Coral reef restoration strategies use natural and human-made methods to repair damaged coral reefs. Reefs are harmed by both natural events and human activities, and people are working to fix this damage. This process includes breaking off pieces of healthy, mature corals and placing them on lines or frames. These pieces are then cared for as they grow, and later moved to their final spots on the reef when they are large enough. These methods can be done in the ocean or on land. Some restoration uses sexual reproduction, while others use asexual reproduction. These techniques sometimes raise concerns about the risk of introducing non-native species that could harm the reef's ecosystem.
Background
Coral reefs are home to more than 25% of all marine life. These ecosystems act as natural barriers between land and water, helping to reduce damage from storms and erosion. They also provide jobs, recreational activities, and food for coastal communities. Tourism related to coral reefs is a major source of income for many coastal areas worldwide. Additionally, coral reefs help store carbon, which is important for the environment. They also support food access and employment opportunities. It is estimated that coral reefs contribute about $375 billion each year through ecosystem services. In the United States, coral reefs contribute nearly $3.6 billion to the country’s economy. Globally, coral reefs are believed to help generate about $9.9 trillion annually through ecosystem services.
The most common type of coral in tropical reefs is stony coral (Scleractinia), which forms hard skeletons made of calcium carbonate. These skeletons provide structure and protection for the reef. Coral polyps have a special relationship with single-celled algae called zooxanthellae. These algae live inside the coral polyps and use photosynthesis to produce energy for the coral. In return, the coral gives the algae shelter and nutrients.
Since 1970, half of the world’s coral has been lost, and all coral reefs are at risk of disappearing by 2050. To help protect coral reefs, scientists are exploring new ways to restore their ecosystems. One common method is fragmentation, which involves breaking off pieces of healthy coral to grow new reefs. This technique is used to create artificial reefs, such as coral trees, nursery lines, and fixed structures.
Threats to coral reefs
Some human activities, like taking coral from reefs, dragging fishing nets along the ocean floor, digging canals, and using explosives to catch fish, break the coral's hard, calcium carbonate skeletons.
Another major problem for coral reefs is chemical damage. Pollution from sunscreens, paints, and inland mining can add harmful chemicals to the ocean, which hurt corals and cause them to weaken. Coral diseases are more common in areas where corals are already stressed, and these diseases have become worse in recent years. Pollution can also lead to too many nutrients in the water, which harms coral reefs by limiting the nutrients corals need. Activities on land, such as cutting down trees, mining, and farming, cause soil to erode and mix with water. This increases the amount of sediment in the ocean, which can cover coral or block sunlight, stopping corals from making food through photosynthesis.
More carbon dioxide from human actions, like burning fossil fuels, makes ocean water more acidic. When too much carbon dioxide enters the ocean, it lowers the water’s pH, a process called ocean acidification. In acidic water, corals cannot build their calcium carbonate skeletons, and some algae that live inside corals cannot survive.
The biggest threat to coral reefs is rising ocean temperatures. Most corals can only survive in water that is between 4–5°C (39–41°F). If the water gets too warm, corals may lose the algae that live inside them, causing them to turn white, or bleach. When ocean temperatures rise beyond what corals can handle, they often die. A study of the Great Barrier Reef found that 50% of the reef died after a heatwave that raised water temperatures by 3–4°C (37–39°F). After bleaching events, injured corals are more likely to get sick, and it can take many years for reefs to recover. Slow-growing corals face extra stress during these events.
Rising global temperatures are caused by releasing large amounts of greenhouse gases into the atmosphere. A study showed that about 655 million people live near coral reefs, which is 91% of the world’s population in developed countries like the United States, the Middle East, and China. The same study found that 75% of people living near coral reefs are from countries that are not highly developed. Even though these countries rely on coral reef ecosystems, they produce only a small amount of greenhouse gas emissions. Studies also show that developed countries release about 11 times more greenhouse gases than less-developed countries.
Propagation methods
Coral gardening is a method used to help restore coral reefs around the world. This process involves growing small pieces of coral through asexual reproduction until they become fully grown. Two main types of nurseries are used: ocean-based and land-based.
Ocean-based nurseries grow coral fragments underwater. These fragments are attached to steel structures and monitored for 6–12 months until they mature. Once mature, the coral is moved to damaged reef areas. Land-based nurseries grow coral in laboratories or farms. This method allows for faster growth techniques, such as micro-fragmentation. Most corals grow about an inch per year, so faster methods are important for reef restoration. Growing corals on land also protects them from temperature changes, predators, and other problems that might harm the restoration process. With the help of NOAA, over 40,000 coral reefs have been restored in the Caribbean region.
Fragmentation is a method used to divide a wild coral colony into smaller pieces. These pieces are then grown into new coral colonies. The new colonies are genetically identical to the original. Up to 75% of the original colony can be removed without harming its growth rate. This allows researchers to restore reefs with little or no harm to the original colony. Fragmentation is used in nearly all coral restoration projects today.
