8. Resilience

Coral trout belong to the genus Plectropomus and are one of the largest and most conspicuous fishes on coral reefs of the Indo-Pacific. They are key reef predators that play an important role in the transfer of energy through the food chain and influence the composition of fish assemblages through top-down control via predation.^1331,2077 Coral trout are also highly prized food fishes that are targeted by commercial, recreational, artisanal and subsistence fisheries throughout their geographic ranges.^2078,2079 Coral trout is one of the major components of the line fishery catch in the Reef. The genus Plectropomus has 8 species, of which 6 occur in the Region.²⁰⁸⁰ 

Common coral trout (Plectropomus leopardus) is the most abundant and widespread coral trout species across most of the Region.²⁰⁸¹ Two closely related species, the spotted (bar-cheek) coral trout (Plectropomus maculatus), and the black-saddled (footballer/blue-spot) coral trout (Plectropomus laevis) are common on inner-shelf and outer-shelf reefs, respectively.^2081,2082 Common coral trout typically have constrained home ranges but can move considerable distances within and sometimes among reefs, particularly when transiting to and from spawning aggregation sites.^{2083,2084,2085}

Common coral trout are protogynous hermaphrodites that transition from female to male throughout their development.²⁰⁷⁸ Females reach sexual maturity at 30 to 37 centimetres total length and 2 to 3 years of age.²⁰⁸⁶ The size and age of transition from female to male is variable but is likely to be influenced by a range of factors including population size structure, behavioural processes, and fishing intensity.²⁰⁸⁶ In the Region, peak spawning periods extend over several days around new moons spanning the Australian summer.²⁰⁸⁷ However, recent evidence also shows that the Reef’s coral trout can spawn and produce viable offspring throughout the year.¹³⁵⁷ Hybridisation among several coral trout species is known to occur, including between common coral trout and spotted coral trout.^2088,2089

Coral trout closely associate with coral reef habitat. Post-settlement juvenile coral trout are commonly found in patchy reef habitats with interspersed live coral colonies that provide them with shelter.²⁰⁹⁰ Adult coral trout often reside under tabular corals and within reef crevices that provide shade, shelter and concealment from which they can ambush prey.²⁰⁹¹ Coral trout population dynamics and resilience can be strongly influenced by changes in coral cover, reef habitat complexity and prey abundance following disturbance events.^{2091,2092,2093} Therefore, coral loss can increase the risk of overfishing of coral trout stocks due to sudden productivity declines.²⁰⁹⁴ For example, on fringing reefs in the Keppel Islands group in the southern region, coral trout abundance was significantly reduced, and tracked the trajectory of declines in coral cover and prey fish abundance, following flood plume disturbances between 2009 and 2013.²⁰⁹²

A recent study has shown that physiological parameters used to assess the health of coral trout were unrelated to coral cover at sites where individual fish were captured. These results suggest that adult common coral trout may be resilient to declines in coral cover, at least over periods of 2 to 5 years following acute coral loss caused by climate-induced mass coral bleaching.²⁰⁹⁵ Given the age at first breeding for coral trout is 2 to 3 years and that adult biomass is dominated by older age classes, the impacts of reduced coral cover on adult biomass are unlikely to manifest for at least 5 years.²⁰⁹⁴ Additionally, past stock assessments for coral trout have not found any clear association between bleaching events and coral trout catchability. However, a previous stock assessment observed that coral loss caused by cyclones correlated with lower catchability of coral trout over the following years.²⁰⁹⁶ A lack of relationship between coral cover and the health of individual adult coral trout likely represents the coral trout’s increased access to prey resources and reduced investment in key energetic process, such as growth. These factors may increase the resilience of coral trout to habitat degradation in the short-term.²⁰⁹⁵

Pressures such as increases in sea temperature can directly affect fish physiology and behaviour, which can ultimately undermine the capacity of coral trout populations to acclimatise or adapt to changes in environmental conditions.²⁰⁹⁷ Loss of coral and habitat degradation are also factors influencing coral trout biology and ecology.

