Coral reefs are under threat. Climate change is having a significant impact, and voracious crown-of-thorns starfish (COTS) are an ongoing major issue. They eat their way through coral and impact restoration efforts.
COTS have phenomenal reproductive abilities. As with pest species like locusts that wreak havoc on crops, COTS numbers can explode. They cause devastation to ecosystems as they move through in waves. This adds stress to coral reefs that are struggling to recover from recurrent bleaching events.
New research incorporating modelling tools outlines how best to manage COTS outbreaks at the Great Barrier Reef; with the potential to stop outbreaks before they start.
Short and long-term modelling tools
Models of Intermediate Complexity for Ecosystem (MICE) assessments are applied as a tool to manage COTS outbreaks. It forms part of ongoing control efforts by the Great Barrier Reef Marine Park Authority (Marine Park Authority). A bold new plan synthesising results of over 50 years of research on the GBR will also help to suppress COTS outbreaks before they start.
Identifying coral ecological thresholds
Since 2012, the Marine Park Authority has been working with research and industry partners to reduce the damaging impacts of COTS on coral populations across the Marine Park. The Control Program involves manual culling of COTS using boats and divers. Given the GBR is of a similar size of Italy, how best to deploy divers to optimise efforts to save corals is critical.
The MICE model uses field and laboratory data to track how the numbers of COTS change over time and what this means for coral declines and recovery. We can identify where COTS consumption of coral outstrips coral growth. This enables us to calculate the ecological thresholds where the culling of COTS is reducing further coral declines. Divers can then move on to the next reef.
The ecological threshold depends on the level of coral cover. When there is good coral cover, the threshold triggering divers to move-on is higher than is the case for poorer coral cover. The worse the condition of the coral, the harder divers have to work to cull as many COTS as possible to help speed up the chance of coral recovery.
Models join vinegar in the fight against COTS
Divers inject an oxbile or vinegar solution into COTS to kill them. To assist divers, our models are used to figure out how many COTS there are and what the damage to the coral is. We use catch rates - how many COTS killed per minute - to determine if it’s enough to make a difference to coral recovery.
In the same way that models of the spread of coronavirus are being used to plan our response to the current pandemic, the Marine Park Authority is using these models to plan its response to COTS and to flatten outbreak curves on the GBR.
For 35 per cent coral cover, COTS culling needs to continue until catch rates decrease to below one COTS per 20 minutes to prevent coral decline. If coral cover is higher (such as 80 per cent), the target threshold is three COTS per 40 minutes. These estimates underpin the pest management rules for the Marine Park Authority’s COTS Control Program.
Understanding when the next COTS outbreak will hit
The fourth wave of COTS is currently propagating south along the GBR. Research carried out during previous outbreaks has enabled us to predict where and when outbreaks will begin. This includes detecting the small COTS populations that act as the source of outbreaks.
The last two outbreaks were detected on the midshelf GBR reefs, between Cairns and Lizard Island. This is often referred to as the ‘initiation box’. The next outbreak is expected to begin around 2025-2027.
We predict that the current outbreak will peter out in the next few years following science intervention. Monitoring and control teams will then focus on suppressing COTS in the initiation box. This will extend the time corals have to recover. It could also result in postponing the next outbreak indefinitely.
Ongoing intervention strategies
There are a wide range of COTS control and other reef intervention strategies being developed and applied to reduce ongoing damage to the GBR.
To support this work, it’s imperative that we reduce greenhouse emissions and the impacts of climate change. This will give us more time to better manage changes to the Reef and protect its unique and diverse ecosystem. Reducing runoff of sediment, dredging and pollution will also contribute to preventing COTS outbreaks.
This will help to ensure the GBR remains an incredible and unique place to visit.
This project is supported with funding from the Australian Government’s National Environmental Science Program.