Assassins, equipped with voracious appetites, deadly spines, and 25 arms, are slowly and silently killing coral reefs. These undercover killers have wrecked havoc on the health and sustainability of coral reefs around the world, and continue to do so, relatively unchecked. These assassins even have a majestic name — the crown-of-thorn sea star (COTS).

Crown-of-thorn seastar. Credit: Carnivora Forum.

The Plight of Coral Reefs

Coral reefs are one of the most bio-diverse ecosystems in existence. However, many coral reefs around the world are under threat, due to the changing climate causing rising seawater temperatures and ocean acidification, and factors driven by human activities such as water pollution from agricultural run-off, destructive fishing, and coastal development. These threats, in combination with naturally occurring tropical storms and COTS outbreaks in recent years, have led to drastic and lasting impacts on the health and sustainability of coral reefs.

Major Coral Reef Regions of the World. World Resources Institute (2011).

The Great Barrier Reef (GBR), located off the North-Eastern Coast of Queensland Australia, is the world’s largest coral reef ecosystem. In recent years, the Great Barrier Reef has experienced significant disturbances to the reef ecosystem such as oil spills, high levels of nutrient runoff, mass coral bleaching, and tropical cyclones. Although these disturbances have had severe impacts on coral health, since the first outbreak recorded at Green Island in 1962, increased severity and frequency of COTS outbreaks have exacerbated coral loss.  

Australia’s Great Barrier Reef. Great Barrier Reef Citizens Science Alliance (2017).

A report published by the Australian Institute of Marine Science (AIMS), on the decline in coral cover on the Great Barrier Reef over 27 years, found that since 1985, mid-shelf and offshore reefs have experienced a decline of over 50%, with crown-of-thorn being responsible for approximately 42% of estimated coral loss. Arlington Reef specifically, located off the east coast of Cairns Australia has experienced a very close case of an active COTS outbreak, where coral loss would have been detrimental.

COTS Outbreaks on the Great Barrier Reef. Credit: Eric Lawrey, eAtlas.

Causes of COTS Outbreaks

In addressing the invasive behaviour of COTS, it’s important to distinguish the difference between introduced species and invasive species. In the case of COTS, these sea stars naturally occur in this region, but their invasive behaviour unfolds when the interaction between species are disturbed and their populations grow at a rate beyond coral recovery. A single starfish can consume up to 6 m² of corals per year, living up to 17 years.

In a typical outbreak, densities reach ~1500 COTS/km². In order to properly manage COTS outbreaks, we must first understand the underlying causes of outbreaks, currently an area of a serious knowledge gap. A research paper assessing different causes of COTS outbreaks, published in 2016, found that there are two leading outbreak hypotheses: the Terrestrial Runoff Hypothesis, assuming that COTS larvae survival increases with increased nutrient availability, and the Predator Removal Hypothesis, assuming the increase in COTS adult survival due to the removal of their predators though human fishing. The cause and effect between hypothesized outbreak drivers and the severity and impacts of these outbreaks on the Great Barrier Reef are highly complex and thus far, no conclusive answer has been achieved as to what driver is ultimately to blame. 

Impacts of COTS Outbreaks

Imagine a balanced coral reef community. COTS primarily feed on fast-growing corals, such as staghorn corals, which allows slow-growing corals a chance to compete and colonize space. In effect, COTS predation helps maintain coral diversity. However, when their first preference of food becomes scarce, they will turn to a wide variety of other corals. Thus when COTS numbers explode, the army leaves a cemetery of coral bones in its path. But it doesn’t stop there. Coral reefs are home to many marine species including fish, sea turtles, and crustaceans. With the destruction of habitat, the inhabitants would be forced to find other ways to survive.

Crown-of-thorn sea star predation on corals. Credit: The Conversation.

From a socio-economic perspective, the outbreak of COTS mainly threatens the reputation of tourism at the GBR. Within the industry, fear exists that news of COTS outbreaks destroying coral reefs would discourage visitors from choosing the GBR as a destination, which has led to the tourist industry actively organizing control teams to eradicate COTS on popular reefs. Whether or not COTS outbreaks indeed affect the influx of tourists is unknown, as statistics show that overall visitation to the GBR continues to increase despite active COTS outbreaks recorded.

