10. What is an outbreak of starfish?

The term "outbreak" is often used by marine scientists when referring to large populations of crown-of-thorns starfish. It is a difficult term to define or quantify because crown-of-thorns starfish populations vary markedly in size and in their effects on corals. Some populations have been estimated to comprise hundreds of thousands of individuals (some have even been estimated to contain several million starfish), whilst others have been composed of only a few hundred starfish (causing moderate to high coral mortality within small isolated areas). In addition, populations comprising tens of individuals have been reported. The crown-of-thorns starfish also may be exceedingly rare on some reefs, with only a few individuals observed over a kilometre of reef. Despite such variability, it is generally considered that an outbreaking population comprises thousands, or indeed tens of thousands of individual starfish (see Fig. 4) which cause high coral mortality within isolated areas or over much of the reef.

Figure 4 An aggregation of crown-of-thorns starfish feeding on staghorn corals (genus Acropora). This feeding front has moved from left (deeper water) to right (shallower water) since those corals which have been dead the longest occur on the extreme left while live corals are present mostly on the extreme right. Recently dead corals occur just to the left of the starfish.

Scientists presently classify outbreaks into one of two broad types based on how they are thought to arise. Those which seem to occur as a result of changes in certain local factors in and around a reef (eg. nutrient conditions, temperature, salinity) are termed primary outbreaks. Those which have resulted from nearby outbreaks of starfish (either due to larval recruitment from reefs upstream or by adult migration) are referred to as secondary outbreaks.

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