3. Ecosystem health

Diseases are infections of plants and animals caused by pathogenic microorganisms, including bacteria, viruses, fungi and parasites. Although many of these microorganisms are naturally present in the environment and usually do not cause widespread disease, outbreaks can occur when microorganism abundance increases rapidly or if host immunity is compromised.^969,970 Pathogens can be introduced or transmitted to new marine areas in various ways, for example, through imported seafood, vessel biofouling or ballast water discharge.⁹⁷¹ 

Disease can have a significant negative effect on population structures, causing rapid population declines and mass mortality or, in extreme cases, hastening species extinctions.^972,973 In habitat-forming species such as corals, disease outbreaks contribute to loss of coral cover and can lead to flow-on effects for associated species, reef structure  and ecosystem function.⁹⁷⁴ Disease can also have sublethal effects on host species, including reduced rates of reproduction and growth.^975,976

At least 8 different coral diseases, including white syndrome, black band and brown band disease, have been reported from various reef building corals within the Region.^974,977,978 Since 2019 major flooding of rivers in the Burdekin region and above-median discharges from some rivers in the Mackay Whitsunday and Wet Tropics regions likely contributed to increased levels of disease observed in inshore areas.¹⁹⁰ Declines in coral cover at reefs in Burnett Mary region in 2022 ¹⁷⁸ and Fitzroy region during 2023 ¹⁹⁰ have also been attributed to disease.

While not detected in the Region or Australia, stony coral tissue loss disease has continued to spread rapidly through the wider Caribbean since 2019,⁹⁷⁹ where it affects at least 25 different coral species and can cause mortality rates of more than 80 per cent in massive corals.⁹⁸⁰ This disease poses a significant risk if introduced to the Region. Bacterial biofilms on ships ⁹⁸¹ and ballast water ^979,982 are both potential vectors of stony coral tissue loss disease.

Prevalence of coral disease and disease outbreaks are frequently linked to environmental stress, including rising sea temperatures,^{978,983,984,985,986,987} storm events,^988,989 decreased water quality ⁹⁹⁰ and nutrient enrichment.⁹⁹¹ Microbial dysbiosis, or a shift in the symbiotic microbiome which favours the establishment of opportunistic bacteria or pathogens, is increasingly implicated in many coral diseases.^{164,974,986,992} Within the Region, reduced microbial diversity has been shown to lower coral immunity to disease.⁹⁹³ Predators can also affect coral disease by damaging protective tissues and exposing corals to infections, transmitting diseases between corals, or acting as reservoirs for pathogens.⁹⁹⁴

White spot disease, which can cause mortality in crustaceans, was first recorded in prawn farms in south-east Queensland.⁹⁹⁵ It was subsequently found in wild crustaceans, including in northern Moreton Bay in 2020.⁹⁹⁶ While there are currently no known occurrences within the Region,⁹⁹⁶ there is potential for transmission from south-east Queensland. Queensland has a movement regulated area (MRA) in place to mitigate the risk of spread. High-risk aquatic animals cannot be moved out of this area. This includes uncooked prawns, yabbies and marine worms. A national surveillance program has demonstrated that all areas of Australia, outside the MRA, have remained free from the virus.⁹⁹⁵

Marine turtles suffer from various pathogenic diseases,^{997,998,999,1000,1001} although disease has only been attributed as the cause of death in a small number of stranded marine turtles in the past 5 years.⁴⁶¹ Higher rates of pathogenic disease tend to be reported in immunocompromised individuals, and there may be links between immunosuppression and exposure to environmental pollution.^1002,1003 Generally, non-infectious diseases (for example, from trauma and injuries associated with collision, gut impaction as a result of plastic ingestion, exposure to contaminants) pose a greater risk than infectious diseases to marine turtles within the Region.¹⁰⁰³ 

Climate change is projected to lead to further disease outbreaks and may also provide increased transmission opportunities as a result of species immigration and range shifts.¹⁰⁰⁴ With increasing global warming and poor water quality, disease prevalence may also increase in fish species.¹⁰⁰⁴ 

Since 2019, disease is known to have affected corals and marine turtles within the Region. The incidence of disease is expected to increase with warming temperatures, and consequences could be significant. There are concerns that the Region could be affected by new outbreaks of disease occurring overseas (for example, stony coral tissue loss disease), but these diseases have not yet been observed in the Region.