Crown-of-thorns starfish (COTS) are marine invertebrates that feed on coral and occur naturally on reefs throughout the Indo-Pacific region. When conditions are right for COTS to multiply, they can reach plague proportions and devastate the hard coral population on affected reefs.

In the past 40 years, three major COTS outbreaks have had a major impact on many reefs of the Great Barrier Reef (GBR). COTS outbreaks are responsible for a greater decline in coral cover than any other threat to the GBR.

AIMS has implemented the most comprehensive COTS monitoring program in the world across the GBR. This long-term program, combined with genetic studies, has shown that COTS outbreaks begin in the north and migrate southward on ocean currents over about a 15 year period. These surveys also show that healthy reefs generally recover between outbreaks, taking 10-20 years to do so. Reefs affected by additional stresses, such as coral bleaching, cyclones or poor water quality, may take a lot longer.

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.

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