There are many methods and techniques of active coral restoration available to reef managers, many of which are very cheap and easy to construct or maintain. But, the science behind coral conservation and restoration is relatively new, and these techniques are constantly being improved. In the case of community managers, each community or region will find that different techniques or materials are more efficient for them, and adapting the techniques is essential for local success. It is important to keep in mind that in any new field, mistakes are bound to be made, but they should not be repeated. This article gives an overview of popular coral restoration methods to review where the field of coral restoration stands today, and what improvements are needed for the future.

Coral reefs around the world are under increased threats due to both local and global stresses, making coral reef restoration more vital to preserve marine resources and local economies. Generally, there are two main types of coral restoration, known as Active and Passive Restoration. Active coral restoration refers to projects whereby time, energy, and resources are devoted to directly increasing the coral reef health, abundance, or biodiversity. Together these three factors constitute what is referred to as the coral reef resilience. A reef which has a high abundance of corals that are healthy and formed from a diverse range of coral genera is said to be resilient, or able to withstand or recover from disturbances.

Ideally, active restoration is done after passive restoration, or the creation of Marine Protected Areas (MPAs) has already been accomplished. The most common objectives of active restoration are to restore habitat and corals which have been lost, or improve reef resilience to mitigate future disturbances. In order to restore an ecosystem effectively, the threat must first be reduced or eliminated. But, in an increasing number of areas around the world today, the likelihood of stopping the consortium of imposing threats is minimal. In these areas, restoration to improve resilience must be implemented to reduce the degree of damage and maintain small areas of the reef to serve as ‘biodiversity banks.'

Charles Darwin has been given credit as being the world’s first coral restorationist. What he realized was that corals dislodged by boat grounding or anchors often died after rolling around or being abraded by sand. Using that observation, he theorized that by securing the coral in place it should recover and regrow. He tested this theory by pounding pieces of bamboo into the sand and tying the unsecured coral fragments to it. After observing the corals over time, he determined that in doing so the coral not only survived, but began to regrow and replace lost tissue areas through the asexual reproduction of coral polyps.

In the time since Darwin, techniques for coral restoration have advanced, but the basic principle is still the same; secure broken corals so they will survive. This can be done due to the fact that corals reproduce primarily through asexual means, and any individual polyp in the colony has the potential to create a new colony in the right conditions. For some corals, such as branching or bushy colonies, asexual reproduction through budding or breakage is a major mechanisms in which they spread out, and is referred to as propagation. Today, we use stronger and more long-lasting materials than Darwin’s bamboo poles to secure the corals onto (concrete, steel, ceramics, limestone, etc.), or simply replant them securely to natural reef areas. Corals can be secured to a solid structure using a wide variety of straps, glues, wedges and other techniques. If a coral has been secured in an area where the physical conditions are conducive for that coral colony’s growth, then it will thrive as long as it is secure in place.

If you have ever been involved in securing coral fragments, or ‘coral gardening,’ then you know that it is not quite so simple as just attaching the coral; the conditions must be conducive for growth. Generally, this means that you maintain the same light levels (depth), and transplant the corals to an area where threats such as sedimentation, pollution, anchor damage, etc are low or absent. Depending on the size of the project, achieving high levels of success and minimizing mortality or unexpected consequences or disasters (risk mitigation), can be quite difficult.

In restoring coral reef through asexual propagation and the securement of coral fragments as discussed above, there are three primary or foundational objectives that are either addressed individually or in unison; (1) Increase solid structures available for coral growth, (2) increase coral coverage (3) alter growing conditions. These objectives can also be referred to structural, biological, and physical conditions (respectively).

Structural restoration generally involves the construction of artificial reefs, sinking of wrecks, or relocation of rocks/dead coral heads. The goal is to increase the amount of reef structure and habitat available for the corals and other reef organisms to grow on. Structural restoration is required in areas were the reef has been lost due to disturbances such as blast fishing, boat grounding, dredging, landslides, etc. In areas which have been reduced to rubble or sand, corals will not have solid structure to attach to and will end up being abraded or buried during high waves. Ecosystem succession in these marginalized areas can take decades to recover, and many never do so on their own.

