Crown-of-thorns starfish are native to the Great Barrier Reef but can occur in plague proportions. Where numbers are high they consume vast amounts of living coral, dramatically reducing coral cover.
The Reef is facing more frequent disturbances, giving reefs less time and opportunity to recover. For example climate change is increasing the frequency of marine heatwaves which cause coral bleaching. It is also probably increasing the intensity of tropical cyclones and floods.
The combination of climate change and crown-of-thorns starfish outbreaks create additional concerns:
- starfish may eat the few corals that survive after heatwaves. These corals potentially tolerate higher temperatures. By removing these corals, the starfish could prevent or slow down reef adaptation
- starfish are likely to hamper reef restoration efforts, if large numbers affect a recently restored area on a reef.
A complex problem, likely to have more than one cause
The causes of crown-of-thorns starfish outbreaks are complex and have been debated for decades.
There is general agreement that outbreaks have always occurred on the Great Barrier Reef.
A variety of biological factors make crown-of-thorns starfish (and several other echinoderm species) prone to rapid population fluctuations – also known as ‘boom and bust’ cycles. The large size of the Reef and the connectivity between reefs also makes it vulnerable to crown-of-thorns starfish outbreaks. Starfish larvae can spread via these reefs once ‘primary’ starfish outbreaks have begun.
However, there is concern outbreaks are now more frequent or more severe due to human activities, adding to other pressures facing the Reef.
While many knowledge gaps remain, the most likely cause of increased frequency of outbreaks is one, or a combination of the following:
- increased nutrient availability increases phytoplankton levels in the Great Barrier Reef lagoon, creating more food for larval crown-of-thorns starfish. This may improve survival of the young starfish, leading to more adults.
- the removal of predators (fish and invertebrates) which reduces predation pressure on the starfish, particularly at the juvenile stage.
Crown-of-thorns starfish outbreaks are a regional issue and can be directly managed to help the Reef. Manual control of starfish takes place across the Reef by the Crown-of-thorns starfish Control Program.
AIMS researchers work with science teams in Australia and abroad to research the causes of causes of outbreaks and to develop better ways of controlling the starfish. Our work contributes to the collaborative Crown-of-thorns starfish Control Innovation Program, funded by the Australian Government’s Reef Trust Partnership with the Great Barrier Reef Foundation.
Investigating water quality as a driver of outbreaks
Laboratory studies, field observations and population models suggest that patterns of starfish outbreaks on the Reef might be related to river floods and regional differences in plankton availability. For instance, the waves of outbreaks on the Reef since the 1960s appeared to follow extreme floods of the Burdekin River and the rivers along the Wet Tropics coast.
Adult crown-of-thorns starfish on the Great Barrier Reef breed mainly in late December, at the start of the tropical wet season. Male and female starfish spawn simultaneously, releasing enormous numbers of egg and sperm into the water to form larvae.
The starfish larvae feed on free-floating microscopic algae (phytoplankton) which ‘bloom’ when more nutrients are available. This occurs when wet season floods transport sediment and nutrients from the land into the ocean, or from oceanic sources of nutrients, such as upwellings along the edge of the continental shelf.
These blooms may improve survival of the young starfish, leading to more adults.
We use lab experiments, field monitoring and modelling to investigate the relationship between water quality and crown-of-thorns starfish outbreaks resulting from increased larval survival.
In the field
Innovative approaches are helping us gain a better understanding of starfish larvae populations across the Great Barrier Reef. Our team collects water samples and analyses them for starfish larvae across the Reef using DNA identification. These samples provide information on the abundance and genetic diversity of crown-of-thorns starfish larvae. Ongoing research will provide detailed insights into how water quality may influence outbreaks.
This research includes working with tourism operators and Crown-of-thorns starfish Control Program vessels.
AIMS water quality monitoring (as part of the Great Barrier Reef Marine Monitoring Program in partnership with GBRMPA and JCU) also provides information on changes in water quality close to the coast. These data clearly show nearshore nutrient and phytoplankton concentrations are elevated in these waters after river flow events.
In the lab
The effects of different nutrient and feeding regimes on larval and juvenile crown-of-thorns starfish are explored in experiments. These experiments, conducted in the National Sea Simulator, complement our field work, providing information on how water quality and increased food might influence starfish survival and condition.
Modelling the data
We use marine hydrodynamic and biogeochemical models in the modelling platform eReefs, much like the models used to get daily weather forecasts. They help us better understand the context and implications of observations and experiments, and to explore scenarios.
Models help target our eDNA field work and assess the potential of innovative intervention approaches. For example, we can visualise water “age” over John Brewer Reef (see below). This tells us where we might find water that has been over the reef for a while and might have been exposed to more starfish DNA.
Models can also help provide information on the relationship between this water quality in the Great Barrier Reef lagoon, and adjacent land use. A recent modelling study investigated the effects of improvements in catchment management on water quality in the ‘crown-of-thorns initiation zone’ – an area between Green Island and Lizard Island where population outbreaks of the starfish have historically been observed to originate. The study suggested reducing land-based loads of sediments and nutrients in this area had little influence on the water quality in this zone. It identified key knowledge gaps such as lack of water quality data within the initiation zone
Using these models, we are also exploring the effects of improvements in catchment management on water quality in the crown-of-thorns initiation zone – an area between Cairns and Lizard Island where outbreaks appear to originate.
Investigating water quality as a driver of starfish outbreaks remains challenging. This is mainly due to the lack of offshore water quality data and limited understanding of how nutrient inputs from land or ocean upwellings contribute to crown-of-thorns starfish population outbreaks.
With their long, sharp toxin-covered spines, it’s hard to think what animals might eat crown-of-thorns starfish. However, starfish predators do exist, and may help to control their densities. Understanding this relationship could lead to new tools to help managers control outbreaks.
Using eDNA techniques, our team found many more reef fish eat crown-of-thorns starfish than previously thought. Traces of starfish DNA are found in gut and poo samples of a variety of reef fish. This indicates they consume starfish at some stage during their lifecycle.
We have also used large scale and long-term starfish and fisheries data to investigate patterns between the two. The study showed starfish densities increase with increased biomass removal of fish important to commercial and recreational fisheries such as emperors, rockcods and tropical snappers.
Fish aren’t the only predator we are investigating. Our team are also assessing the capacity of invertebrates such as shrimp and the large giant triton to help control starfish through predation using aquaria and field trials.