The Importance and Fragility of Natural Coral Recruitment  

By Alyssa Allchurch

It doesn’t take a degree in marine biology to know that coral reefs around the world are struggling. Today it is common knowledge that coral reefs are in decline due to a multitude of factors including rising sea surface temperatures, nutrification, sedimentation and a plethora of other human induced factors. While the causes of global coral reef declines are numerous, so are the solutions. There are multiple avenues different scientists, conservationists, reef managers and interested parties have taken over the years to try and assist the recovery of corals. These avenues include coral fragmentation, a coral propagation method which consists of the out planting of coral fragments after being broken off a larger colony. This method, while useful in areas of the world where coral decline has led to incredibly limited coral abundance and diversity, fragmenting adult colonies can often have severe impacts (Scott 2019). Solutions in areas of the world which still host high coral diversity, such as the Gulf of Thailand where ATMEC is based, should focus on maintaining that high diversity at all costs. The best way to try and maintain this high natural diversity is enhancing and assisting natural coral recruitment. Supporting coral recruitment is a cornerstone of ATMEC’s work and is something all coral conservation programs should strive to support.

There is value in understanding and supporting natural coral reef spawning and recruitment. Coral spawning is an often simultaneous event, cued by lunar cycles and water temperature, in which coral colonies release egg bundles and sperm into the water column (Keith et al., 2016). These gametes are either broadcasted into the water column and mix together at the surface causing external fertilization, or in certain brooding species are brought into the coral body, fertilized and then released back into the water column as an embryo. These coral larvae are called planulae, which drift through the ocean until they are ready to settle, during which time they seek out stable substrate to attach themselves to. In areas of the world which still have large coral spawning events, such as the annual spawning event that occurs in the Gulf of Thailand, this event is essential for the survival of reefs. Under current global conditions, supporting this step is crucial in allowing coral reefs to recover and grow. However, there are many roadblocks to a successful coral spawning.

First, a reef needs a variety of large coral colonies in close proximity to each other for a coral spawning event to be successful. Only mature, healthy colonies are able to devote the energy needed to create egg bundles. Colonies which need to use energy to repair their tissue after bleaching or other physically damaging events are not able to create gametes. Gamete creation is metabolically demanding, as is the rapid growth required after the breakage of a coral colony. In this way coral fragmentation takes a healthy adult colony which would have been capable of contributing to coral spawning and stops that spawning from happening (Baums 2008). If a coral colony undergoes stress during the creation of egg bundles, these bundles may be reabsorbed to give the colony enough energy to combat that stress (Torres et al., 2008). Only colonies which have access to surplus energy can contribute to the creation of new colonies through spawning events.

Once these gametes are fully developed and ready to be released, the next step occurs, spawning. Coral spawning, which traditionally happens during a narrow window of time, must be synchronized and the conditions must be right. At this step most reef managers have very little control over if coral spawning events will be successful. Storms flush the ocean with fresh water which can be lethal to coral babies who cannot handle the change in salinity. Another much more sinister change happening in our oceans is the occurrence of synchrony loss. Corals use multiple environmental factors to synchronize their gamete release, and these environmental cues are being altered by anthropogenic activities. Multiple changes are responsible for spawning cues no longer being clear for corals, including shifting temperature, impacts from light pollution, and high levels of hormonal pollutants (Shlesinger and Loya 2019). This has resulted in broadcasting corals at certain locations losing their ability to synchronize their gamete release, resulting in ejaculation at different times and low levels of fertilization.

If a coral colony is able to grow big enough to spawn, manages to time it correctly with its neighbours and the gametes are able to survive long enough in the water column to create planula, the final step is settlement. This is a crucial step where conservationists can make a big impact, especially in places like the Gulf ofThailand. The Gulf is basically a giant sand pit, with sparse amounts of stable substrate that isn’t already occupied by other benthic organisms. In the Gulf, high levels of sedimentation due to terrestrial human activities are reducing the amount of stable, hard substrate available, dramatically reducing available space. Many dedicated conservationists are tackling this issue through the use of deploying recruitment friendly artificial substrate. Large concrete structures, which provide stability, rugosity and protection from sedimentation burial, are being designed with natural coral recruitment in mind. At ATMEC we are studying what factors such as depth, material type, complexity and distance from natural reefs promote the best conditions for natural coral recruitment. Through the deployment in our first semester of approximately 4 tons of artificial substrate, we are actively working towards assisting coral settlement. We are aiming, through our research and initiatives, to enhance natural recruitment levels around our island and to support the recovery of our local coral reefs.

Coral spawning is one of the great natural wonders of the world, and sadly an event which is becoming more and more rare. While the threats to a successful annual coral spawning are growing, dedicated conservation organizations such as ATMEC are working to fight back. If we want our coral reefs to survive and thrive into the future, we need to combat big threats like climate change and ocean acidification, while also combating threats to spawning success. ATMEC prioritizes supporting natural recruitment around the Koh Mun Nai archipelago through studying coral spawning timelines in the Gulf, deploying stable substrate and researching the factors that influence recruitment of coral communities. Help us continue our fight by not only supporting our activities, but by advocating for science-based solutions in environmental science and not being fooled by quick fixes.  


  • Torres, J. L., Armstrong, R. A., Weil, E. (2008). Enhanced ultraviolet radiation can terminate sexual reproduction in the broadcasting coral species Acropora cervicornis Lamarck. Journal of Experimental Marine Biology and Ecology 358: 39-45.

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