In the past 40 years, three waves of crown-of-thorns starfish outbreaks have had a major impact on the many reefs that make up the Great Barrier Reef.
Crown-of-thorns starfish (also known as COTS) are marine invertebrates that feed on coral. They occur naturally on reefs throughout the Indo-Pacific region, and when conditions are right, they can reach plague proportions and devastate hard coral communities.
Our 2012 study revealed that crown-of-thorns starfish, along with tropical cyclones, have been the two leading cause of coral cover losses over the past 27 years.
AIMS runs a major crown-of-thorns starfish monitoring program on the Reef. This long-term program has shown that outbreaks have begun in the north and migrated southward over about a 15-year period, with ocean currents transporting larvae between reefs.
The surveys also show that healthy reefs generally recover between outbreaks, taking 10 to 20 years to do so. However, recovery takes longer on reefs that are affected by additional stresses, such as coral bleaching, cyclones or poor water quality, so the coral may not fully recover before the next wave of outbreaks occurs.
Laboratory research at AIMS has shown that survival of crown-of-thorns starfish larvae increases dramatically when phytoplankton, their food source, becomes more abundant. Phytoplankton numbers are usually low in reef waters, but production can increase rapidly if early-season monsoonal and cyclonic floods carry fertilisers and other pollutants into the Great Barrier Reef lagoon.
Once dense breeding populations of starfish develop on some reefs, the huge numbers of larvae that they produce can establish outbreaks on mid-shelf reefs in the central Reef, even though these reefs are hardly ever affected by runoff.
Overfishing may also contribute to the formation or persistence of crown-of-thorns starfish outbreaks.