Coral Reef Restoration Methods, Strategies, & Approaches
Methods of coral restoration can be grouped into three categories:
(1) Asexual propagation methods
(2) Sexual propagation methods
(3) Substrate enhancement
Asexual propagation methods
Asexual propagation refers to any coral restoration technique that results in the transplant of coral colonies (coral fragments covered with live coral polyps) from a healthy reef or coral farming operation to a degraded reef. These methods typically do not harness any elements of the natural coral spawning process. Since each colony is composed of genetically identical organisms, the importance of maintaining genetic diversity within each species—key to improving the odds that some of the transplanted corals will survive disease outbreaks, warming seas, or other threats—is not addressed by this approach unless combined with other strategies. For example, the need for greater genetic diversity can be met by cultivating colonies of each coral species from stock that originated from a variety of "donor" locations.1
Direct Transplantation
The direct transplantation of coral colonies from healthy reefs to degraded ones—or new environments entirely—is one of the oldest, simplest, and most common methods used to counteract coral reef decline. This approach involves removing live coral from an existing reef, which can significantly degrade the donor reef if too much coral is harvested from a limited area within a short time period. In some cases, unexpected events such as vessel groundings on reefs, violent storms, or other circumstances physically dislodge fragments of coral colonies, providing "corals of opportunity" that are broken anyway and can be transplanted to other reefs without harvesting additional healthy coral.
The success of transplantation depends on a variety of factors, such as size and health of coral fragments, the manner of transportation to the target site, water conditions (turbidity, light), species composition, density of "outplanted" coral, and the solidness of the attachment to a substrate, as well as the presence of algae and predatory fish that feed on coral.[FN] For example, in a degraded reef that has a heavy algae cover, algae will quickly overgrow and smother transplanted corals without ongoing human intervention to control algae growth.
A comprehensive report ("the Report") on coral restoration methods jointly published in 2020 by a diverse body of experts suggests that a 70 percent survival rate is a reasonable benchmark for success.2 In analyzing the outcomes of a large number of restoration efforts, the authors found that coral transported underwater in cages tended to result in superior survival rates, but stated that this was often precluded by the distances to recipient reefs. In those cases, coral fragments must be transported by boat in containers of seawater, protected from air and sun. The Report also noted that, while there is commonly an assumption that the transplantation of a few coral species will attract other species that comprise a healthy reef system (e.g., fish, echinoderms, crustaceans, other invertebrates), this has not been borne out by existing data.
The Report also evaluated methods used to attach transplanted coral to various types of surfaces (substrata) in recipient reefs. It is widely documented that coral must grow on a solid foundation. The mere placement of coral on the sea bottom generally results in a near-total mortality rate. Common methods for attaching transplanted coral to a solid foundation (e.g., degraded coral, artificial structures, or metal stakes) include epoxy (in the form of a putty that is mixed underwater by a diver from two components), zip ties, and cement (prepared in advance).
Coral Farming
Coral farming involves cultivating coral colonies from small fragments of live coral in nurseries and then outplanting those colonies, attaching them to a degraded reef after they have grown large enough to be viable without additional human intervention. A portion of the cultivated coral can be retained and subdivided into smaller fragments to seed the next cycle of coral cultivation. This avoids the need to continuously harvest coral from healthy reefs to support restoration efforts elsewhere. Coral fragments can be farmed in underwater nurseries (in situ), which take advantage of natural conditions, or propagated in terrestrial facilities (ex situ) that consist of arrays of tanks equipped with carefully controlled water quality management systems.
1. Conventional methods
Coral farming has been pioneered not only by research organizations, but by the aquarium, tourism, and hotel industries. Historically, most coral farming operations involve the cultivation of small pieces of donor coral, which are raised in nurseries until they are large enough to survive on their own, without intensive human intervention. The coral cultivars may be placed on racks, cement bases, frames, or midwater "trees" (composed of metal or PVC pipes). Farm-cultivated coral is then outplanted to degraded reefs or to new locations whose conditions are suitable for coral growth.
"Conventional" coral farming has for the most part involved species that grow and heal quickly, or which reproduce both asexually (through budding or fragmentation) and sexually (spawning) in the wild. This ensures a satisfactory survival rate and measurable results in a relatively short time period. The Report observed that it is difficult to compare the success rate of direct transplantation with coral farming, since there are mortality rates inherent in each phase of the latter, while direct transplantation involves risks only during the transport and transplantation phases. Furthermore, the authors noted that many coral farming organizations do not actively monitor coral survival and growth after the first year or two, making it difficult to evaluate and compare the long-term outcomes of different projects.
