Crown of Thorns Starfish on the Great Barrier Reef

The AIMS Long Term Monitoring Program Surveys:

Crown of thorns starfish (COTS) populations are surveyed by towing an observer with a manta board behind a small boat around the perimeter of a reef. After towing for two minutes, the boat stops and the observer writes down how many COTS were seen. Crown-of-thorns starfish feeding

In principle, COTS populations are described as 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 status of COTS on the Great Barrier Reef is to look at the proportion of reefs with outbreaks. Of the reefs surveyed in 2007, 4% have outbreaks. This compares with 10% in 1988 when COTS caused widespread destruction on reefs in the central Great Barrier Reef. The graph below shows the population of reefs with outbreaks in each year since 1986.

Another way to look at COTS populations is to consider the average COTS density across the entire Great Barrier Reef. This is expressed as COTS per tow.

On the reefs surveyed in 2007, the average density is .04 cots per tow. This compares with 1.17 in 1988. The graph below shows the average COTS density since 1986.

Record your observations - have you seen COTS on a recent visit to the GBR?