2 crown of thorns starfish on coral

Crown-of-thorns starfish

Investigating the causes of crown-of-thorns starfish outbreaks

Since 1962, crown-of-thorns starfish outbreaks have had a major impact on the many reefs that make up the Great Barrier Reef. A fourth outbreak is currently underway in the World-Heritage Area.

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 research has revealed crown-of-thorns starfish are a major cause of coral loss on the Great Barrier Reef, after coral bleaching.

AIMS monitors crown-of-thorns starfish as part of the Long-Term Monitoring Program. This Program has shown that outbreaks begin in the north and migrate 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.

Starfish outbreaks over the years

Map of Great Barrier Reef showing starfish outbreaks
Animation of COTS outbreaks from 1985

This visualisation is based on data from the AIMS Long-Term Monitoring Program.

Data analysis: These data were interpolated using IDW (inverse distance weight) function to create a grid for each year. IDW is available in the package gstat for the language R. The interpolation used a fixed radius of 1 degree. Any cell in the grid which contained less than three points was assigned a ‘no data’ value.