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  • By Raquel Bagnol

Crown-of-thorns starfish outbreak decimates coral reefs in Cook Islands

Crown of thorns

Scientists who conducted coral reef surveys in the Cook Islands were shocked to witness the devastating effects of a crown-of-thorns-starfish (COTS) outbreak, according to Alexandra Dempsey, director of Science Management for the Khaled bin Sultan Living Oceans Foundation.

The foundation conducted a five-year Global Reef Expedition in 2013 with a team of international scientists to study the coral reef crisis in the Pacific islands. In a recently released report, the foundation noted that over 80 percent of the live coral community on the reefs surrounding the island of Aitutaki was decimated by these corallivores.

COTS are naturally occurring organisms on the reefs of the Indo-Pacific Ocean that primarily eat coral. “Finding a few COTS living on a coral reef is a normal and healthy part of the coral reef ecosystem. However, at times their population can drastically exceed normal levels and a COTS outbreak can occur,” Dempsey wrote on the foundation’s website.

She said COTS outbreaks can cause extremely rapid destruction of coral reef ecosystems in a matter of months.

“In recent years, COTS outbreaks have increased in scale and frequency, causing marine resource managers and scientists to take action. We saw outbreak numbers in the Cook Islands, specifically on the reefs surrounding Aitutaki. Our science team decided to take the initiative and physically removed COTS from the reefs to better study their size class structure and record the number of starfish. But this was no small task.”

The expedition involved over 400 surveys of the coral and reef fish communities in 30 dive sites surrounding Rarotonga, Aitutaki and Palmerston Atoll. The team used an underwater digital video camera to record real time at each location and analyze the fish communities, as well as determine how fishing pressures might be affecting the fish communities.


The survey data showed that the benthic communities of the Cook Islands appear to be in moderate to good condition, except for Aitutaki, which showed a lack of live coral cover and was also experiencing a COTS outbreak when the foundation did the survey. In Aitutaki, the foundation helped train Cook Islanders to recognize COTS and its dangers, recognize an outbreak and established practices on how to minimize the impact.

Palmerston had the healthiest reef system of the three atolls surveyed. The ranking showed Palmerston had a good reef system with high coral cover and a healthy assemblage of algae and invertebrates. Rarotonga showed moderate reef health with moderate coral cover and high algae.

Besides the COTS outbreak, there are many other threats to the health of coral reefs. These include pollution, climate change, overfishing, storm damage and natural disasters such as cyclones and warm water induced coral bleaching. With all these threats, it is therefore very important to protect the reefs from other natural and anthropogenic stressors, the scientists said.

The survey showed that with continued efforts to protect and preserve the fish and benthic communities, it is possible for these reefs to become some of the best in the South Pacific.

Despite the moderately healthy fish and benthic communities of the Cook Islands, the lax or nonexistent nearshore fishing regulations and management makes the fish populations vulnerable to collapse from overfishing.

With this, a complete participation, cooperation and enforcement of fisheries regulations by the community and the government is necessary to preserve fish populations. This should involve monitoring and recording catch sizes, establishing size and catch limits, and enforcing quotas for important fish species.

Aitutaki and Rarotonga follow a clear trend where the more populous the island, the more degraded the reef and fish health.


The KSLOF survey showed that fish community health is inversely correlated with human population. There is a lack of large predatory fish around the more populous islands.

Palmerston, the most sparsely populated island, had a noticeably higher number of large predatory fish. This is an indication that fishing pressure on the islands of Aitutaki and Rarotonga which has a higher population has likely decreased the number of top predators.

Rarotonga, which is the most populous at Cooks Island had the lowest mean value for all fish population metrics--this includes species richness, fish density, and biomass), and this is most likely due to high fishing pressure.

On a global comparison of the fish density and biomass, KSLOF data shows that the fish populations of the Cook Islands are comparable to other nearby countries such as French Polynesia and Fiji.

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