Tracking Crown of Thorns Culling at Rainbow Reef
A reef can look healthy from the surface, even while a slow-moving threat is feeding underneath. Crown of Thorns starfish are natural reef animals, and endemic to Fiji, but when their numbers climb too high, they can strip living coral faster than the reef can recover. When coupled with other threats such as warming sea temperatures, removal of natural predators due to overharvesting/overfishing, and increasing cylclone activity, the reef can suffer serious long-term impacts. Rainbow Reef Fiji Foundation is working hard to thwart these threats - but we need your help.
At the Rainbow Reef, that threat is no longer something to notice only when damage becomes obvious. Dive operators, conservation-minded yachties, visiting divers, and local reef watchers are helping track sightings, culling activity, and site conditions across different dive sites. The goal is simple: protect coral cover, preserve biodiversity, and keep reef health from sliding quietly out of view. By tracking sightings and culling activities, we are able to see the relative abundance of COTS and potential for future damage - and the need to mobilize more resources to keep populations under control.
Culling Crown of Thorns starfish is not glamorous work. It takes careful diving, patient searching, safe handling, and good records. Yet it is one of the most direct tools available when outbreaks put coral communities under pressure.
A divemaster from Sau Bay Resort injecting vinegar into a COTS.
Why Crown of Thorns outbreaks matter for reef health
Crown of Thorns starfish feed on live coral tissue. A single starfish moves across a coral colony, everts its stomach onto the coral surface, digests the living tissue, and leaves behind a pale skeleton.
The bleached coral is visble on the right, showing the damage one COTS can do in a matter of minutes.
In normal numbers, this behavior is part of reef ecology. In outbreak numbers, it becomes a serious threat.
The biggest concern is the loss of live hard coral. Hard corals build the structure that supports the reef community. When coral cover drops, the impact spreads beyond the corals themselves.
Healthy coral structure provides:
Shelter for juvenile fish
Feeding grounds for reef species
Attachment space for invertebrates
Breaks and surfaces that slow water movement
Habitat complexity that supports biodiversity
When Crown of Thorns starfish heavily feed on a site, the reef can become less complex. Branching corals, plate corals, and other exposed growth forms may suffer visible damage. Fish that depend on coral shelter may move away or become more vulnerable to predators. Algae can take hold on dead coral surfaces if herbivorous fish and invertebrates cannot keep up.
That shift affects more than scenery. Reef health depends on balance. Coral loss can reduce biodiversity, weaken natural recovery after bleaching or storms, and make the ecosystem less resilient to future stress.
Rainbow Reef is known for rich coral life, clear water, and varied dive sites. That variety is one reason active observation matters. Culling records from one site may not tell the whole story. Some sites may show scattered individuals. Others may show clusters, fresh feeding scars, or repeated sightings over time. Tracking those patterns helps teams prioritize effort where it can do the most good.
What divers are seeing across Rainbow Reef dive sites
Two years ago, there were virtually no signs of COTS on the Rainbow Reef. Over the past two years, we have seen increasing numbers of COTS on various dive sites.
COTS culling works best when it starts with good observation. Divers are often the first to notice changes because they return to the same reefs again and again. A dive guide who visits a site weekly may spot a new feeding scar before it becomes widespread. A yacht crew anchored nearby may report repeated sightings while moving between sites.
Our current effort brings those observations together. Dive operators in the area, transient yachties dedicated to conservation, and trained or experienced recreational divers are contributing reports from various Rainbow Reef dive sites. The value comes from consistency. At RRFF, we are assembling those observations and data from culling into a comprehensive database that will be used to track reef health and the impact from the culling activities.
Useful reports include:
The dive site name or clear location reference
Date and approximate time
Number of Crown of Thorns starfish observed
Whether the animals were culled, photographed, or left untouched
Depth range and general habitat type
Evidence of fresh feeding scars
Notes about coral condition nearby
This kind of tracking changes culling from a one-off reaction into a shared reef health record. It also helps avoid wasted effort. If a site has repeated observations, teams can plan targeted dives. If a site shows no recent activity, operators can keep monitoring without sending limited culling resources there too often, and instead can focus on reefs where COTS are more widespread.
The human side matters as much as the data. Divers involved in this work often describe the same mix of urgency and care. They do not want to remove animals unnecessarily, but they also know what unchecked feeding can do. Many have watched a small patch of damage grow between visits. Others have seen how quickly a coordinated culling day can reduce visible pressure on a reef section.
One recurring insight from dive operators is that culling is not just about finding the obvious large starfish. It is also about learning the site. Crown of Thorns starfish may shelter under coral ledges during the day, sit partly hidden in rubble, or become easier to spot by following fresh feeding trails. Experienced local eyes make a real difference.
Several COTS lurk beneath a table coral
Transient yachties add another strength: presence. Many are already moving through remote or lightly visited areas. When they understand what to look for and how to report it safely, they extend the reef watch network beyond scheduled commercial dives.
How Crown of Thorns culling is done
Culling methods vary by location, training, available equipment, and environmental rules. No method should be used casually. Crown of Thorns starfish have venomous spines, and poor culling practices can harm divers, corals, or other marine life. For example, if a diver is hanging onto the reef while attempting to cull the COTS, he/she may harm the healthy coral in the process.