Management 

Coral trout are the primary targets of the Queensland Reef Line Fishery.¹²⁷⁰ The fishery makes no distinction between coral trout species, and they are collectively marketed as coral trout, but common coral trout are the primary target species.²⁰⁹⁸ Coral trout comprise approximately 35 to 55 per cent of the commercial catch, 20 to 25 per cent of the charter catch and 15 to 20 per cent of the recreational catch of demersal reef fish from the Reef.²⁰⁹⁹ 

Coral trout are managed as a single stock using a range of fishery controls, including individual transferable quotas (ITQs) and a total allowable commercial catch (TACC) for the commercial sector, as well as a minimum length limit and a recreational in-possession (bag) limit.²¹⁰⁰ Annual coral trout harvest peaked at approximately 2000 tonnes in the early 2000s before the introduction of ITQs and a TACC of 1350 tonnes in 2004 and a further reduction to 1280 tonnes in 2005 (Figure 8.6).¹²⁷⁰ In 2022, a common coral trout stock assessment identified that total retained coral trout catch averaged 1002 tonnes between 2017 and 2021. This included 812 tonnes by the commercial sector, 68 tonnes by the charter sector, 111 tonnes by the recreational sector and 11 tonnes by First Nations (Traditional Owner) fishers.¹²⁷⁰

The Great Barrier Reef Marine Park Zoning Plan 2003 has generated significant benefits for coral trout stocks and other reef-associated species.²¹⁰¹ Coral trout are more abundant, larger, older, and more fecund on Marine National Park Zone (no-take reserve) reefs than on reefs that have remained open to fishing.^58,2102 Furthermore, no-take reserves also export coral trout larvae and deliver recruitment subsidies to fished reefs, contributing to population replenishment and fishery sustainability across the Region.^2102,2103 The existing management regime that mixes Marine Park no-take zones and catch quotas is robust to regional variation in ecological dynamics, overfishing and climatic change.⁶⁵⁶

The Queensland Reef Line Fishery is also subject to short seasonal closures during summer new moons that apply to both commercial and recreational sectors. The closures are based on the presumption of seasonal and lunar spawning of the common coral trout.¹²⁷⁰ However, a recent study has shown that year-round spawning activity generates distinct recruitment cohorts that span several weeks to months in the Region. In some instances, peak spawning had little or no alignment with existing seasonal fisheries closures around the new moon.¹³⁵⁷ Given the recent indication of variability and uncertainty in peak spawning times,¹³⁵⁷ the coral trout fishery may further benefit from additional and longer seasonal closures, or alternative fisheries management strategies, to maximise the recruitment contribution from periods of greatest reproductive success.²⁰⁸⁸ 

Evidence for recovery or decline

The latest stock assessment modelling for coral trout indicated that between 1953 and 2011 biomass declined to 46 per cent relative to unfished spawning stock biomass. Spawning stock biomass has been steadily increasing since 2012. In 2022, the stock level was estimated to be at 60 per cent unfished biomass.¹²⁷⁰ Coral trout stock assessment modelling draws upon Reef fishery catch and effort data and fishery-independent underwater visual census data. The use of fishery-independent data is fundamental to accounting for differences in coral trout abundance and size between reefs that are open and closed to fishing within the Marine Park.¹²⁷⁰ 

It is important to note that these modelling results do not take into consideration environmental factors known to affect stock levels, and they incorporate limited information on recreational catches. Nevertheless, the stock assessment provides the best evidence that fishery controls on catch and effort, combined with Marine Park zoning and compliance, have led to an increase in coral trout population size and enhanced the sustainability of reef line fishery catches. There is growing concern about the impacts of climate change on coral trout populations.¹²⁷⁰ While coral cover in the Region has recovered recently to record highs in some reefs,¹⁶¹ continuing increases in cumulative disturbances, such as cyclones and increased sea temperatures, can result in significant coral loss. Habitat degradation may increase the vulnerability of common coral trout to exploitation and undermine fisheries sustainability.^58,1646,2105