The Great Barrier Reef is under attack from a range of enemies including climate change effects (coral bleaching, increased severe storms, and ocean acidification), pollutant discharge from the land, coastal development, and damage from fishing.
As a result, coral cover is in severe decline, seagrass has declined dramatically in the last few years, while numbers of megafauna including dugong, turtles, sharks and some dolphins have greatly reduced population numbers.
In particular for coral, analysis of coral cover data from about 1960 onwards suggests that cover across the GBR has fallen from about 50% in the 1960s to about 16% now. As yet unpublished estimates by Dr Glenn De'ath and his colleagues suggest that if current trends continue coral cover could be as low as 5% in 20 years.
Coral-eating crown of thorns starfish (Acanthaster planci) (COTS) have caused widespread damage to many coral reefs in the Indo-Pacific over the past five decades as population “explosions” have occurred at regular intervals. On the GBR the greatest cause of coral mortality in recent decades is COTS ahead of other major causes such as cyclones, bleaching and coral diseases. COTS were probably the major cause of coral mortality in the period from 1960 to 1985 also but our data is less complete for this period. Kate Osborne and her co-researchers found that COTS were responsible for 36.7% of the coral damage above all other causes including storms (33.8%), disease (6.5%), bleaching (5.6%) and unknown or multiple causes (17.4%).
There have been three major periods (“waves”) of COTS outbreaks on the GBR: 1962 – 1976; 1978 – 1991; 1993 – 2005; and it is now accepted that we are at the beginning of the next wave which appears to have started off Cairns in 2009. Each wave started near Cairns and spread through larval dispersion up and down the GBR generally as far as Princess Charlotte Bay in the north and Mackay in the south. If the current wave moves in a similar way we can expect starfish populations to progress throughout the central GBR over the next 10 years or so.
The impact of outbreaks on the GBR is a major concern to the multi-billion dollar tourism industry. Over a number of years, there was an outbreak on reefs between Cairns and the Whitsundays which was estimated to cost tourism operators, and the Queensland and Australian Governments about $3 million a year for control measures.
The cause of the outbreaks remains a controversial issue despite years of research. Hypotheses have included that (1) population outbreaks are a natural phenomenon due to the inherently unstable population sizes of highly fecund organisms such as COTS; (2) outbreaks are due to anthropogenic changes to the environment of the starfish with a range of possible anthropogenic causes including: removal of adult and/or juvenile predators; destruction of larval predators e.g. corals, by construction activities on reefs; and larval food supply (phytoplankton) enhancement from nutrient enriched terrestrial run-off.
It is now well established that the large scale outbreaks seen on the GBR since 1962 are most likely to have been caused by nutrient enrichment associated with increased discharge of nitrogen and phosphorus from the land due to increased soil erosion and large scale fertiliser use. Increased nutrients drive phytoplankton blooms with increased biomass and also a shift to larger phytoplankton types more palatable to COTS larvae as food. Removal of predators (especially fish) is also implicated as a secondary cause.
There is some evidence that the increase in the area of no-take zones in 2004 has had significant success, as COTS numbers on closed reefs are lower than on reefs open to fishing. Site specific management (through removal) has been successful at a local scale, although it is very labour intensive. With the initiation of the fourth wave of outbreaks now confirmed it is clear that water quality management under the Reef Water Quality Protection Plan (implemented in 2008) has not had time to prevent further outbreaks. However further water quality management will be critical to minimise future outbreaks.
In conclusion we can state unequivocally that COTS remain the greatest threat to the coral of the GBR and thus also indirectly to coral reef fish, although obviously of lesser threat to seagrass, dugongs, and some other megafauna.
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