What causes crown-of-thorns starfish outbreaks?

While there have been many theories on the causes of outbreaks of the crown-of-thorns starfish, there are three theories that are supported by scientists. These theories have neither been proved nor disproved. The theories are:

• Fluctuations in the crown-of-thorns starfish population are a natural phenomenon.

• Removal of natural predators of the crown-of-thorns starfish has allowed populations to expand.

• Human use of the coastal zone has increased the nutrients flowing to the sea and resulted in an increase in planktonic food for larvae of the crown-of-thorns starfish. The improved survival of larvae has led to an increase in the number of adult starfish which results in outbreaks.

Natural fluctuations

Like many marine invertebrates, crown-of-thorns starfish produce large numbers of eggs - potentially 1,000 million eggs in the lifetime of a single female. Fluctuations in the environment can change the survival rate of the larvae. If larval survival is improved from one larva in 100 million, to one larva in 10 million, there would be a 10-fold increase in the population of crown-of-thorns starfish within one generation. Once there was a small increase in the starfish population, the success of subsequent spawning aggregations would ensure that even more larvae could survive in successive generations.

Natural fluctuations in temperature, salinity or availability of planktonic food could all contribute to improving the survival of crown-of-thorns starfish larvae. There has been some suggestion that outbreaks of the crown-of-thorns starfish are linked to the timing of the El Nino-Southern Oscillation (ENSO) system which can cause dramatic shifts in local climate around the Pacific Ocean.

Removal of predators

Although crown-of-thorns starfish have few predators, one theory suggests that they play an important role in keeping starfish populations in check. Predators of adult crown-of-thorns starfish include the giant triton snail, the humphead maori wrasse, starry pufferfish and titan triggerfish.

The giant triton snails was highly prized and heavily collected prior to its protection in 1969. Numbers of the triton snails on the reef remain low. However, the triton shell can eat only about one crown-of-thorns starfish per week so its capacity to prevent starfish outbreaks seems limited.

Humphead maori wrasse are reputed to be active crown-of-thorns starfish predators. Whilst maori wrasse have historically been taken in the Great Barrier Reef line fishery – and more recently in the live fish fishery – from December 2003, it became illegal under Queensland fisheries legislation to fish for, or keep this species.

Predation by other reef fish on juvenile starfish might also limit crown-of-thorns starfish populations. Juvenile starfish are most likely to be eaten by fish about six months after settling when they start to feed on coral. If numbers of the predator fish were depleted by fishing activities this might allow an abnormally large number of starfish to survive to maturity. There is no substantial evidence at this time to show that commercially exploited fish eat significant numbers of juvenile crown-of-thorns starfish.

Because the numbers of starfish larvae that usually survive and then settle is unknown and it is difficult to estimate feeding rates of predators in the field, it is unclear what is the minimum predation rate that would be needed to prevent outbreaks.

Human influence on water quality

Many reports from around the world have suggested that there is a relationship between periods of high rainfall and the beginning of crown-of-thorns starfish outbreaks. Periods of high rainfall after drought or extended dry periods cause water with low salinity, high sediment and high nutrient loads to be washed into the waters of the Great Barrier Reef.

High nutrient levels can cause an increase in microscopic algae in the water, providing food for the developing crown-of-thorns starfish larvae. This can increase the number of larvae that survive and lead to larger adult starfish populations. Low salinity also increases the survival of crown-of-thorns starfish larvae.

Therefore, flood events could be a natural cause of outbreaks. However, the amount of nutrients reaching the Great Barrier Reef lagoon from the adjacent rivers has increased several-fold since European settlement. This increased nutrient load could improve survival of larvae which could possibly cause outbreaks or could increase the frequency or intensity of outbreaks. Recent mathematical models that mimic a 10-fold increase in larval survival shows that this would lead to more frequent outbreaks of crown-of-thorns starfish over 200 years.