Spines, Stars, and Surveys: Managing a Crown-of-Thorns Outbreak
Armed with dive slates and syringes of vinegar, our survey teams have been hitting the water to monitor and manage a local Crown-of-Thorns outbreak. It’s all about spines, strategy, and helping maintain reef balance.
You may be wondering, ‘What on earth is a Crown-of-Thorns’? Crown-of-Thorns (COTs) is a species of starfish native to the Philippines and occurs naturally across coral reef ecosystems. They get their name from the sharp, toxic spines that cover their upper body and are notorious corallivores — feeding primarily on hard coral polyps. And boy, do they eat! A single adult COT can consume up to 10 m² of reef-building coral per year, so when populations boom, they can pose a serious threat to reef health.
As well as having a ferocious appetite, these starfish have evolved two other effective survival strategies. Firstly, their infamous toxic spines act as a defense mechanism, so they don’t have many natural predators, and the ones they do have tend to occur in low numbers. Secondly, female COTs are egg-releasing machines, producing up to 200 million eggs each year — talk about a reproductive powerhouse! Put simply, these guys have survival figured out to a T.
Things started looking suspicious during our regular monitoring dives… COTs are usually nocturnal, hiding under ledges or in reef crevices during the day. But at Andulay MPA (one of the oldest MPAs in the region), our survey teams began spotting them out and about in broad daylight — a telltale sign of a Crown-of-Thorns outbreak.
Readying syringes before the dives
In response to this, a team of 12 staff and volunteers carried out a site-wide COT survey to assess the full extent of the outbreak. With the outbreak threshold being 15 individuals per hectare, our first survey recorded a staggering 164 COTs within just 0.6 hectares! This was unexpectedly high, and one thing was clear — action was urgently needed.
Over the following weeks, and with permission from the DENR, our survey teams carried out multiple intervention dives to reduce the COT population back to a healthy level. For this, each COT was injected with 20 ml of household vinegar — a surprisingly simple but effective solution that doesn’t have any negative impact on the wider marine environment.
As marine conservationists, we don’t take the decision to cull any marine organism lightly — we recognize the importance of every species in the ecosystem, and our goal is to preserve marine life. However, in this case, culling is necessary to safeguard the future of this reef for the local community, and this is an effective, globally utilized approach.
During each intervention dive, our divers were careful not to inject every COT they saw — after all, these starfish are still a natural part of the reef! By feeding on fast-growing corals, COTs help maintain balance, giving slower-growing species the space they need to thrive. Our intention is to restore the population to a healthy level, not wipe them out completely — it’s all about balance!
So, what causes COT outbreaks? Scientists have been trying to figure that out for years, and the answer seems to lie in a mix of human-induced pressures. When there’s more farming or development along the coast, extra nutrients can wash into the ocean, boosting algae growth — a favorite food of baby COTs, increasing their survival. Fewer predators like Triton’s Trumpet or Horned Helmet snails, due to overfishing, also means fewer COTs are being eaten. And with ocean temperatures rising, young COTs can grow faster and survive more easily. Most likely, this outbreak was driven by a combination of these different factors — a reminder of just how closely our actions are tied to the health of our oceans.
Interestingly, this does seem to be a regional issue, rather than limited to just this one reef. Fortunately, we have the resources at MCP to continue monitoring and managing this COT outbreak for the foreseeable future. Through our long-term monitoring program, we will also be keeping an eye on the neighboring reefs and surveying any suspected outbreaks elsewhere. In the meantime, our Science Officers will be busy collating data after each dive and writing up a report for the relevant Local Government Units.
At MCP, our long-term monitoring program serves many purposes… At the moment, it is helping us keep the reef balanced — one survey (and one starfish) at a time!
