Coral reef restoration

Natural and anthropogenic processes are leading to the global destruction of coral reefs, resulting in the urgent need for coral reef restoration strategies.
Coral reefs are important coastal defenders, standing as a buffer between the coast and waves, to reduce storm damage and coastal erosion. Additionally, they provide jobs, recreational opportunities, and are a major source of food for coastal/global communities.
The most prevalent coral in tropical reefs are stony corals (Scleractinia), which build hard skeletons of calcium carbonate that provide protection and structure to the reef. Coral polyps have a mutualistic relationship with single-celled algae referred to as zooxanthellae. These algae live in the tissue of coral polyps and provide energy to the coral through photosynthetic products (carbohydrates and oxygen). In turn, the coral provides shelter and nutrients to the zooxanthellae.
As this delicate relationship becomes more threatened, methods for restoring reef ecosystems are being investigated. Fragmentation is the most common strategy for restoring reefs, often used to establish artificial reefs like coral trees, line nurseries, and fixed structures.
Threats to coral reefs.
Coral reefs are fragile ecosystems that are threatened by both physical and chemical stressors. These threats make reef restoration efforts imperative.
Some anthropogenic activities, such as coral mining, bottom trawling, canal digging, and blast fishing, cause physical disruption to coral reefs by damaging the corals' hard calcium carbonate skeletal structure. Another major threat to coral reefs comes from chemical degradation. Marine pollution from sunscreens, paints, and inland mining can introduce chemicals that are toxic to corals, leading to their decay. Often a result of pollution, eutrophication can occur in coral reef ecosystems, limiting nutrients from the corals. Additionally, increased CO2 emissions from human activities such as fossil fuel burning can effect the acidity of ocean waters. Ocean acidification occurs when excess CO2 reacts with ocean water and lowers the pH. Under acidic conditions, corals cannot produce their calcium carbonate skeletons, and certain zooxanthellae are not able to survive.
Perhaps the biggest threat to coral reefs comes from rising global temperatures. Most corals can only tolerate a 4-5oC range in water temperatures. As ocean waters warm beyond the tolerated temperature range, corals are dying. One study of the Great Barrier Reef found the reef mortality rate to be 50% after an extreme heatwave with 3-4oC temperature increase. Due to bleaching events similar to this one, injured corals continue to die after the event due to increased disease susceptibility, it takes decades after bleaching events for the reef to recover, and the slow growing corals are put under an immense amount of stress.
Propagation methods.
Fragmentation.
Fragmentation is a method used to divide a wild colony of coral into smaller fragments, and these smaller pieces are grown into additional coral colonies. These fragmented colonies are genetically identical to the host colony. Up to 75% of the host colony may be removed without negative effect on its growth rate. This allows researchers to move forward with restoration projects with minimal impact, if any at all, on the growth rate or survivorship of the original colony. Fragmentation practices are used in virtually every kind of coral restoration strategy used today. Several different methods of growing fragmented corals are outlined below.
Fragmentation allows for about an 8x increase in productivity compared to that of the original donor coral. The amount of fragmentation done to the donor coral is determined based on the amount of space available for attachment. Although fragmentation has great potential, it should be avoided when risk for disease and storms are high as it increases the potential risks from these stressors. This strategy may not be optimal for certain species that are less adapted to fragmentation or have slower growth rates.
Line nurseries.
Vertical.
In vertical line nurseries, coral fragments are tied to a line suspended in the water. One end of the line is attached to a buoy while the other is anchored to the seafloor. The corals in this type of nursery are linked directly to the vertical line in the water column.
Suspended.
In suspended line nurseries, two vertical line nurseries are placed apart from each other so they are parallel vertically in the water column. They are then connected together with rope tied perpendicularly between the two. Coral is then attached to this rope, but it is partially dangling off the lines so there is less contact with the rope itself.
Less contact between the coral and the suspension lines leads to lower the partial mortality of the corals. Although these structures have some partial mortality, studies show high survival of the whole nursery (in both vertical and suspended). Raising corals on line structures increases the distance between the coral colonies and potential predators, benthic diseases, and there is less space to compete for. Corals grown in line nurseries need to be moved to fixed substrates after an initial growth period, while those propagated on fixed structures can grow indefinitely. 
Fixed structure nurseries.
Fixed structure nurseries are frames attached to the seafloor. These nurseries are often made from materials like PVC, plastic mesh, and cinder blocks.
There are likely no differences in growth rates between corals grown horizontally in fixed nurseries, versus those grown vertically in line nurseries. Although, the survival rate of these nurseries are lower than line nurseries. A 2008 study found that fixed structure nurseries had a 43% survival rate, while line nurseries. Initial mortality of fixed structure nurseries is also likely dependent on the time of year that the corals are transplanted. It is important to limit stressors that newly grafted corals are exposed to.
Coral tree nurseries.
A “coral tree” is the first type of nursery of its kind where coral is completely suspended in the water column. Low cost and availability of materials to create these coral trees, makes them an ideal method for propagation. These nurseries are less susceptible to damage from wave action, here is less interaction between benthic predators and disease, and reduced entanglement risk for other marine life (compared to line nurseries). Because these nurseries are only anchored in one place, there is minimal impact to the seafloor, they are portable and easily transported by one person, and they can be easily adjusted if depth is an issue.
Considering restoration strategies.
When determining which restoration strategy is best for a given location, it is important to compare and contrast all methods. The effectiveness of a strategy can be dependent on the habitat a nursery resides in, the conditions of the environment, how the conditions vary annually, and the structure of the nursery chosen.
Coral gardening for reef restoration, on any scale, may not be capable of saving a depleted species. Instead, restoration strategies should be used to aid natural recovery in the re-establishment of a larger genetic pool of a species of coral. This allows corals to sexually reproduce and recover naturally with time. Coral gardening and propagation of corals is important because it is much easier for a fragment of coral to survive than it is for the early life-stage of coral to establish itself in reef environments.
Creating repositories for corals can aid in species reintroduction after coral die-off events. Not only do these repositories serve as a method for recovery, but they can also greatly enhance the genetic pool of isolated populations of corals. Through enhancing these genetic pools, we can expect higher future survival rates for the corals. One study found used an "Acropora cervicornis (A. cervicornis)" nursery as a repository after an extreme cold-water event occurred that wiped out roughly 43% of its population in the area. The reintroduction of corals from these repositories reintroduced healthy coral tissues to the coral population, aiding in natural reproduction.
These practices should be used simultaneously with practices such as watershed management, sustainable fishing practices, and the establishment of Marine Protected Areas. Coral gardening also offers indirect benefits, like the rapid creation of new fish and invertebrate habitat on depleted reefs. These reef restoration methods also create citizen science opportunities, getting the community involved in coral restoration and conservation.