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Indonesia is the global coral reef restoration leader by number of projects, yet these remain diverse and disparate. This study reviews the status of Indonesian coral reef restoration within a framework of international common best practice (CBP) that incorporates internationally-recognised Standards for Ecological Restoration (SER). This framework is used to formulate recommendations for a formal network of reef restoration practitioners with the purview to develop and implement a national restoration roadmap. Forty-five projects were surveyed to determine how projects have been planned and implemented. This was compared with recommendations from CBP. There is particular scope to increase quantitative data collection, reinforce community involvement, improve ecological data collection, and standardise monitoring protocols. While 84% of projects reported quantifiable goals, 64% did not quantify goals during planning and 61% did not incorporate climate-smart design features. Quantitative reef monitoring surveys were absent in 22% of projects. The majority of projects did not quantify important ecological metrics like coral community composition/diversity (96%), coral health/bleaching (89%), benthic community (62%), and coral survival (62%). Indonesia has the capacity, regulations, and networks to position itself as a reef restoration driver in the Coral Triangle region; this will require increased coordination, alignment, and quantification of restoration. A structured, collaborative, and iterative national network of various stakeholders would facilitate the development of a national restoration roadmap based on adaptive management strategies. This would aid in standardising project planning, monitoring, and reporting. Efforts should include an increased focus on climate change adaptation goals.

Recent discussions have raised concerns about the long-term effectiveness of coral reef restoration efforts, questioning whether current interventions can effectively address the ongoing loss of reef ecosystems. However, details matter and vary greatly with respect to scale, social context and benefits, and diverse approaches are needed to maintain functional coral reef ecosystems.

Interest is growing in developing conservation strategies to restore and maintain coral reef ecosystems in the face of mounting anthropogenic stressors, particularly climate warming and associated mass bleaching events. One such approach is to propagate coral colonies ex situ and transplant them to degraded reef areas to augment habitat for reef-dependent fauna, prevent colonization from spatial competitors, and enhance coral reproductive output. In addition to such \"demographic restoration\" efforts, manipulating the thermal tolerance of outplanted colonies through assisted relocation, selective breeding, or genetic engineering is being considered for enhancing rates of evolutionary adaptation to warming. Although research into such \"assisted evolution\" strategies has been growing, their expected performance remains unclear. We evaluated the potential outcomes of demographic restoration and assisted evolution in climate change scenarios using an eco-evolutionary simulation model. We found that supplementing reefs with pre-existing genotypes (demographic restoration) offers little climate resilience benefits unless input levels are large and maintained for centuries. Supplementation with thermally resistant colonies was successful at improving coral cover at lower input levels, but only if maintained for at least a century. Overall, we found that, although demographic restoration and assisted evolution have the potential to improve long-term coral cover, both approaches had a limited impact in preventing severe declines under climate change scenarios. Conversely, with sufficient natural genetic variance and time, corals could readily adapt to warming temperatures, suggesting that restoration approaches focused on building genetic variance may outperform those based solely on introducing heat-tolerant genotypes.

In 2019, the United Nations Environment Assembly requested that the United Nations Environment Programme (UNEP) and the International Coral Reef Initiative (ICRI) define best practices for coral restoration. Guidelines led by the UNEP were prepared by a team of 20 experts in coral reef management, science, and policy to catalog the best-available knowledge in the field and provide realistic recommendations for the use of restoration as a reef management strategy. Here, we provide a synthesis of these guidelines. Specifically, we present (1) a case for the value of coral reef restoration in the face of increasing frequency and intensity of disturbances associated with climate change, (2) a set of recommendations for improving the use of coral reef restoration as a reef management strategy, tailored to goals and current methods. Coral reef restoration can be a useful tool to support resilience, especially at local scales where coral recruitment is limited, and disturbances can be mitigated. While there is limited evidence of long-term, ecologically relevant success of coral reef restoration efforts, ongoing investments in research and development are likely to improve the scale, and cost-efficiency of current methods. We conclude that coral reef restoration should not be seen as a “silver bullet” to address ecological decline and should be applied appropriately, with due diligence, and in concert with other broad reef resilience management strategies.

Reef restoration activities have proliferated in response to the need to mitigate coral declines and recover lost reef structure, function, and ecosystem services. Here, we describe the recent shift from costly and complex engineering solutions to recover degraded reef structure to more economical and efficient ecological approaches that focus on recovering the living components of reef communities. We review the adoption and expansion of the coral gardening framework in the Caribbean and Western Atlantic where practitioners now grow and outplant 10,000’s of corals onto degraded reefs each year. We detail the steps for establishing a gardening program as well as long-term goals and direct and indirect benefits of this approach in our region. With a strong scientific basis, coral gardening activities now contribute significantly to reef and species recovery, provide important scientific, education, and outreach opportunities, and offer alternate livelihoods to local stakeholders. While challenges still remain, the transition from engineering to ecological solutions for reef degradation has opened the field of coral reef restoration to a wider audience poised to contribute to reef conservation and recovery in regions where coral losses and recruitment bottlenecks hinder natural recovery.