As COTS outbreaks continue to be a leading source of coral reef degradation on the Great Barrier Reef, short-term and long-term management interventions have been implemented in attempts to manage the severity and frequency of COTS outbreaks, these interventions, however, have had limited success.

Attempts at Management: Case Study at Arlington Reef

While different reefs on the GBR tell a different experience with COTS, Arlington Reef has been heralded as a leading example of effective management response. When an incipient COTS outbreak was declared at Arlington Reef in 2014, the Association of Marine Park Tourism Operators swiftly launched control interventions with funding from the Australian government. A team of trained divers scanned the reef for COTS and injected them with euthanizing chemicals using a modified drench gun. In the past, sodium bisulfate was commonly used but it required multiple injections in each sea star. A newer, more powerful, and time-efficient method involving a single shot of bile salts began to replace the multi-shot sodium bisulfate method and is the main COTS control strategy on Arlington Reef. Fortunately, this injection method has no known adverse effects on the marine environment.

Diver injecting a crown-of-thorn sea star with a lethal chemical. Credit: Cairns Post.

Management Success?

27 000 COTS dead in just eight days.” – This was the triumph of Arlington Reef in 2014. While it surely is an outstanding report card for the single-shot bile salts injection method, what is the report card for the ecosystem? The most direct measurement of the success for control interventions is the density of COTS. Is there a decrease in the density of COTS after management? Arlington will answer yes! Since the year of incipient outbreak, COTS density has reduced to 10% of that level according to a report by AIMS, avoiding actual active outbreaks. But how are the corals doing? It turns out that coral cover has remained at 10-20% cover since the incipient outbreak. Although its predators have dropped in numbers, other sources of pressure, such as cyclones, may have suppressed coral recovery. Overall, COTS control at Arlington has been met with some success, but the future of coral health requires continued monitoring.

Support and Critiques of Management

Management interventions at Arlington Reef have been both supported by local communities, members of government, but critiqued by independent researchers.  Listed below are the opposing views.

Support of Management Critiques of Management

Local communities: 

Divers find it “rewarding, doing [their] bit to save the Reef” –Steve Moon, project manager of the COTS control plan.


Usually supportive of intervention programs because science is somewhat driven by political motives. 

 Independent Researchers: 

“There are no current methods that control these outbreaks” – Dr. Dworjanyn.
Believing that these plans are ineffective at controlling active COTS outbreaks on a population level.

Supporters of the current management plan generally agree that this plan allows a step-by-step basis for controlling COTS outbreaks, and allows local communities to participate in contributing to reef protection. The opposing views of the current management plan tend to deny the existence of any current “methods” since they are only successful at minimal spatial scales, which are miniscule in comparison to the Great Barrier Reef as a whole.

Other Attempts at Management

Other attempts to manage outbreaks proactively on the reef have been implemented. The Australian Government has introduced a long-term management plan aimed at protecting live coral cover and the integrity of the ecosystem from future COTS outbreaks. Two specific initiatives, the Reef Water Quality Protection Plan and the Australian Government’s Reef 2050 Plan are both aimed to address human-caused sources of reef degradation, such as reducing nutrient heavy agricultural run-off and improving reef water quality. Both of these plans, although on paper are relatively promising and provide metrics from which success of these plans can be measured, they have made relatively little progress as of yet in managing the anthropogenic factors contributing to COTS outbreaks since being implemented.

Where do we go from here?

Future management should be aimed at addressing COTS outbreaks in a proactive rather than reactive manner. It is clear that short-term removal methods may have some limited success at controlling current COTS outbreaks, but these current reactive control methods are not enough to reduce the occurrence of future outbreaks. Current attempts at long-term management of COTS outbreaks consists of changes to the current practices, including the Reef Water Quality Protection Plan and the Australian Government’s Reef 2050 Plan. Although these plans are a step in the right direction, they are designed to target a limited scope of anthropogenic drivers, and surprisingly do not address concerns regarding other drivers to the outbreaks. If long-term management is to be effective, it is important that it recognize and attempt to address multiple sources of COTS outbreak drivers, rather than only addressing what the government deems to be relevant. Overall, it is clear that the large majority of management methods of addressing crown-of-thorns sea star outbreaks on the Great Barrier Reef have been and remain largely ineffective.

Blog created by:

Gabriella Schauber (@schauber_g)

Sarah Ho (@SarahHo420)

Elizabeth Herbert (@eliizabethrae)