2. Micro-fragmentation
A specialized, land-based form of coral farming has emerged that has revolutionized the asexual propagation process and has enabled significantly faster growth of large, reef-building coral species that normally have very slow growth rates, such as brain, boulder, star, or mounding corals.3 The technique was developed and perfected by Dr. David Vaughan and staff biologist, Christopher Page, at the Mote Marine Laboratory, on Summerland Key, Florida. The process involves cutting larger fragments of live coral into micro-sized pieces (1 cm square or less) using a diamond-coated saw blade and attaching them to small tiles or concrete "pucks," on which they will grow until they are large enough to be outplanted to a reef. The micro-fragments and their bases are cultured in shallow tanks of seawater, known as raceways. 
The technique is a product of an accidental discovery. In 2006, while removing a fragment of live elkhorn coral from a saltwater tank at the Mote facility, Dr. Vaughan noticed that a piece of the fragment had accidentally broken off, leaving two or three coral polyps behind. After a few weeks, Dr. Vaughan happened to examine the tank where the breakage had occurred and saw that the polyps that remained in the tank had multiplied, more than doubling the colony size and the number of polyps—an astonishing event, given the slow growth rate of coral in the wild. Being reduced to such a small scale triggered an accelerated growth response in the corals, causing them to reproduce quickly to recover their size. (Note: in this context, corals reproduce asexually by budding—sending out new "buds "—offshoots that become separate, genetically identical polyps.)
In recent years, Dr. Vaughn and Mr. Page have leveraged the fast growth rate (from 25 to 50 times faster than in the wild) of micro-sized fragments and refined an optimal process for cultivation.[FNx] When the cultured corals are large enough to transplant, they are epoxied or cemented to a surface in the degraded reef, such as a dead coral head that has been thoroughly cleaned of algae or an artificial reef substrate. Additionally, they have found that multiple cultured coral "plugs," when attached close together on a dead coral head, will grow outwardly towards one another, eventually fusing together and forming a continuous coating of live coral (referred to as "re-skinning").
Challenges and solutions
Early in the process of refining the micro-fragmentation process, Dr. Vaughn and his team of researchers and volunteers encountered a serious challenge: parrotfish and other coral-eating fishes preferred the farm-cultivate corals to the corals already living in the reefs, causing a near-total loss of transplanted corals. In response, a solution was devised that involves an additional step—an initial period of acclimation near the transplant area, during which the cultivated corals are held in underwater tents, protected from predators. After a period of adjustment, the corals changed from the bright green color they had when growing in the terrestrial tanks (which appeared to attract parrotfish) to the duller color of the same species of coral already living on the reef.
Lack of genetic diversity is a drawback of coral farming techniques, including micro-fragmentation. High genetic diversity within a single species greatly improves the odds that some cultivated corals will survive disease outbreaks or evolve to tolerate changed conditions, such as warmer sea temperatures. This can be addressed by combining the tank-grown cultivation methods, which yield genetically identical clones, with measures that exploit the sexual spawning process.
Going forward
Due to the success of the micro-fragmentation process utilized at the Mote Marine Laboratory, Dr. Vaughn is hoping to train many other coral aquaculturists in the use of the micro-fragmentation method, in order to allow a broad, collective restoration of reefs on the massive scale necessary to prevent the disappearance of live coral reefs.
Sexual propagation methods
Sexual propagation involves measures to restore coral reefs by implementing techniques that utilize the natural coral spawning process but aim to significantly improve recruitment—the process by which some coral larvae are able to settle to the sea bottom and attach themselves to a secure substrate where they can continue to grow into coral polyps. Hard coral larvae that recruit successfully form part of a colony and create hard calciferous skeletons that are fused to the surface on which they land.
Breeding coral colonies reproduce through once-a-year events, during which they simultaneously release millions of gametes (eggs and sperm) into the water, creating an underwater "blizzard" of spawn that rise near the surface, where eggs and sperm are intermixed in the water column. The factors that determine the timing of this remarkable synchronous event are still poorly understood.
