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Crown of thorns menaces Guam reefs

Become Guam’s “eyes of the reef” to help combat coral-eating sea stars

This year, local coral reef managers, scientists, and community members have recorded sightings of a coral-eating predator, the crown-of-thorns sea star (known as COTS), on Guam’s reefs.

Outbreaks of the venomous crown-of-thorns sea star can cause severe damage

to coral reefs. The spiny sea stars eat coral and have few predators. (Photo: Whitney Hoot)

One adult COTS can eat more than 100 square feet of coral per year – the size of a 10 ft x 10 ft canopy. This can make a big impact on Guam’s coral reefs, which have declined in recent years due to local impacts, such as pollution, heavy fishing pressure and coral bleaching caused by global climate change. Although COTS are not invasive and are a natural part of Guam’s coral reef systems, sometimes their numbers explode. COTS outbreaks can cause severe damage to coral reefs over a just a few weeks.

The global community is still working to address climate change, but here on Guam we can help protect our reefs by preventing damage caused by these hungry, venomous sea stars. By attending a training and participating in the Eyes of the Reef Marianas program, you can join forces with Guam’s Coral Reef Response Team and other Guam residents to protect our reefs from COTS outbreaks. The next EOR training session will be held at the Piti Church Social Hall on Wed., Nov. 14th from 6:00 to 8:00 p.m.. All ages are welcome and participation is free. Through this training you will learn how to identify coral reef impacts – including COTS – and report them online. Learn more or sign up at

Anyone who spends time on Guam’s reefs while swimming, snorkeling, diving, paddling, or fishing can make a report online. If you see any COTS while you are out enjoying Guam’s coral reefs, the response team hopes you will submit a report. Please note the location, depth, approximate size of the COTS, and the number of individuals. You can fill out a quick report at the EOR website (, email, or call 475-9684.

The Guam Coral Reef Response Team – a partnership between local and federal agencies – aims to protect Guam’s reefs from acute impacts, such as coral bleaching events, boat groundings, and COTS outbreaks. The response team relies on community members to provide reports on reef impacts so the team can respond as quickly as possible.

This year, the team has received over 50 reports of COTS sightings from participants. With a better understanding of where COTS numbers are highest, the team is applying an innovative method to manage COTS populations. Divers use a special injection system to put a small amount of ox bile – a byproduct of the cattle industry – into each COTS. The mixture kills the COTS without harming the environment or other reef creatures. The response team has conducted successful COTS removal efforts at five sites so far, including Gun Beach and Hap’s Reef.

COTS spines can deliver a painful sting and handling them can be dangerous. Also, if you injure one, it may spawn and release a new generation of sea stars, making the outbreak worse. Do not remove COTS yourself. The best thing to do is to report your sighting so the team can decide how to manage any outbreaks. With more “eyes of the reef,” we can work together to preserve Guam’s valuable coral reefs for current and future generations.

Whitney Hoot is the Coral Reef Resilience Coordinator at the Guam Bureau of Statistics and Plans (, or call 475-9684.]


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