Natural means by which reef structure could return in areas with physical reef damage includes the settlement and long-term growth of corals from the Fungidae family which are better adapted to survive without being attached to the substrate. Eventually these fungidae corals grow large, and die. Other corals can then settle on the large, and relatively stable, dead skeletons successfully. Larvae from the Porites genus seem to be particularly successful in recruiting to marginalized areas, as they tend to be more resilient towards physical abuse by wave action. Giant clams, rock oysters, coralline algae, and even marine debris can sometimes assist in this process. By adding artificial structures in these areas where the physical and biological conditions for coral growth are still good, and the natural levels of coral recruitment are high, the reef can quickly and effectively be restored. In areas that are recruitment limited or greatly marginalized, artificial structures will need to be ‘seeded’ with coral transplants to facilitate and speed development.

In addition to restoring damaged reefs, this technique can also be used to extend the reef boundaries, create new reefs in sand flats, improve fisheries, or create alternative dive sites to mitigate the negative impacts of diving tourism. In areas where there is already an abundance of reef structure, as in reefs impacted by coral bleaching or disease, then structural restoration is not necessary, and may be a waste of resources and time that are usually limited to reef managers.

Biological restoration usually involves increasing the amount of living corals on the reef in areas were structure is already available. This is generally achieved through methods such as collecting and rehabilitating naturally broken coral fragments, propagating coral colonies, culturing coral larvae, or transplanting living coral colonies. The general goal of biological restoration is to regrow corals in areas where populations have been diminished or lost. This is most generally required in areas which have been impacted by bleaching, disease, predation (COTs and Drupella snails), algae overgrowth, sedimentation, etc. Following these types of mass mortality events, small coral colonies can be used to ‘seed’ the reef that will eventually grow large enough to become sexually reproductive and return the balance on the reef. Biological restoration may also include integrating fish, giant clam, or other nurseries and aggregation devices to restore reef balance and return vital symbiosis necessary for reef health.

Without human intervention, the coral reef would naturally recover from mass morality events through both asexual and sexual means. Single corals such as branching colonies can propagate and create stands of healthy, but mono-specific (all the same DNA) stands of reef. Primarily though, the area will require that coral larvae successfully recruit to the area from connected reefs and are able to grow genetically unique colonies which are each suited to withstand different types of disturbances in the future. In areas where the reproductively viable coral population has been destroyed, connectivity to other reefs is low, or larvae are unable to settle successfully (due to grazing, turf cover, etc.) than the reef may never recover on its own, and biological restoration is necessary.

Biological restoration can also be used to assist in the development of artificial reefs, transplant corals to areas where threats are reduced, create ‘coral banks’ for risk mitigation, or take proactive measures to increase coral reef resilience. To be successful, the restoration areas must still have ample light, temperature, and water quality to be conducive for coral growth. In areas where the threats to the corals are still present, or the ecosystem has completely collapsed, biological restoration may not be effective and could even have negative consequences for surrounding reefs. In these areas restoration techniques that address the physical conditions must be employed.

Physical restoration involves addressing the conditions in which the corals are growing to improve their health, growth rates, or fecundity (reproductive ability). These methods have generally been developed more recently, and some are still in the experimental stages. Methods include mid-water coral nurseries or mineral accretion devices such as Biorock ™ technology. Mid-water nurseries are used as a staging area for rehabilitation of damaged or propagated corals before they are placed back out onto artificial or natural substrates. By floating in mid-water, the nurseries can be placed in areas with high water quality (such as in the open ocean), but still maintain the same ambient light levels which the corals and their zooxanthellae are adapted to. Generally, corals growing on mid-water nurseries have very high survival rates and tend to grow faster than similar colonies on the natural reef due to decreased stress caused by sedimentation, eutrophication, predation, or pollution.

Mineral accretion devices are an advanced reef restoration technology which uses low voltage electrical energy to change the water chemistry directly around the structure. The process of electrolysis utilized by the technology increases sea water pH and causes carbonate salts to precipitate out of the water (CaCO3, MgCo3, etc). In these conditions, corals can devote less energy to forming skeletal structure and divert that energy into other processes such as tissue growth/repair, immune system, mucus production, lipid storage, or reproduction. Corals growing on these devices tend to grow 3-5 times faster than their natural counterparts, and will survive in an extended range of physical conditions. This means that corals can thrive in areas where temperature, water quality, or light levels would otherwise be outside the range for that specific coral’s survival.

Generally, physical restoration is expensive and requires a high amount of regular maintenance compared with basic structural biological restoration methods. In many areas though, focusing on only one type of restoration will not led to success, and the various forms of restoration must be implemented together proportionally as is needed. This is most effectively done through a three-part strategy involving collecting a ‘feed stock’ of corals, the rearing of mature colonies, and transplanting to natural or artificial structures.