14. How long do outbreaks of crown-of-thorns starfish last?

The duration of outbreaks on individual reefs varies widely: some may last for 4-5 years whilst others may have run their course within 1-2 years. The duration of an outbreak probably depends on the size and shape of the reef, the number of starfish in the population, the amount of corals available and certain physical factors (eg. weather conditions).

In general, reefs pass through a series of 5 distinct stages during an outbreak of crown-of-thorns starfish. This is termed an outbreak cycle. These stages represent the main changes in abundance of the crown-of-thorns starfish and its coral prey:

Stage 1.	Before the outbreak (high live coral cover and very few crown-of thorns starfish) (see Fig. 5)
Stage 2.	Start of the outbreak (increasing numbers of crown-of-thorns starfish and declining amounts of coral) (see Fig. 6).
Stage 3.	Height of the outbreak (very high numbers of crown-of-thorns starfish and large amounts of recently dead coral).
Stage 4.	Decline of the outbreak (rapidly declining numbers of crown-of-thorns starfish and low live coral cover).
Stage 5.	Recovery from the outbreak (few crown-of-thorns starfish and increasing amounts of live coral) (see Fig. 7).

Figure 5 A coral reef community prior to an outbreak of crown-of-thorns starfish. Note the high coverage of live corals.

Figure 6 The beginning of an outbreak of crown-of-thorns starfish. Note the numerous white corals. A number of starfish may be seen hidden amonst the corals.

Figure 7 A coral reef community after a large outbreak of crown-of-thorns starfish. Note the many drab, algal covered corals. The plate like coral in the bottom left-hand corner is mostly still alive.

The time taken to progress through each of these stages varies from reef to reef. It may take several months for reefs to go from stage 1 (before the outbreak) to stage 2 (start of the outbreak) and it usually takes a further 6-/2 months before the crown-of-thorns population reaches its peak (stage 3). The decline of an outbreak is signified by the rapid disappearance of the majority of starfish usually once there are very few live corals remaining (although this is not always the case). This rapid decline may occur within a few weeks to several months.

Despite this, it may take considerably more time before pre-outbreak levels of starfish abundance are reached as moderate numbers of starfish may persist on reefs for several years. At least 12-15 years is required before pre-outbreak levels of live coral cover are achieved (see 34). The changes in abundance of crown-of-thorns starfish and corals during this cycle are depicted in Figure 8. They provide a good example of the ways in which the abundance of a predator alters in relation to the abundance of its prey. Note that the cycle of the starfish lags behind that of its coral prey.

Figure 8 A generalised plot of the change in abundance of the crown-of-thorns starfish and its coral prey during an outbreak cycle.

Whilst outbreaks of crown-of-thorns starfish may be present on individual reefs for up to 4-5 years they may last for a considerably longer time in complex reef systems, which comprise many hundreds of reefs separated by relatively short distances (ie. up to 100 km) of shallow water (ie. up to 40-50 m in depth). For example, outbreaks which took place on the Great Barrier Reef during the 1960's and 1970's lasted for about I 5 years (see 36). Outbreaks have been taking place in the Ryukyu Islands for approximately 20 years. The duration of outbreaks in such highly complex systems may well depend on the rate of transport (via water currents) (see 28) of material (eg. larvae) between reefs and the extent to which individual reefs are linked by these transportation processes.

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