Fragmentation can increase coral production by about 8 times compared to the original coral. The amount of fragmentation depends on the space available for the coral to attach. However, fragmentation should be avoided when disease or storms are likely, as these risks increase. Some coral species may not grow well with fragmentation or have slower growth rates.
In vertical line nurseries, coral fragments are tied to a line that is suspended in the water. One end of the line is attached to a buoy, and the other is anchored to the seafloor. The corals are directly linked to the line.
In suspended line nurseries, two vertical lines are placed parallel to each other and connected by ropes. Coral is attached to the ropes but dangles slightly, reducing contact with the ropes. This helps lower the chance of coral death. Studies show that both vertical and suspended line nurseries have high survival rates. Growing corals on line structures keeps them farther from predators, diseases, and competition. After an initial growth period, corals in line nurseries must be moved to fixed surfaces, while corals on fixed structures can grow indefinitely.
Fixed structure nurseries are frames attached to the seafloor. These structures are often made from materials like PVC, plastic mesh, and cinder blocks.
Studies suggest that corals grown horizontally in fixed nurseries may grow at the same rate as those in vertical line nurseries. However, fixed nurseries have lower survival rates. A 2008 study found that fixed nurseries had a 43% survival rate, while line nurseries had a 100% survival rate. The survival of fixed nurseries may also depend on the time of year corals are transplanted. It is important to reduce stress for newly transplanted corals.
A "coral tree" is a type of nursery where coral is completely suspended in the water. These structures are low-cost and easy to make. They are less likely to be damaged by waves, reduce contact with predators and diseases, and are less likely to entangle marine life. Coral trees are anchored in one place, causing minimal damage to the seafloor. They are portable, easy to move, and can be adjusted for depth.
Land-based nurseries allow corals to grow to a suitable size before being moved to the ocean. Tanks with circulating seawater provide a safe environment for coral seedlings. Like plant nurseries, these facilities protect corals from storms, predators, and other dangers. They also allow for selective breeding of corals that are more resistant to stress. Techniques used on land include both sexual and asexual reproduction. Combining these methods can help corals grow faster and be more resilient.
Corals reproduce asexually when one polyp creates a new, identical polyp through budding. A method called micro-fragmentation, developed by Dr. David Vaughan in 2006, uses this ability to grow coral. Micro-fragmentation involves cutting a coral into small pieces (larger than 1 cm) and attaching them to a ceramic or cement base called a plug. These plugs are placed in land-based tanks.
Massive reef-building corals are often used in this method because it speeds up their growth. Instead of waiting decades for corals to grow large, months are enough to produce strong coral specimens. A 2014 study by Dr. David Vaughan showed that corals like Orbicella faveolata and Montastraea cavernosa grew 6.5x and 2x larger, respectively, over 2.5 years. Without micro-fragmentation, this growth would take decades. This happens because corals heal quickly after being cut. The wounds created during micro-fragmentation cause the coral to expand outward, cover the plugs, and eventually be placed in the ocean. Combining multiple fragments of the same type can create larger areas of coral.
Corals reproduce sexually through broadcast spawning. During this process, coral eggs and sperm are released into the water, where they combine to form larvae. In land-based nurseries, scientists can control which corals reproduce, allowing them to breed corals that are more resilient.
The availability of coral gametes (eggs and sperm) in the wild depends on environmental factors. Studies show that most spawning occurs at the same time each evening and is influenced by the moon’s cycle. Scientists are trying to trigger spawning in nurseries by mimicking these natural conditions.
Concerns with Propagation
Restoring coral reefs by growing new coral has some possible risks. A major concern is that changing the genes of coral could cause too much influence on the ecosystem. This can happen in different ways, such as when coral is moved from other reefs or from special growing areas. These changes might give certain species traits that help them compete better with other species in the ecosystem. This could lead to problems with invasive or exotic species, which can affect other species in two ways. Direct interactions are when one species harms another through eating it or competing for resources. Indirect consequences are when diseases spread because of these changes.
Restoration strategies
Coral restoration has been happening for more than 40 years. When choosing the best way to restore coral reefs in a specific area, it is important to compare different methods and understand their differences. How well a method works depends on where the nursery is located, the environment’s conditions, how these conditions change each year, and the type of nursery used.
Coral gardening, whether done on a small or large scale, might not be enough to save a coral species that has very few members left. Instead, restoration efforts should help natural recovery by increasing the variety of genes in a coral population. This allows corals to reproduce naturally over time. Coral gardening is helpful because it is easier for a small piece of coral to survive than it is for young coral to grow in reef environments.
Creating places to store coral can help bring back coral species after they die off. These storage areas not only help recovery but also increase the variety of genes in small, separated coral groups. A greater variety of genes can lead to better chances of survival for future coral populations. One study used a nursery for Acropora cervicornis after a very cold event killed about 43% of the coral in the area. Returning corals from the nursery helped the population recover by adding healthy coral tissue.
These methods should be used along with other actions, such as managing water areas near land, using fishing practices that protect the environment, and creating Marine Protected Areas. Coral gardening also helps in other ways, like quickly building new homes for fish and other sea creatures on damaged reefs. These restoration efforts also give people chances to help with research and conservation work.