Crown-of-thorns starfish FAQs
The Pacific crown-of-thorns starfish (COTS), Acanthaster cf. solaris, is a marine invertebrate that is native to the Great Barrier Reef. These starfish also naturally occur on coral reefs throughout the Indo-Pacific region, and closely related starfish species also occur across reefs in the Indian Ocean and Red Sea.
These starfish feed exclusively on live coral as adults, can grow rapidly to reach maturity, and produce large numbers of offspring once they mature. Because of these life history characteristics, populations of these starfish can grow to reach ‘outbreak’ numbers where they become a pest and cause significant damage to coral reefs.
Four contemporary major crown-of-thorns starfish outbreaks have been documented on the Great Barrier Reef — beginning in the 1960s, the late 1970s, the early 1990s and the current outbreak, which was first detected in 2010.
Oral histories and anecdotal accounts also suggest there were crown-of-thorns starfish outbreaks on the Great Barrier Reef prior to the 1960s, although the frequency and severity of these historical outbreaks is not well understood.
The four contemporary major crown-of-thorns starfish outbreaks on the Great Barrier Reef have followed a similar pattern of initiation and spread. The outbreaks begin about every 15 years on reefs between Lizard Island and Green Island, in the northern management region of the Marine Park. This region is generally referred to as the ‘initiation box’. The outbreaks then spread southward down the length of the Great Barrier Reef over a period of at least 10-12 years. The southern spread of these outbreaks happens mainly through the transport of crown-of-thorns starfish larvae on ocean currents.
Crown-of-thorns starfish outbreaks have also been documented periodically in the remote Swain Reefs, located at the far southern end of the Marine Park. These outbreaks are thought to develop separately from the outbreaks that begin in the north.
Taken together, this means that crown-of-thorns starfish outbreaks are impacting the Great Barrier Reef most of the time, although the spatial location of that impact changes over time.
Further information on patterns of outbreaks over time across the Great Barrier Reef is available through the Australian Institute of Marine Science Long-term Monitoring Program.
This question has long been researched and debated in the scientific community. Through this research it has become increasingly clear that there is unlikely to be one single factor that causes outbreaks to start. Instead, it is likely to be a combination of factors that together provide the conditions for outbreak initiation, including:
- the biological traits of the starfish themselves, which enable crown-of-thorns starfish populations to grow and reproduce rapidly in the right conditions
- reductions in the abundance of natural predators of crown-of-thorns starfish, which may enhance the ability of crown-of-thorns starfish populations to grow and reach outbreak numbers
- excess nutrients from run-off or natural up-welling that provides extra food for crown-of-thorns starfish larvae to survive and multiply
- patterns of ocean currents that spread crown-of-thorns starfish larvae in such a way that promotes the development of outbreaks
- the availability of sufficient live coral as a food source to support outbreak numbers.
Crown-of-thorns starfish usually occur on coral reefs in low numbers (for example, a few starfish per hectare of reef habitat). When in low numbers, starfish feeding causes mortality of individual coral colonies on a local scale, but is unlikely to cause decline in coral cover across an entire reef.
Outbreaks occur when starfish numbers become unsustainable for coral growth and recovery. This means that starfish numbers increase to levels where the impact of their feeding is expected to cause decline in coral cover across an entire reef.
Individual reefs across the Marine Park experience varying severity of crown-of-thorns starfish outbreaks, with some reefs becoming severely infested, some reefs mildly infested, and some reefs not impacted at all by an outbreak. This variability in crown-of-thorns starfish impact amongst reefs poses a challenge to managers, particularly given the immense spatial scale of the Great Barrier Reef Marine Park (~3,000 reefs across 344,440 km2). It is impossible to know the ‘Outbreak Status’ of every individual reef in the Marine Park at any one time.
Regular monitoring of crown-of-thorns starfish numbers helps managers understand the current ‘Outbreak Status’ of a reef, and identify when crown-of-thorns starfish numbers are beginning to increase to unsustainable levels. The two main survey techniques used to monitor crown-of-thorns starfish numbers across reefs in the Marine Park are manta tow surveys and Reef Health and Impact Surveys (RHIS).
The ‘Outbreak Status’ thresholds used by the Marine Park Authority are derived from scientific research, and are tailored to the particular survey technique that is used to estimate the number of crown-of-thorns starfish on a reef. These thresholds indicate the average number of crown-of-thorns starfish on a reef that are unsustainable for coral.
Outbreak Status of a reef
Impact on coral at that reef
Average number of crown-of-thorns starfish per manta tow survey
Average number of crown-of-thorns starfish per RHIS
Crown-of-thorns starfish numbers are sustainable for coral
Crown-of-thorns starfish numbers may cause coral decline
Crown-of-thorns starfish numbers will cause coral decline
Crown-of-thorns starfish numbers will cause severe coral decline