3. Ecosystem health

Recruitment is the process of adding new individuals to populations, and it is fundamental to the maintenance and replenishment of depleted populations. New recruits can be sourced locally (when parents are from the same population) or externally (parents are from a different population).^869,870 Depending on an organism’s characteristics, recruits can be live young (for example, dugongs), hatchlings (for example, turtles), planktonic larvae settling onto a reef (for example, corals) or seeds (for example, seagrasses) (Figure 3.14). In the Region, the levels of monitoring and types of data on recruitment vary across different components of the Reef. 

Dugong populations south of Mission Beach show evidence of long-term decline. During the 2022 urban coast dugong survey,⁵²² a few dugong calves were seen in the southern region of the Reef and another 13 were seen in the central region. The proportion of calves, a population health measure, was 6.7 per cent along the urban coast of the Reef, within the range recorded for previous surveys but at the low end. 

Humpback whale populations have continued to increase since whaling was banned in Australia in 1985.⁵⁰⁴ No monitoring data exist on the abundance of live young, but breeding and calving continue to occur in the Region. The resulting continued growth of the humpback whale population means it is predicted to reach carrying capacity in 2021 to 2026.⁵⁰⁴ 

Many seabird breeding populations from islands and cays across the Region have shown a decline over the past 4 decades,⁴⁸⁶ but key information on nesting success and recruitment to breeding populations is lacking for most species.

Of the 6 species of marine turtle found in the Region, only green, flatback, loggerhead, and hawksbill turtles are known to regularly nest there (Section 2.4.10). The southern population of green turtles is considered to be moderately recovered.⁴⁴³ The breeding population of flatback turtles is considered stable or increased;⁴⁷³ however, some decreases in nesting populations at Peak Island have been observed.⁴⁴³ The northern green turtle population is now considered to be in significant decline and very few males are now recruiting to the breeding population.⁴⁴⁶ The annual recruitment of young female loggerheads into the adult breeding population on the mainland has declined over the past 2 decades,⁴⁷⁰ and recovery of the loggerhead nesting populations on the island rookeries has been negligible.^443,470 Since 1990, the number of hawksbill turtle eggs laid and the number of females at the significant Milman Island rookery have steadily declined.⁴⁶² Models have projected that the current rate of decline could lead to loss of nesting at Milman Island within the next 10 to 15 years.⁴⁶²

Organisms with a planktonic larval stage, such as corals, crown-of-thorns starfish and fish, tend to produce large numbers of individuals with high likelihood of mortality. Rates of survival at key life stages can make a large difference to population outcomes, and identifying the factors influencing these life stages is crucial for understanding population dynamics.⁸⁷¹ For example, it has recently been discovered that larvae from crown‑of‑thorns starfish and some corals cue on specific crustose coralline algae to find suitable settlement substrates.^280,872 Presence of particular coralline algae can promote higher or lower settlement of recruits, depending on the algal species.^410,411

Recruitment of coral is highly variable across the Region.^178,873 Some reefs and certain coral types are showing recovered recruitment levels,⁸⁷⁴ while others remain very low.⁸⁷⁵Major reductions in coral recruitment that occurred across the Region following mass coral bleaching events in 2016 and 2017 highlighted that sensitivity of adult brood stocks to repeated episodes of bleaching can seriously impede subsequent recovery.⁸⁷⁶ Longer recovery windows since 2019 have led to improvements in recruitment. For example, in 2021, substantial recovery of coral recruitment had occurred at some sites, linked to increases in coral cover driven by branching and tabular Acropora.^161,874,875 Inshore reefs often have lower densities of juvenile corals than offshore reefs, particularly in the Burdekin, Mackay Whitsunday and Fitzroy regions.^161,185 The impacts of widespread coral bleaching during the 2023–24 summer on coral recruitment are yet to be determined.

At the end of 2023, crown-of-thorns starfish populations were still at outbreak levels in the southern section of the Reef (Section 3.6.2), while an emerging primary outbreak in the northern region ⁸⁷⁷ had likely initiated as a result of several years of elevated recruitment.¹⁸³ Recent research has revealed that reproduction and settlement of crown-of-thorns starfish can be enhanced by warming events during the typical summer spawning period.⁸⁷⁷

Marine heatwaves are causing coral bleaching and fundamentally changing habitat structures for coral-associated fishes.^365,878 Habitat loss and fragmentation affects early post-settlement survivorship of some reef fishes.⁸⁷⁹ Recruitment of other coral-associated fishes was found to persist following loss of habitat-forming coral species at Lizard Island from consecutive mass coral bleaching events in 2016 and 2017.³⁷⁰ Recruitment was unaffected 2 years after the loss of corals, and fish were found in a range of alternate reef microhabitats, including live, non-branching, non-preferred corals, algal-turf-covered dead corals and coral rubble.³⁷⁰ A 10-year study conducted at Lizard and One Tree islands further suggests that some reef fish are resilient to environmental perturbations, with recruitment remaining stable over the studied period.⁸⁸⁰

Monitoring of 4 giant clam species from 1982 to 2017 across 5 reef sites showed that populations of all species declined at 3 sites due to low recruitment and death of old individual clams. However, at 2 sites, a significant juvenile recruitment cohort of giant clam and smooth giant clam followed a mass mortality event and returned the population to pre-disturbance abundance. Some of these individuals lived long enough to reproduce successfully in some years, as evidenced by recruitment.⁸⁸¹ Population abundance of fluted giant clam and horse hoof clam also increased slightly at these 2 sites.

Seagrass recruitment is assessed on the basis of reproductive effort, which is calculated by identifying the presence of reproductive structures (flowers, fruits) and the size of seed banks in seagrass meadows.⁸⁸² Seed banks in the Region’s inshore meadows have been consistently present and abundant in all regions, particularly in the Burdekin region. Reproductive effort in the Fitzroy and Burnett Mary regions has remained low over the past 5 years in most habitats and declined in 2021–22 relative to the previous period.¹⁴⁵ 

The persistence of the macroalga Sargassum on reefs, especially those located in the Region’s inshore area, is likely enhanced by the reluctance of herbivores to consume holdfasts and propagules, though some fish species do. Sargassum can also regrow from holdfasts that have sustained significant damage.⁸⁸³ Overall, since 2019, cover of macroalgae has increased or remained high at several inshore reefs across the Region.¹⁹⁰ Increased cover of macroalgae in some reefs in the Mackay Whitsunday region has been driven by recruitment and settlement of propagules since cyclone Debbie in 2017.¹⁸⁵ In offshore reefs, macroalgal cover across the Region has remained stable over the past 5 years.²⁷⁷

The level of knowledge of the condition of, and pressures on, recruitment processes within the Region varies substantially between species and species groups. There are very few data on recruitment for most organisms. Trends in recruitment, where known, are also highly variable between different groups: some are stable (for example, seagrass) and some show declines (for example, dugongs and marine turtles). Improvements in coral recruitment occurred on many reefs; however, trends were highly variable and the impacts on recruitment of widespread severe bleaching during the 2023–24 summer have not yet been determined.