Background and COTS numbers

Coral-eating Crown-of-Thorns Starfish

It was during the 1960s that the first predictions of the ultimate destruction of the GBR were made. They followed the discovery that crown-of-thorns starfish (CoTS) (Figure 3.1) were in plague proportions on some reefs, literally eating the coral and leaving devastation behind. It was argued that these plagues were almost certainly unnatural and, if they continued, the GBR could be gone forever.

Figure 3.1 Crown of Thorns Starfish (CoTS) grazing on coral. White scars are areas already eaten by the CoTS. (Photo: Roger Steene)

Very little was known about the GBR in those days so it was not unreasonable to be worried. The initial culprit blamed for the plagues was overfishing of triton shells (Pearson and Endean, 1969).[1] These monstrous trumpet shaped shells can easily grow more than 30cm (1 foot) long and are a predator of CoTS. It is notable that half a century after the initial dire predictions of scientists, the same news headlines about the death of the GBR by CoTS are repeated each year in the media. The culprit is increasingly now seen as farmers rather than shell fisherman.[2]

COTS are NOT a feral Animal

Many people in Australia think that CoTS are a feral animal introduced to the GBR after European settlement. The Australian environment has suffered greatly from introduced animals such as cats, foxes, cane toads, and rabbits which can devastate the unique native animals and plants of the Australian continent. There is even a plague of feral Northern Pacific Starfish that originally came from around Japan, which are devastating ecosystems in southern Australia and Tasmania.

CoTS are, however, a native species to Australia and belong to the GBR as much as kangaroos or wombats belong on the Australian mainland. If CoTS were an introduced feral animal, with no predators, it would not be surprising if their numbers were out of control – this has happened in almost every country in the world where either feral animals or noxious weeds have adversely affected the environment. Native species have reached plague proportions in Australia, on many occasions damaging agriculture. It is much less common for them to reach plague proportions and damage their native environment, which is what CoTS are claimed to be doing to the GBR.

Documentation of CoTS plagues focused the world’s attention on preservation of the GBR and, together with concerns about mining and oil exploration on the Reef, precipitated declaration of the GBR Marine Park in 1975. In addition, major outbreaks of CoTS plagues in the early 1980s motivated the first long-term monitoring of coral cover on over 200 reefs each year by the Australian Institute of Marine Science.[3] This has become one of the most valuable pieces of scientific data on the GBR as it tracks the coral cover and the number of CoTS. It provides a better perspective than was available in the 1960s and shows that CoTS outbreaks come and go (see Figure 3.2).

Figure 3.2: Density of CoTS on the GBR. The outbreaks of the 1980s and 1990s are clearly evident as is the recovery. There is a smaller outbreak occurring at the moment. The density is defined as the number of CoTS per 130 m transect across the Reef.[4]

The “official” reason now given for CoTS outbreaks can be found in the 2017 Scientific Consensus Statement.[5] It was written by eminent scientists representing the major science institutions that work on the GBR. This Scientific Consensus Statement is similar to the highly influential reports produced on climate change by the Intergovernmental Panel on Climate Change (IPCC).  IPCC reports on climate are prepared by large numbers of scientists representing governments and science institutions from around the world. Their reports greatly influence government policy.

The GBR Scientific Consensus Statements are prepared by scientists mostly from Australian institutions and government bodies. The consensus statements are effectively the foundation documents upon which governments base legislation that affects the reef, and claims that the cause of CoTS plagues is nutrients from agricultural fertilizer.

The argument is as follows: much of the fertilizer applied to farms washes down the rivers during the wet season (December to April) and triggers an increase in phytoplankton (microscopic floating plants) that are eaten by the larval stages of the CoTS. The extra phytoplankton supposedly allows far more of the CoTS larvae to survive and because an adult CoTS can produce tens of millions of larvae, a small increase in survival could produce a plague.

This link between agricultural fertiliser and CoTS has led to very strict legislation regulating use of fertilizer on farms adjacent to the GBR. Until recently, sugar cane farmers were the primary target but, in late 2019, even more draconian regulations have now been imposed on all agricultural industries adjacent to the GBR. These will affect productivity and profitability. In short, the CoTS is a huge issue for agriculture along the Queensland coast.

The purpose of this section is to examine the “agriculture hypothesis” that the 2017 Scientific Consensus Statement proposes. The analysis will show that it is tenuous at all levels. The proposition that CoTS plagues are unnatural in the first place, and only started in the last half century, ignores the geological evidence. Likewise, any link with agriculture is weak and has many contradictions.  

[1] Pearson, R.G. and Endean, R. (1969). A preliminary study of the coral predator Acanthaster planci (L.) (Asteroidea) on the Great Barrier Reef. Fisheries Notes, 3, pp.27–55.

[2] Great Barrier Reef Marine Park Authority (2019). Great Barrier Reef Outlook Report 2019. [online] Available at:

[3] Australian Institute of Marine Science (2020a). Reef monitoring sampling methods. [online] Available at:

[4] Australian Institute of Marine Science (2020b). Monitoring Crown-of-Thorns Starfish on the Great Barrier Reef. [online] Available at:

[5] Waterhouse, J., Schaffelke, B., Bartley, R., Eberhard, R., Brodie, J., Star, M., Thorburn, P., Rolfe, J., Ronan, M., Taylor, B. and Kroon, F. (2017). 2017 Scientific Consensus Statement: Land Use Impacts on Great Barrier Reef Water Quality and Ecosystem Condition. [online] Brisbane: The State of Queensland. Available at:

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