An egg and sperm gamete that join together as an embryo develops into coral larva, referred to as planula.4,5 Planulae float near the surface for days or weeks, depending on the species, until they are ready to attach themselves to a solid foundation on the floor of the reef. When this development phase is complete, they descend to the bottom and—if successful—permanently affix themselves to a suitable surface, where they begin to grow into a new coral colony.
In nature, however, a high proportion of larvae never succeed in making the transition to sedentary adulthood, being swept away by currents, eaten by predators, or lacking suitable substrates. Furthermore, the fertilization of coral in the water column may not occur if there is asynchronous spawning, with coral eggs and sperm being released at different times. These challenges are exacerbated by human-caused threats that upset normal spawning conditions. By leveraging the ability of corals to generate millions of gametes (eggs and sperm) and greatly reducing the mortality rate of coral spawn that is inherent even in healthy, ideal reef conditions, larval (sexual) propagation aims to compensate for increasingly adverse conditions that hinder natural propagation.
1. Ex situ larval enhancement methods
This approach to larval propagation involves cultivating (ex situ) or collecting coral gametes, nurturing them during the larval stage, and then allowing them to settle on artificial objects in land-based saltwater tanks. In this approach, it is possible to exercise greater control over the recruitment of coral polyps (attachment to substrate) than in the wild (in situ), where ocean currents and predators reduce success rates. After the polyps have made the transition to their sedentary phase of their life-cycle, they can be outplanted to degraded or artificial reefs.
2. In situ larval enhancement methods
In situ larval enhancement methods begin by collecting coral gametes on coral reefs during spawning events. The timing of collection must be perfect, since the opportunity occurs only once a year. The gametes and fertilized embryos are held in saltwater pens or booms until they become planulae that are ready for the settlement stage. When the embryos are ready to settle, they are directed into a floorless mesh tent or enclosed curtain over the target reef restoration area. The planulae descend to the portion of the reef floor within the tent perimeter. The enclosures may be moved to multiple locations, releasing only a portion of the planulae at each target site. 
In situ larval enhancement projects have already demonstrated success in the Philippines and on the Great Barrier Reef in Australia. In Australia, large-scale larval propagation studies involved patches of reef 100 meter2 were undertaken in 2017. These areas are still in the process of being monitored. Another larval enhancement project on the Great Barrier Reef, spearheaded by the Queensland University of Technology, involves the use of underwater robotic drones ("LarvalBots") that "spray" recruitment-ready planulae more precisely onto promising substrates.6,7 Scientists are trying to learn more about the factors that trigger annual spawning events, with the hope that it may be possible to induce the events with greater frequency, speeding up efforts to counteract the effects of climate change and other sources of coral reef decline.
Substrate enhancement
Reef restoration approaches in this group aim to provide favorable conditions for coral recruitment without the cultivation or propagation of coral colonies, although transplanting coral fragments are often carried out in conjunction with substrate enhancement efforts.
1. Artificial reefs
Creating artificial reefs involves the placement of human-made objects on the sea floor to simulate the structural characteristics (projections, overhangs, and hiding places) of a natural coral reef and to attract marine life, including corals and all the other forms of marine life present on a healthy coral reef.8,9 Artificial reefs have been made from sunken vessels; concrete pipes, cubes, hollow balls (such as ReefBalls), and blocks; modular assemblages of steel rods and pipes; and granite, ceramic, and other materials.10,11 Corals of opportunity (e.g., coral fragments rescued after vessel groundings or hurricanes) or coral from coral farming operations are often used to begin colonizing the artificial reef.
It is important to keep in mind that natural reefs are complex ecosystems composed of enormous numbers of species of fish, hard corals, soft corals (corals with no calciferous skeletons, such as sea whips and sea fans), and countless other marine invertebrates. The mere transplantation of a limited number of coral species onto artificial structures does not assure that a diverse range other species will follow and populate the reef, unless other measures are taken.
2. Stabilization of substrates
Restoration efforts involving the reinforcement and stabilization of substrates is important in areas where previous coral formations have been extensively damaged by storms or ship groundings, resulting in a bottom characterized by unstable fragments and rubble. Since most corals need a solid foundation on which to grow, the sea floor is stabilized with mesh or netting, large rocks deposited onto unstable bottoms, or spikes driven into piles of loose rubble. Substrate stabilization does not by itself favor the regrowth of healthy coral. It is often combined with artificial reef-building and coral transplantation efforts.
8. NOAA, What is an artificial reef? https://oceanservice.noaa.gov/facts/artificial-reef.html.