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Hungry sea stars threaten Guam coral reefs

A crown of thorns sea star (COTS) perches on a coral in Asgadao Bay

in the Achang Marine Preserve off Merizo village. These starfish are

hungry coral predators that can destroy large areas of coral reefs in

a very short time. (Photos: Dave Burdick)

A voracious, venomous predator may be coming for our corals, and Guam’s coral reef response team is hoping you can help.

The crown of thorns sea star, or COTS for short, is a large, spiny sea star that eats coral. Though these starfish are a natural part of Guam’s reef systems, sometimes their populations explode. Scientists don’t fully understand why these huge population spikes happen, but it may have to do with water pollution.

The coral reef response team, which includes local and federal government agency members who respond to reef threats, has noticed an increase in COTS on local reefs recently. If you see any COTS while you are out enjoying Guam’s coral reefs, the team hopes you will let them know. Please note the location, depth, approximate size of the COTS, and the number of individuals. You can fill out a quick report at the Eyes of the Reef (EOR) website,, send an email to, or call (671) 475-9684. For more information about our reefs and the COTS threat, you can attend an EOR training at from 6:00-8:00 pm on Thursday, April 26 in the Fort Soledad Ballroom at the Outrigger Guam Resort. You can sign up for training at

COTS spines can deliver a painful sting and handling them can be dangerous. Also, if you injure one, it may release a new generation of sea stars, making the epidemic worse. But reports of where the COTS are will help the reef response team decide which areas may need COTS control efforts.

With a better understanding of where COTS numbers are highest, the response team will test a COTS control method used in American Samoa and Australia. Divers use a special injection system to put a small amount of ox bile into individual COTS. The mixture kills the COTS without damaging the environment or other reef residents. Once divers are comfortable with the equipment and the process, the response team will conduct a control project to remove COTS from special reefs that are important fish nurseries, dive spots, or fishing grounds.

One medium-sized COTS can eat an area about the size of a 10-foot by 10-foot canopy in a year. This doesn’t sound like a lot, but if you have thousands of hungry sea stars on a reef, they can devastate an area very quickly. COTS are especially fond of the delicate branching corals that have recently been weakened by warmer ocean waters. COTS reporting and removal could help keep our coral reefs healthier and diverse by protecting vulnerable types of coral.

For more information, please contact Whitney Hoot at the Bureau of Statistics and Plans at or (671) 475-9684.

A crown of thorns sea star (COTS) eats a large Porites coral near Ga’an Point in

Agat. Although these starfish prefer to eat delicate, branching corals, on Guam they

have started eatingvery large coral species that are not normally their preferred food source.


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