History of crown-of-thorns starfish outbreaks

Spicules from crown-of-thorns starfish skeletons have been found in old reef sediments which suggests that starfish have occurred on the Great Barrier Reef during the past 3,000 – 7,000 years. However, because the sediments become mixed over time and the spicules are difficult to date accurately, it is unclear whether starfish outbreaks are a new phenomenon or not.

The first documented case of large numbers of crown-of-thorns starfish on the Great Barrier Reef was noticed at Green Island off Cairns in 1962. It’s possible that starfish outbreaks are more likely to be noticed now than in the past because of increased tourism and the popularity of SCUBA diving.

Starfish outbreaks generally have started in the north of the Great Barrier Reef and moved southward. This pattern is believed to result from the starfish larvae being carried with the southerly flowing ocean currents in the area. For example, reefs off Townsville were affected in 1970 and by the mid-1970s, the starfish had reached reefs north of the Whitsundays, more than 300 km further south. This southward progression of outbreaks has been a consistent pattern in subsequent events in the Great Barrier Reef.

During the first outbreak in the 1960s, only a few reefs in each region carried large populations of crown-of-thorns starfish. As with the following outbreaks, mid-shelf reefs were the most likely to be affected. On some reefs, only sections around the perimeter were affected. This outbreak on the Great Barrier Reef ended about 1975, with subsequent good recovery of corals on most of the affected reefs.

A new outbreak was recorded in 1979, again starting in the Green Island region. Large Porites corals that were metres across and hundreds of years old, were eaten by the starfish, even though this type of coral is not usually preferred as food. Once again, this outbreak took about a decade to move south to the Swains Reefs, but largely missing the Whitsunday and Pompey regions. About 17% of reefs were affected by crown-of-thorns starfish during this outbreak.

In 1994, about 15 years after the start of the previous outbreak, large numbers of crown-of-thorns starfish were found in the Cooktown region. As expected, this was followed by reports of large populations further south along the Great Barrier Reef. By 2003, the highest densities of starfish were recorded in the area northeast of Townsville.

There have been persistent outbreaks on reefs in the Swains area. These starfish populations are genetically similar to northern populations but do not follow the pattern of southward spread of crown-of-thorns starfish outbreaks. In 2002-03, four of the seven reefs surveyed in the Swains sector by the Long-Term Monitoring Program had active outbreaks.