What is the problem?

Corals can readily recover from crown-of-thorns predation when numbers are low (for example, when there are roughly less than 15 starfish per hectare). This estimate applies when there are moderate to high levels of coral cover (about 40–50 per cent). If coral cover is lower than this, fewer crown-of-thorns starfish are needed to have a negative impact. If crown of thorns numbers are low, management activities are not required.

When the density of crown-of-thorns starfish is greater than roughly 15 starfish per hectare, starfish consume coral tissue faster than the corals can grow (and a decline in coral cover is likely to occur). This is known as an outbreak.

Outbreaks are of concern to all Marine Park users, in particular the tourism industry. This is why significant efforts are made to manage outbreaks at tourism sites, and why the tourism industry has for many years played a key role in protecting coral from the crown-of-thorns starfish.

Recently, the Marine Park Authority and key stakeholders have extended targeted control to reefs of significant ecological value. These ecologically important reefs spread coral larvae, which helps corals recover from the impacts that we cannot manage locally (i.e. cyclones, bleaching).

How do outbreaks occur?

Flood plumes during the wet season can carry additional nutrients and sediments into the Reef. This promotes plankton blooms, which increase the food for many animals including crown-of-thorns starfish larvae.

Scientific evidence indicates elevated nutrient levels may exacerbate crown-of-thorns starfish outbreaks. The timing of a spike in nutrient levels is critical, because when a dramatic increase coincides with the crown-of-thorns starfish spawning season (around October to February) this has the potential to fuel outbreaks.

However, given outbreaks have occurred on the Reef for decades, nutrient enrichment is unlikely to be the sole reason for population explosions.

Another hypothesis relates to the removal of predators. This proposes that fishing and shell collecting have led to lower numbers of predators of the crown-of-thorns starfish, allowing populations to increase beyond natural levels.

Predators such as the giant triton snail, humphead Maori wrasse, sweetlip emperor and starry puffer fish feed to some extent on crown-of-thorns starfish adults. The first two are now protected on the Great Barrier Reef; however, giant triton numbers have not fully recovered to pre-harvesting levels. Many other reef fish will feed upon larval or juvenile starfish throughout their lifecycle.

While none of the starfish’s predators appear to feed exclusively on it, predation pressure is thought to be an important way to keep crown-of-thorns starfish populations in check.

Reef zoning has been shown to influence the prevalence of outbreaks. Green zones that are protected from fishing have lower frequency of crown-of-thorns starfish outbreaks compared to blue zones. This is likely because the green zones help keep the natural food webs intact.