A Closer Look at How Predator Management Influences Coral Recovery Efforts
Three years ago, a team of coral restoration experts, including Marine Biologist Matthew Walker, began a coral restoration project at Soneva Fushi in the Maldives. This joint initiative between the Soneva Foundation and Coralive aimed to regenerate the Soneva Fushi reef and improve the resilience of the marine ecosystem around the resort.
What seemed like a straightforward coral propagation effort soon faced an unexpected challenge that threatened the survival of their newly planted corals, sparking the first study of its kind in the Maldives on how predator management can positively influence coral restoration success.
Restoring Essential Corals
By mid-2022, an eight-month project to establish a coral nursery was completed. The project evolved in many directions, centring on growing essential corals, mainly in a typical coral nursery or ‘gardening’ approach. To populate a coral nursery, small coral fragments are harvested from parent colonies. In the nursery, these fragments are carefully nurtured to grow into larger corals, which can then be outplanted onto degraded reefs.
This process is known as asexual coral propagation. It’s an effective and fast way to increase coral abundance and boost coral cover in areas where the reef is threatened. Since it’s an asexual process, it doesn’t immediately offer genetic diversity, but it can play a crucial role in building a resilient reef.
A nursery table (Image credit: Daniel Bichsel)
In the Maldives, two key coral genera, Acropora and Pocillopora, have suffered greatly due to repeated bleaching events in 1996, 2010, 2016, and 2024. These fast-growing, colourful corals are vital to the reef’s structure and biodiversity, but their rapid growth makes them much less resilient to heat stress than slower-growing species. The team recognized the need to grow Acropora and Pocillopora in the coral nursery at Soneva Fushi, prioritizing these species to help restore the degraded reef’s vital ecosystem.
An Unexpected Setback
In addition to their work in the coral nursery, the team regularly cements offcuts from parent colonies directly onto the reef. By monitoring these offcuts, the team gains insights into how well the coral fragments from the nursery are likely to fare once outplanted on the reef, since the process is very similar.
The team began daily inspections to monitor the health and growth of the offcuts. It quickly became clear that something was interfering with the development of these small corals. The corals were turning white seemingly overnight, a common sign of coral bleaching, yet no bleaching event was occurring. It was even more puzzling that the larger Acropora and Pocillopora corals on the reef remained unaffected.
They quickly discovered that cushion starfish were the culprit. These cushion-shaped starfish invert their stomach over small corals and strip them of their living tissues, leaving only their white skeletons behind. Only corals that are too big for the cushion star to invert their stomach over escape this nocturnal feeder with an appetite so big that it can eat several corals each night.
A cushion starfish (Image credit: Sabrina Inderbitzi)
The Need for Research
Thriving ecosystems generally maintain balanced predator-prey dynamics, but in disturbed ecosystems, this balance can be disrupted. The team suspected this was happening on the reef: its degradation may have shifted the predator-prey balance in favour of the cushion starfish. They realized that this could have far-reaching consequences for both their current and future restoration efforts, and that this consequence may have been occurring on the reef for many years unnoticed.
While research papers had been published on cushion starfish predation in the Maldives, none of the studies included coral restoration; they were all based on the natural reef. To fully understand the extent of the problem, the team realized they would need to conduct their own research.
This prompted Matthew to investigate the impact of cushion starfish predation on coral restoration efforts. The study aimed to determine whether the removal of cushion stars positively affected the survival rate of outplanted coral fragments and, in addition, natural recruits.
Matthew Walker monitoring the reef (Image credit: Sabrina Inderbitzi)
The research spanned 27 weeks and was divided into two main phases. The first phase, lasting 9 weeks, compared coral health under two conditions: with cushion starfish present and without them. During this phase, the starfish were frequently relocated by diving teams to simulate culling. The second phase, lasting 18 weeks, monitored predation after the relocation process ended, aiming to assess its effects in both a positive and a negative control.
Setting up the Experiment
The research sites
Five research sites were created on the reef, each site containing two identical square plots: one test square and one control square.
Each square contained naturally growing Acropora and Pocillopora corals of various sizes, including natural recruits small enough for the cushion starfish to eat, with only corals less than 10cm being monitored.
For the study, 100 transplants (coral fragments from the nursery) were added to each square, using cement as an adhesive to attach them to a hard substrate. All fragments were less than ten centimetres in size, ensuring they were small enough for the cushion starfish to eat.
Monitoring the reef for predated corals within research sites (Image credit: Matthew Walker)
The research phases
In phase 1 (weeks 1 to 9), in the control squares we allowed the cushion starfish to behave naturally, without human intervention. In the test squares, we actively searched for and removed all starfish in all five research sites.
In phase 2 (weeks 10 to 27), we allowed the cushion starfish to move back into the research sites and behave naturally, without human intervention in any of the squares.
