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, 6 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)

Sector Reef ID Date Surveyed Outbreak
Cape Upstart Showers 19076SMar 2007Incipient
Torres Strait Aureed 00082SFeb 2013Incipient
Cooktown / Lizard Island Two Isles 15002SOct 2012Active
Cooktown / Lizard Island Three Isles 15005SOct 2012Incipient
Cooktown / Lizard Island Eye 14134SDec 2014Active
Cooktown / Lizard Island Marx 15027SDec 2014Active
Cooktown / Lizard Island Irene 15084SDec 2014Active
Torres Strait Waier And Dauar 00088SFeb 2014Incipient
Princess Charlotte Bay Clack 14017SAug 2016Active
Swain 22-084 22084SJan 2020Active
Swain Small Lagoon 21572SJan 2020Active
Innisfail Flora 17010SDec 2016Incipient
Innisfail Gilbey 17057SDec 2016Active
Swain East Cay 21305SSep 2020Incipient
Swain Gannet Cay 21556SSep 2020Active
Swain Horseshoe 22104SSep 2020Active
Swain Snake 22088SSep 2020Active
Cape Grenville 11-049 11049SDec 2020Incipient
Torres Strait Erub 00084SMay 2021Incipient
Torres Strait Erub 00084SMay 2021Incipient
Torres Strait Mer 00085SJun 2021Incipient
Pompey 21-187 21187SSep 2019Incipient
Pompey 21-064 21064SSep 2019Active
Pompey 21-062 21062SMar 2018Active

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