Monitoring Crown-of-Thorns Starfish on the Great Barrier Reef

AIMS monitors crown-of-thorns starfish populations on the Great Barrier Reef to determine the current state of outbreaks along the Reef, which factors influence the likelihood of a crown-of-thorns starfish outbreak, and how long it takes coral to recover after an outbreak.

Crown-of-thorns starfish feeding

We survey crown-of-thorns starfish by towing a snorkeler with a manta board behind a small boat around the perimeter of a reef. The boat stops every two minutes (which is approximately every 200 m) to allow the observer to write down what they have seen.

Crown-of-thorns starfish populations are considered to be 'outbreaks' when they reach densities such that the starfish are consuming coral tissue faster than the corals can grow.

There are various ways of estimating this level, but when populations on a reef average one per tow the coral cover will certainly be reduced; this is referred to as an active outbreak. Average densities as low as 0.22 COTS per tow can cause damage; this is referred to as incipient outbreak.

One way to get a historical view of the numbers of crown-of-thorns starfish on the Great Barrier Reef is to look at the proportion of reefs with outbreaks. Of the reefs surveyed in 2021, 4 per cent have outbreaks. This compares with 10 per cent in 1988 when crown-of-thorns starfish caused widespread destruction on reefs in the central Great Barrier Reef. The graph below shows the percentage of surveyed reefs with outbreaks in each year since 1986.

Another way to look at crown-of-thorns starfish populations is to consider their average density across the entire Great Barrier Reef. This is expressed as crown-of-thorns starfish per two-minute tow.

In 2021, the surveyed reefs had an average density of 0.13 crown-of-thorns starfish per two-minute tow. This compares with 1.17 in 1988. The graph below shows the average crown-of-thorns starfish (COTS) density since 1986.

The following reefs had outbreaks of crown-of-thorns starfish when last surveyed:
(click on reef name for detailed information)

More information

• Crown-of-thorns starfish monitoring is carried out by the Long-term Monitoring Program, led by Dr Mike Emslie.
• Results of crown-of-thorns starfish monitoring discussed