The coral holobiont contains a diverse community of bacteria that have been widely acknowledged as a major contributor in the maintenance of host health and in promoting reef resilience under changing environments. However, little is known regarding the spatial distribution of these communities or the processes and mechanisms that are responsible for creating these patterns. Here we show that bacterial communities associated with the reef-building coral, Pocillopora acuta, from nine offshore islands in an urbanised coral reef ecosystem (Singapore) can diverge sharply and are significantly different among sampling locations. We suggest that small-scale environmental factors such as prevailing surface currents and wind direction, even over short distances (< 1 km), are responsible for generating bacterial community structure. Considering the sharp differentiation we observe among bacterial communities from different sites, we recommend that future coral reef restoration projects consider the microbial aspect of the coral holobiont as this may affect the success of coral transplants in recipient populations.

Successful restoration of coral reefs depends on the survival of outplanted species. Research shows that outplanting survival is mixed, with outplants often experiencing rapid mortality in response to various stressors. We used published results on outplant monitoring to investigate the role of sea surface temperature in the survival rates of corals. We find that the maximum temperature experienced at an outplanting site is very important in determining outplant survival, with ∼50% mortality occurring if temperatures reach 30.5 °C. Some genera, however, are more tolerant than others. Outplant survival increases when sites experience greater variability in temperature, where outplants are exposed to temperatures both warmer and cooler than the long-term mean. Similar results were found when considering temperature conditions of the site in the year prior to outplanting. Thus, sea surface temperature data can be used as a tool to assess whether a restoration site is appropriate, with sites chosen to increase outplant survival.

Coral reefs are experiencing global decline, and their recovery relies heavily on asexual reproduction through fragmentation, the success of which hinges on self-sustaining attachment to the reef substrate. However, despite decades of research into coral biology, we still lack a comprehensive understanding of the attachment process and how to optimize efforts exploiting it. We recently proposed a model explaining the attachment process in Acropora millepora (Lewis BM, Suggett DS, Prentis PJ, Nothdurft LD. 2022 Cellular adaptations leading to coral fragment attachment on artificial substrates in Acropora millepora (Am-CAM). Sci. Rep . 12 , 18431. (doi: 10.1038/s41598-022-23134-8 )). To determine if the model is conserved across coral species, we employed cutting-edge integrated optical and electron microscopy techniques to observe attachment development in two key reef-forming coral genera, including Montipora mollis and Pocillopora verrucosa , comparing them with the previous model, A. millepora . Although developmental steps were broadly conserved, we identified taxonomically distinct variations in immune responses, behaviour, tissue development and skeletal microstructure. These differences explain why certain coral species, like M. mollis and A. millepora , can exhibit faster and stronger attachment compared with P. verrucosa. These findings provide critical diagnostics for asexual success and offer actionable insights into coral fundamental biology and for enhancing reef restoration efforts.

Coral reef restoration is an attractive tool for the management of degraded reefs; however, conventional restoration approaches will not be effective under climate change. More proactive restoration approaches must integrate future environmental conditions into project design to ensure long-term viability of restored corals during worsening bleaching events. Corals exist along a continuum of stress-tolerant phenotypes that can be leveraged to enhance the thermal resilience of reefs through selective propagation of heat-tolerant colonies. Several strategies for selecting thermally tolerant stock are currently available and range broadly in scalability, cost, reproducibility, and specificity. Different components of the coral holobiont have different utility to practitioners as diagnostics and drivers of long-term phenotypes, so selection strategies can be tailored to the resources and goals of individual projects. There are numerous unknowns and potential trade-offs to consider, but we argue that a focus on thermal tolerance is critical because corals that do not survive bleaching cannot contribute to future reef communities at all. Selective propagation uses extant corals and can be practically incorporated into existing restoration frameworks, putting researchers in a position to perform empirical tests and field trials now while there is still a window to act.

Larval settlement is a critical bottleneck in the process of coral sexual propagation. Promoting coral larval settlement by inducers is a promising strategy in coral reef restoration engineering. In this study, the settlement-promoting effect of Ca2+ on larvae of the brooding coral Pocillopora damicornis was investigated for the first time. Treatment with 40 mM CaCl2 for 24 h effectively promoted coral larval settlement (~ 80%). Moreover, CaCl2 is comparable with the natural inducer, crustose coralline algae (CCA), in both promoting coral larval settlement and post-settlement growth. CaCl2 showed toxic effects on larval survival and growth at high concentrations, and this could be minimized by optimizing CaCl2 concentration and shortening the exposure period. Our study suggests that applying Ca2+ to effectively and efficiently induce coral larval settlement is viable for laboratory research and small-scale aquaculture systems, and it might become a useful tool in future coral reef restoration engineering.