Two broad methods dominate reef response work: manual removal and chemical treatment.
Manual removal can be useful but demanding
Manual removal means physically collecting Crown of Thorns starfish from the reef. Divers may use tongs, hooks, bags, bins, or other purpose-built tools to lift the animals without touching the spines. The starfish are then brought to the surface or to a boat for land disposal.
The main advantage is control. Divers can see exactly what they remove. There is no chemical injection, and the method can work well for small numbers or sensitive sites where teams prefer not to inject animals underwater.
But manual removal has limits.
Crown of Thorns starfish are awkward to handle. They can be large, heavy, and difficult to bag without damaging nearby coral. Moving them through the water can be slow. The risk of spine injuries is real. If a team is facing many animals across a large area, manual removal may require a lot of divers, bottom time, and boat support.
Manual removal also depends on careful disposal. Fragmenting or mishandling starfish is not a solution. The goal is removal, not scattering tissue or creating extra disturbance.
Manual collection works best when:
Starfish numbers are low to moderate
The site is accessible and safe for repeated dives
Divers have proper tools and training
Boats can support collection and disposal
Coral density allows removal without avoidable contact
Chemical treatments can be efficient when used correctly
Chemical treatment usually involves injecting each Crown of Thorns starfish with an approved substance that kills the animal in place. Administration of substances by means of an injection gun, such as vinegar or bile salts, are proven methods.
The strength of injection methods is efficiency. A trained diver can treat multiple starfish without lifting them from the reef. That reduces handling time and avoids carrying spiny animals through coral structures. For outbreak control, this can make a major difference.
Chemical treatment also has limits. It requires the right equipment, careful dosing, and approved protocols. Poor technique can lead to incomplete treatment. Use of unapproved chemicals can harm non-target organisms or damage trust in conservation work.
Injection programs work best when:
Divers are trained
Teams record each treated starfish
Follow-up surveys check whether numbers decline
Sites are revisited to find missed animals
The method is not a replacement for monitoring. It is a response tool. Culling removes pressure, but reef recovery depends on coral survival, water quality, herbivore populations, and future stress events. "We will never be able to control them completely - but we can give the reef a fighting chance", says Leo Rebele, owner of Sau Bay Divers.
COTS are part of the natural habitat in the South Pacific, but overpopulation can cause severe damage to critical coral reefs.
Tracking the Culling
A useful culling record does more than count dead starfish. It helps answer practical questions:
Question | Why it matters |
Which sites show repeated sightings? | These areas may need scheduled follow-up dives. |
Are the starfish clustered or scattered? | Clusters may point to outbreak pressure or recent movement. |
Are feeding scars fresh and widespread? | Fresh scars show active coral loss. |
Which method was used? | Method records help compare effort and results. |
Was the site checked again later? | Follow-up shows whether culling reduced visible pressure. |
For Rainbow Reef, active tracking across dive sites creates a clearer picture than isolated reports. A single sighting matters, but patterns matter more. If operators, yachties, and conservation divers report in a shared format, the reef community can respond faster and learn over time.
Community effort turns culling into reef stewardship
Crown of Thorns culling is a practical task, but it also builds a conservation culture. The work asks divers to slow down, look closely, and treat the reef as a living system rather than a backdrop.
Dive operators bring site knowledge, boats, trained staff, and frequent access. Yachties bring mobility, time on the water, and a willingness to help watch less-visited areas. Visiting divers bring extra eyes, photos, and sometimes professional skills from other conservation programs. When these groups share observations, the reef gains a wider safety net.
The best community efforts have a few common habits:
Clear reporting channels
Safe handling rules
Respect for local permissions and environmental regulations
Training for identification and culling methods
Follow-up surveys after culling
Honest records, including sites where no starfish were found
The “no starfish found” reports matter too. They help build a baseline. They show where monitoring has happened and prevent false assumptions that quiet sites are simply being ignored. Knowing which parts of the Rainbow Reef remain less impacted by COTS is valuable information to guard against future spread.
This work also reminds us that culling is only part of reef protection. Crown of Thorns outbreaks can be influenced by broader conditions, including nutrient runoff, predator loss, and reef disturbance. Protecting coral biodiversity also means supporting clean water, responsible boating, careful anchoring, sustainable fisheries, and reef-safe dive behavior.
A diver who removes one Crown of Thorns starfish helps. A community that records sightings, returns to sites, trains new volunteers, and protects the reef from other stressors helps much more.
The takeaway for Rainbow Reef
Rainbow Reef’s coral biodiversity is worth steady, organized care. Crown of Thorns starfish are part of the marine ecosystem, but outbreaks can upset the balance and weaken reef health. Culling, when done safely, can reduce immediate pressure on live coral. Tracking makes that work smarter.
The most useful reports are specific, repeatable, and shared. The most effective culling programs combine local knowledge with safe methods and follow-up checks. The strongest conservation efforts welcome many hands, from dive operators to transient yachties to visiting divers who care enough to report what they see.
Protecting a reef is not one dramatic act. It is a pattern of attention. One survey, one careful removal, one accurate record, and one return visit at a time, the community around Rainbow Reef can give its corals a better chance to recover, grow, and keep supporting the life that depends on them. Support Rainbow Reef Fiji Foundation today to help us preserve the Rainbow Reef!
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