What is the problem?

When crown-of-thorns starfish numbers are low (for example, when only one starfish is seen during a 20-minute swim), corals can readily recover from the limited predation. At these times, management activities are not required.

However, when the density of crown-of-thorns starfish increases to a point where the starfish consumes coral tissue faster than the corals can grow, a decline in coral cover is likely to occur.

An outbreak is considered to have occurred when there is roughly more than 15 starfish per hectare or more than one starfish spotted per 20 minute swim.

These estimates apply 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.

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.

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. The first two are now protected on the Great Barrier Reef, however giant triton numbers have not fully recovered to pre-harvesting levels.

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