Controlling outbreaks of the Crown-of-thorns Starfish

The Crown-of-Thorns Starfish (COTS) is not an alien species but it is a native marine pest that needs either effective containment or eradication. Over the last 30 years, this starfish has been responsible for almost half of a... [ view full abstract ]

The Crown-of-Thorns Starfish (COTS) is not an alien species but it is a native marine pest that needs either effective containment or eradication. Over the last 30 years, this starfish has been responsible for almost half of a significant decline in coral cover on the Great Barrier Reef. In 2012, the fourth cycle of starfish outbreaks since the 1960’s emerged on the northern GBR and the tourism industry responded with an aggressive program of diver-based eradication of starfish hotspots informed by a comprehensive surveillance program. AIMS has analysed abundance data from the surveillance program and other sources to estimate the size of the target confronting the control program. In 2013, outbreaks were common on mid-shelf reefs along more than two degrees of coast between Lizard Island and Cairns but were more frequent and more intense north of 16 degrees South. Our estimate is that reefs north of this latitude were holding 4.7 million starfish (confidence limits 3.2, 9.7) compared with 0.8 million (CL 0.5, 2.0) on reefs from a similar area south of this latitude. This is the strongest evidence to date that primary outbreaks, which are precursors to major outbreak cycles, build up between Lizard Island and Cape Tribulation. At current levels of abundance, where many millions of starfish are distributed over more than 2,000 km perimeter of reef slopes, hand control can do little more than defend local assets (high-value tourism sites, key reservoirs of coral brood stock) while this cycle of outbreaks runs a natural course. The lesson from this episode is that regional surveillance of ground zero (the area with primary outbreaks) should begin when the starfish is again rare following the self-destruction of its food base. This is the only time when reactive control has a realistic chance to contain local populations and prevent the inflationary expansion of the meta-population that follows spawning in flood years. If this strategy of containment proves ineffective, then it may be time to consider options for eradication.