seawater quality across the study region. Rather, connectivity models predicted that many of the more turbid reefs in the Gulf are predominantly self seeded, receiving limited supply under degraded reef states. Overall, we find that the influence of oceanography can overwhelm the influences of physical and biological interactions on recovery potential at locations where environmental stressors are high, whereas populations in relatively benign physical conditions are predominantly structured by local ecological drivers. Such context-dependent information can help guide expectations and assist managers in optimizing strategies for spatial conservation planning for system recovery.

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Accelerating the recovery of marine coastal ecosystems is a global challenge that has been attempted on many systems around the world. Restoration efforts have shown varying levels of success at localized-scales, but developing techniques for large-scale application are still in their nascent stage for many systems. For seagrass meadows and marsh plants, large-scale successes have been realized by distributing seeds from moving boats or planes, respectively. Similarly for coral reefs, the harvesting, culturing and releasing of wild coral-spawn slicks to targeted reefs is anticipated to achieve cost-efficient, large-scale restoration of coral communities with low-impact technology. Yet, operational protocols for full-scale application still require development by practitioners. In this study we conducted a field trial to evaluate the actual feasibility of harvesting wild coral-spawn slicks for large-scale restoration activities, incorporating technologies used in oil spill remediation, dredging operations, and land-based aquaculture. Testing the potential for scalability to commercial vessels, our trial focused on concentrating and collecting wild coral-spawn slicks for culturing until settlement competency using an experimental 50,000 L aquaculture facility built on a tugboat. Five objectives were set and all were achieved successfully, with only one requiring further optimization. Overall, this restoration approach allows for long-distance translocation of genetically diverse coral assemblages, and may be combined with other larval conditioning techniques that are being developed to increase the resistance to stress and survival of coral recruits. Most importantly, it is fully scalable to produce billions of coral larvae for delivery to target reefs, with negligible impact to source populations.

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Accelerating the restoration and recovery of coral populations is a global challenge that has been attempted on many coral reefs around the world. Previous approaches have shown varying levels of success at localised scales, but the comparison of cost and benefits to develop large scale restoration concepts has so far been lacking. Here, we compare two large scale restoration approaches: the harvesting, development, and release of wild coral spawn slicks and the transplantation of fully fecund adult coral colonies. Comparisons incorporate the best available information on the demographic rates to estimate coral population growth beginning at settlement to maturity five years following deployment. Cost effectiveness is also considered in a coarse manner. This contribution elaborates on the apparent optimal approach for large scale application in the current context of the Great Barrier Reef: coral spawn slick harvesting and release. The coral spawn slick harvesting and release approach 27 appears optimal because it achieves large scale restoration of coral communities with low impact technology and at lower cost per colony. Overall, it has the potential to (1) transport billions of thermally tolerant larvae up to 1000s of km’s that (2) are relevant to coral restoration efforts at the geographical scales of the Great Barrier Reef while (3) benefitting from the use of technology that likely has extremely low impact on wild populations and (4) retaining the natural genetic and species diversity needed to enhance the resilience of restored coral communities. The proposed technology and application of using industrial techniques for large scale coral spawn slick harvesting and release is presented. Our contribution also provides valuable insights into critical elements of each concept. We highlight information gaps and the relative sensitivities of different approaches to parameter uncertainties such as levels of larval retention, which could result in different assessments of cost effectiveness. @en

Accelerating coral reef restoration is a global challenge that has been attempted around the world. Previous attempts show varying levels of success at localized‐scales, but comparisons of cost and benefits to evaluate large‐scale reef restoration approaches are lacking. Here, we compare two large‐scale restoration approaches: the harvesting, development, and release of wild coral spawn slicks onto a target reef, with the transplantation of gravid coral colonies to provide a seed population and local source of larvae. Comparisons incorporate the best available information on demographic rates to estimate population growth, beginning at embryo production to colony maturity four years following deployment. Cost effectiveness is considered in a coarse manner. The harvesting, development, and controlled release of coral spawn slicks is anticipated to achieve large‐scale restoration of coral communities with low impact technology at low cost per colony. Harvesting wild spawn slicks has the potential to (1) transport billions of larvae up to 1000s of kms that (2) are relevant to coral restoration efforts at vast geographical scales while (3) benefitting from the use of technology with extremely low impact on wild populations and (4) retaining natural genetic and species diversity needed to enhance the resilience of restored communities. Transplanting colonies is most useful from reefs designated to be impacted by infrastructural development by providing an opportunity for transfer to high value zones, from dedicated nurseries, and for brooding species. Our contribution provides insights into critical elements of both concepts, and we highlight information gaps in parameter uncertainties.@en