The human intervention
In phase 1 (weeks 1 to 9), a diving team manually removed cushion starfish from the test squares at all five research sites. This was done twice a week, with two dives per square, totalling 10 dives per week. This process was halted in week 10 (phase 2) to observe the difference.
In total, around 400 cushion starfish were relocated during the nine weeks in phase 1. For the study, the starfish were released outside the research perimeter, effectively simulating their removal from the ecosystem without culling them.
Aiden removes cushion starfish from the research site (Image credit: Matthew Walker)
Study Insights
The results per phase
In phase 1 (weeks 1 to 9), we observed an average of 24.6% predation of transplants and 9.5% predation of recruits across all control squares. Contrastly yet unsurprisingly, we observed nearly 100% survival from predation in the test squares where cushion stars were actively removed, with minor mortality from other natural processes.
In phase 2 (weeks 10 to 27), predation continued to mount up, rising to an average of 32% in control squares. In test squares, predation rose from 0.4% to 25% in a matter of months, without the protective element of frequent cushion starfish removal. In one of the sites, starfish predation rose beyond 62% of all outplants in the time.
Key Findings
Phase 1 showed nearly 25% average predation of transplants in all control squares. For the control squares, this percentage increased to 32% by the end of phase 2. In one site, outplant mortality rose to 92%, with the likelihood being that most of the death was caused by cushion starfish. These findings indicate that, over a year, transplants and recruits in particularly high-population sites are likely to experience close to 100% predation in the absence of any human intervention.
In phase 2, mortality peaked in one site at 92%, with 62% directly attributed to cushion starfish. The actual impact of the cushion starfish is believed to be closer to 85%, though this could not be fully recorded due to reduced monitoring frequency. These findings indicate that cushion starfish predation is the leading cause of background mortality for transplants and recruits.
A cushion starfish among white corals that have been eaten (Image credit: Matthew Walker)
Conclusion
Matthew concluded that, without human intervention, the chances of coral transplants and natural recruits surviving their critical early phase, during which they must grow large enough to avoid predation by cushion starfish, are minimal.
The findings highlight a critical challenge for coral restoration efforts in areas suffering from reef degradation, where the predator-prey balance has shifted in favour of the cushion starfish.
It is possible to protect small corals without removing or culling the starfish, for instance by keeping them out of the cushion starfish’s reach during their critical development phase. However, such methods are likely too time-consuming and costly to make restoration efforts feasible.
Silent Predator and Reef Ally
Our research does not advocate removing or culling entire cushion starfish populations from degraded reefs. Every species plays an important role in its ecosystem, the cushion starfish is no exception. It helps regulate coral growth by feeding on weak or overgrown coral colonies and, in dense areas, clearing space for new coral generations to settle. While the cushion starfish has a reputation as the ‘silent predator of the Maldives’, it is also often referred to as a ‘coral frenemy’.
If our research has made one thing clear, it is the need for human intervention when cushion starfish populations become too high. Studying the population dynamics further can help determine an outbreak threshold. This threshold represents the population level of cushion starfish on a given reef that exceeds a sustainable limit and may warrant culling. Such a threshold can help ensure that most small corals survive while leaving enough cushion starfish to continue their role in the ecosystem.
While culling may sound drastic, there is no risk of endangering cushion starfish populations. With over 1,120 islands in the Maldives, there are cushion starfish populations surrounding each island. Starfish release their larvae into ocean currents, enabling them to repopulate neighbouring island reefs naturally and replenish their population quickly.
Soneva Fushi (Image credit: Daniel Bichsel)
Managing Predation for Coral Resilience
Matthew’s study highlights the challenges of restoring coral reefs in areas where predator-prey dynamics have been disrupted. It offers a foundation for future coral restoration efforts in the Maldives and beyond, providing a practical framework that can be adapted and evolved to fit local conditions.
Coral reefs around the world are facing three main threats: bleaching, storms, and predation. While we can’t directly control the first two, we can take action against predation to reduce some of the pressure on these already stressed ecosystems.
The team at Soneva Fushi plans to extend the research to a reef-wide scale. This research is also being extended to Coralive projects elsewhere in the Maldives and around the globe. By monitoring reefs over several years, Matthew and the team aim to uncover the long-term impact of cushion starfish removal and gain deeper insights into how active management strategies can support the recovery and resilience of coral reefs worldwide. Since the study was completed, nearly 1,000 cushion starfish have been removed from Soneva Fushi’s reef.
Matthew’s study has been published in Springer Nature (Coral Reefs) and now guides practitioners in the Maldives. It has been presented at the European Coral Reef Symposium 2024 in Naples, Italy, the Reef Futures Symposium 2024 in Cancun, Mexico, and the Maldives Marine Science Symposium 2024.
Click to read the full study or watch the Cushion Star Dilemma video by Sabrina Inderbitzi on YouTube. To contact Matthew Walker, please email [email protected].
