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, algal turf cover, etc.) then the reef may never recover on its own, and biological restoration is necessary.
Biological restoration usually involves increasing the number of living corals on the reef in areas where the structure is already available and physical conditions are Making a certain situation or outcome likely or possible More for growth.
This is generally achieved through collecting and rehabilitating naturally broken coral fragments, propagating coral colonies, culturing coral larvae, or transplanting living coral colonies. Biological restoration aims to regrow corals where populations have been diminished or lost.
This is most generally required in areas impacted by mass mortality events such as bleaching, disease, predator outbreaks, algae overgrowth, or other large or chronic disturbances. Following these mass mortality events, small, surviving coral colonies can be used to ‘seed’ the reef. These will eventually grow large enough to become sexually reproductive and return the balance on the reef by attracting coral-associated organisms.
Biological restoration may also include integrating fish, giant clams, or other nursery and aggregation devices to restore reef balance and return vital key species necessary for reef health.
Without human intervention, coral reefs would naturally recover from mass mortality 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 genes) stands. But this type of recovery is quite limited and does not create resilient reefs.
Resilient coral reefs generally have a high influx of coral larvae successfully recruiting to the area from connected reefs, with each having the potential to grow into healthy, thriving, genetically unique colonies suited to withstand different types of disturbances. These larvae will need several years to establish and grow, and any further disturbances can set this process back or prevent it. Biological restoration can expedite this process and lead to more resilient reefs.
Biological restoration can also be used to assist in speeding the development of artificial reefs, transplanting corals to areas where threats are reduced, creating ‘coral banks’ for risk mitigation, or taking proactive measures to increase coral reef resilience.
Coral gardening is the practice of caring for coral reefs in a way similar to tending to a garden at home. This can include affixing corals that have been broken or knocked loose, protecting them from predators, fixing damage from storms or human activities, and preventing algae from growing over them. Though it’s difficult to measure the success of coral gardening in numbers, many people who care for the reefs believe it’s one of the most important things they can do for their local environment.
In-situ coral nurseries can assist local managers in coral gardening activities by providing an area where broken or unsecured coral fragments can be rehabilitated and grown out to a size suitable for transplantation. Coral nurseries create an area where corals can be looked after, removed from threats, and cared for. We will cover more about them in the next few topics,
Coral transplantation refers to moving a coral that is healthy enough to survive when secured to natural or artificial reefs. We use this term primarily to refer to corals grown in nurseries, under threat due to construction/dredging, or naturally broken but large enough to negate the nursery stage. Some programs also use ‘direct transplantation’ to refer to the act of removing large ‘donor’ corals from one reef and moving them to another. But we do not condone this, as it is stealing genetic material from somewhere it is doing well and moving it to areas where other corals have already proven they cannot survive.
Larval culturing is a method of producing feedstocks for coral restoration that provides the greatest genetic diversity by utilizing the sexual reproductive cycle. While more difficult than asexual propagation, this method is the most effective at creating resilient reefs that can withstand future threats. Larval culturing programs seek to assist in adapting corals to changing climates by increasing genetic mixing during fertilization, increasing offspring survival, reducing generation times, and selecting for advantageous traits.
In some cases, land-based holding facilities and nurseries can also be part of the cycle of biological restoration by providing an area of respite, research, or facilitating coral larval and microfragmentation projects. For many years these techniques required extensive knowledge of aquarium systems and coral husbandry that were outside the scope of most local reef managers, but with advances in system technology and the rise of the internet, this has become more accessible.
Microfragmentation is cutting coral colonies into smaller pieces to stimulate tissue repair and growth while taking advantage of geometric increases in growing boundaries. This is most commonly done in land-based holding systems, but the fragments can be returned to the sea after a short time to grow out in in-situ nurseries. This technique has been shown to increase coral growth rates by about two to three times and is primarily used for massive and submissive coral growth forms (slow-growing corals).
We will cover more of these methods in much more detail later in this course, and we also have courses on Coral Larval Culturing, Predator Management, and Giant Clam Nurseries that you can take after this one.
For biological restoration to be successful, the restoration areas must still have ample light, temperature, and water quality to be conducive to coral growth. In areas where the threats to the corals are still present, or the ecosystem has completely collapsed, biological restoration may be ineffective and could even have negative consequences for surrounding reefs. Restoration techniques that address the physical conditions must be employed in these areas.