3. Ecosystem health

Crown-of-thorns starfish are a native coral predator on the Reef, as evidenced by their long fossil record.^1005,1006 Scientists have long debated whether Reef-wide outbreaks of crown‑of-thorns starfish are a natural occurrence or whether they are caused or exacerbated by human activities.^{900,1007,1008,1009} Life history characteristics of the species, such as exceptional fecundity, naturally encourage population fluctuations and enable crown-of-thorns starfish to quickly reach outbreak proportions when conditions are favourable.^{735,1010,1011}

The causes of crown-of-thorns starfish outbreaks on the Reef are complex and remain a subject of ongoing research interest.^735,1010 In addition to the influence of nutrients, oceanographic processes and reduced predation,^1010,1012 temperature has recently been shown to affect reproduction and settlement of crown-of-thorns starfish.⁸⁷⁷ 

Continued uncertainty around the causes of outbreaks, and the likelihood that the convergence of multiple factors is critical to their initiation and spread,⁷³⁵ presents a dilemma for Reef managers as to the most effective approach to reduce their impact.^1010,1013 Recent evidence indicates that manual removal of starfish from reefs is the most effective way to mitigate outbreaks,¹⁰¹⁴ and can achieve individual reef- and regional‑scale coral benefits with timely detection and response.^187,1015 

The Reef has experienced 4 destructive outbreaks of crown-of-thorns starfish since the 1960s. Once established, an outbreak spreads when adult starfish spawn and their larvae are carried by currents to other reefs. Once the larvae settle on a reef, they initially feed on crustose coralline algae before transitioning to a diet of coral tissue and increasing rapidly in size.^735,809,1016 The herbivorous juvenile starfish hide among coral branches and crevices and are mainly active at night, making them difficult to observe and monitor. 

They can remain in this small, cryptic phase for months to years,^1009,1017 and experimental evidence suggests this juvenile stage has high thermal tolerance.¹⁰¹⁸ The survival rate of juvenile starfish is key to generating outbreaks, but considerable knowledge gaps persist about this stage of their lifecycle.⁸⁰⁹

An outbreak of crown-of-thorns starfish that began between Lizard Island and Cairns in 2011, has moved south and is still (as of 2023) ongoing in some southern and central reefs. Importantly, some outbreaks in the southern reefs likely initiated independently of the northern outbreak.¹⁰¹⁹ Mounting evidence suggests that the Swain Reefs,¹⁰²⁰ Cape Grenville ¹⁸⁴ and Townsville–Innisfail sectors ^1020,1021 are particularly susceptible to crown‑of-thorns outbreaks and may play crucial roles in overall outbreak wave dynamics or initiation. Since 2019, a buildup of crown-of-thorns starfish at Lizard Island ^183,184 and Cape Grenville ¹⁸⁴  has been identified using a novel survey method, and it may represent the start of a new primary outbreak. This research highlights the potential of new survey methods for detecting outbreaks in the early stages when they are most amenable to management intervention.^184,1022

Surveys in 2022 indicate there are still ongoing outbreaks on some reefs in the southern region, including in the Swain Reefs. Three surveyed reefs were classified as having severe outbreaks, 2 reefs had established outbreaks and 3 reefs had potential outbreaks. In the southern region, coral cover has decreased since 2021, largely due to ongoing crown-of-thorns starfish outbreaks.¹⁰²³ Active targeted culling of crown-of-thorns starfish in the central region has likely contributed to the low numbers of starfish recorded during recent surveys and the increasing coral cover in that region from 2018.¹⁸⁷

Active management of crown-of-thorns starfish outbreaks began in 2002 with the introduction of the Crown-of-Thorns Starfish Control Program. Culling efforts have become much more effective over the past decade due to several improvements, including adopting an integrated pest management approach,¹⁰²⁴ developing a single shot injection technique, making advances in computer-aided modelling and prediction, and increasing vessel capacity. Since 2019, increased effort has led to suppression of outbreaks at both reef and regional scales, and new evidence demonstrates that the control program is minimising coral loss at target reefs (Figure 3.15).¹⁸⁷ 

Crown-of-thorns starfish outbreaks continue to affect reefs across the Region. However, in the past 5 years, increased crown-of-thorn starfish control efforts have reduced the impact of outbreaks on coral reefs, at both local and regional scales.