Crown of Thorns Starfish (COTS):
Summary: See these links for more information.
Are farmers to blame for plagues
Crown of thorns Starfish occasionally reach plague proportions killing a large fraction of coral on a given reef.
The first “scare” about the GBR dates back to the late 1960’s.
Although nothing was known about them before the ‘60’s the plagues were immediately attributed to human impacts. It was assumed that the plagues never occurred before this time although there is little evidence for this assumption.
It was first blamed on the fishing of Triton shells which are a predator of COTS.
More recently, the blame has shifted to the extra nutrients (fertilizer) that come from agriculture.
However COTS are also found in areas where there is no agricultural pollution.
The tenuous hypothesis that links COTS with agriculture is based upon a single data set that claims to show much higher nutrient levels in the Central GBR where intensive agriculture exists compared with the far northern section of the reef where there is little agriculture.
This data actually shows no difference if a genuine comparison is made completely invalidating the original hypothesis.
The hypothesis that agriculture upsets the nutrient balance in the GBR lagoon is surprising given that the water quality of the GBR is completely dominated by water flushing from the clean waters of the Pacific Ocean, not from rivers. The Pacific Ocean flushes as much water into and out of the GBR in about 8 hours as all the rivers on the entire Queensland coast for a whole year.
Rivers plumes, while looking spectacular in satellite and aerial images, have an impact for only a few days each year and rarely reach the main reef matrix where 99% of the corals are located. For large parts of the Southern region, which is a long way offshore, the plumes never reach the reef.
Returning to COTS. The geological evidence from cores drilled into the reef, which are analyzed to find ancient COTS skeletons, indicates that COTS are an entirely natural phenomenon, i.e.there is no evidence that they are in greater numbers than in the past.
Before the 1970’s marine biologists were a very rare species on the GBR so just about everything they saw was new to science. Several phenomenon as spectacular as COTS were not detected before the biologists studied the reef. It took until the 1980’s for them to discover mass coral spawning which is when every coral on the GBR releases its eggs in one or two nights causing spectacular slicks on the surface which can be seen from space. But nobody would claim that spawning never occurred before the 80’s.
Is one of the reasons that there are so many more problems with the GBR today that did not exist before the 1970’s the explosion in numbers of marine biologists?
If a reef was decimated by COTS, bleaching, or a cyclone in 1923., who would have known, and would anybody have cared?
Despite the overwhelming probability that COTS are entirely natural, it makes sense to do limited culls of COTS close to tourist sites. It is impossible to control COTS on the entire 2000 km long GBR and attempting to do that is a complete waste of money. However for the small areas where tourists visit, COTS can be controlled and this ensures that the tourist operations can be kept going while a plague moves through the general area. Visitors are concentrated in very small areas, for example Moore Reef takes 20 percent of all GBR visitors, and they only see a tiny fraction of that reef.
Great Barrier Reef Science Commentary 