Insights into reproductive trends, predation patterns, and climatic triggers observed before a crown-of-thorns (COTS) outbreak are crucial for developing effective mitigation strategies to protect coral reefs, according to Ciemon Caballes, Ph.D., a senior research associate at the University of Guam’s NSF EPSCoR and assistant professor at the UOG Marine Lab.
Caballes shared his insights during the “Guam Crown-of-Thorns Starfish Outbreak Response Strategy” workshop in December at the UOG Marine Lab.
COTS are marine invertebrates known for their predation on corals. They can cause devastating impacts on coral reefs by depleting coral populations. They are found in reefs across the Indo-Pacific region.
Caballes’ research, supported by a grant from the National Fish and Wildlife Foundation, in collaboration with UOG’s NSF EPSCoR and Sea Grant, and other local and federal resource management agencies, focuses on understanding and mitigating these outbreaks.
“When we try to catch a serial predator, like the crown-of-thorns starfish, we need to build a profile,” Caballes said noting that understanding COTS and the trends leading to an outbreak is crucial for an effective mitigation strategy.
Caballes outlined several hypotheses regarding the formation of COTS outbreaks. The first suggests that outbreaks are a natural biological predisposition, given the species’ high fertilization rates.
According to Caballes, even a single mating pair can initiate a population surge. He said a female starfish could potentially produce over 100 million eggs in one spawning season while a male can produce around 50 billion sperm.
Surprisingly, he pointed out that, despite a male-skewed sex ratio, crown-of-thorns starfish maintain high fertilization rates, achieving up to 30% success even when separated by significant distances.
The second hypothesis links outbreaks to the overfishing of natural predators, leading to the release of COTS from predatory pressure and triggering a trophic cascade.
He also discussed the nutrient hypothesis, which posits that elevated nutrient levels from river runoff result in increased phytoplankton densities. Since phytoplankton provide a source of food for COTS larvae. the spike improves their survival and development rates. However, he also said recent findings indicate that excessive phytoplankton might lead to overfeeding, affecting larvae survival negatively.
Caballes also recalled several significant outbreaks, particularly one in 2009, and noted the possible influence of temperature and strong El Niño-Southern Oscillation (ENSO) events on COTS populations. He illustrated the correlation between high precipitation and river discharge events, such as those in 2003 and 2004, and subsequent COTS outbreaks in 2006 and 2007.
Caballes noted, “With back-to-back coral bleaching events compounded by COTS outbreaks, corals are struggling to recover.” He emphasized the need for effective management strategies to address these dual threats.
“This workshop is the culmination of our work since 2022. The project began in March of that year to assess the crown-of-thorns starfish populations around Guam and explore the feasibility of coral rehabilitation in these reef areas.” Caballes stated.
Caballes described their research efforts, which included testing various plots to determine if culling crown-of-thorns starfish would enhance the survival of transplanted corals.
Input from workshop participants have been integrated into the Guam COTS Outbreak Strategic Management Plan, which will be available to the public and distributed to all local and federal partners.
Fran Castro, associate director for operations and development at the UOG Center for Island Sustainability and Sea Grant, highlighted the significance of the project in addressing COTS. She described multi-agency efforts that involved training local agencies and coordinating monitoring efforts.