When considering the effect of dredging on the GBR, it is important to recall that all the ports, and especially the major ports, are a long way from the GBR (see figure below). Hay Point is the most distant at 110 km; and Cairns, the closest port to any reef is 25 km. And these are the distances to the closest reefs – the distances to the rest of the reefs of the GBR averages around 1 000 km.
Measurable affects even in the down current direction are confined to a few kilometres, and none of the ports is up-current of the GBR. It is simply inconceivable that the GBR is significantly affected by dredging.
Although the GBR itself is not affected by dredging, it is not obvious that the Mediocre Fringing Reefs are so safe. Some are much closer to dredging. In the case of Townsville, the nearest coral is only a few kilometres from the shipping channel. At Hay Point, there is a very small, but healthy, coral reef growing right beside the wharf. As mentioned above, these reefs are very small, often little more than a few tennis courts in size.
How far away does mud drift from a dredge?
Dredging can kill coral within a few kilometres, but the nearest reefs of the GBR are generally ten or a hundred times further away.[1]
During the first major study in the world into of the effects of dredging on corals, at Townsville Port in 1993, the expectation was that much of the coral on the nearby fringing reefs would be killed. At the closest point, the Mediocre Fringing Reef coral is only a 1.5 km (1 mile) from the dredged channel for Townsville although the GBR itself is much further away. James Cook University’s involvement was to use its newly developed instruments to measure the plumes of sediment reaching the reef while the biologists monitored the corals over the dredging campaign that lasted around four months. It was expected that large spikes in the concentrations of sediment would be seen on the fringing reefs.
Fortunately, this was not so. The fringing reefs emerged unharmed. Moreover, months of data only occasionally picked up a distinct signal of extra sediment concentrations caused by the dredge. When it did, it was not on the fringing reefs, but at sites much closer to the dredge. This does not mean that there was no effect on the fringing reef, but that effects were so small as to be virtually undetectable. Every time the wind increased, causing waves, the muddy seabed of Cleveland Bay was resuspended (Figure 4.2 and 4.3) causing very high sediment concentrations – far higher than had been previously recorded with the previous method of using water bottle samples. The situation on the fringing reefs was more complicated, but wave resuspension was the dominating factor determining concentrations of sediment.
Biologists monitoring the corals on the fringing reefs were also pleasantly surprised. Of the 500 corals that were tagged and repeatedly visited to monitor their health, only one died and the conclusion was that this was not due to dredging (Kaly et al., 1994,[2] Benson et al., 1994). Partial mortality (when part of the coral dies, a little like a tree losing a branch) occurred on less than 10 percent of the corals with most loss occurring on a very distant “control” site. This demonstrates that mortality was not increased by the dredge activity.
In the quarter century since that first landmark study at Townsville Port the amount of data collected about the effect of dredging on reefs, much of it with instruments designed and built by my group at JCU, has increased significantly and it was notable how little coral was being killed. An example of how little damage from dredging has been measured comes from the Hay Point expansion. 8.6 million cubic metres of material were dredged over six months (SKM, 2013).[3] It was claimed to be the largest dredging project to occur in the southern hemisphere in 2006 using world’s biggest trailer hopper suction dredger.
The muddy plume was occasionally discernible with satellite images up to 50 km away (Islam, 2007)[4] along the coast. Average distance was 15 km to the north and 10 km to the south. No measurable reduction in coral cover was found at the sites near the dredge; the nearest two fringing reefs (Victor Islet and Roundtop Island) showed partial mortality of about five percent due to accumulation of sediment. This was, it needs to be stressed, partial mortality where a part of the coral died like a tree losing a branch. No whole corals died (SKM, 2013). The GBR, 100 km away, could not possibly have been affected.
Perhaps the biggest study, and in many respects the most useful, was associated with construction of a new port on Barrow Island in Western Australia in 2010-11. The dredging was totally different to that at GBR ports; it cut a channel straight across a fringing coral reef. Instead of mud and sand being dredged, which is the case at GBR ports, for Barrow Island, a small part of a fringing reef was also dredged. In addition, although the corals on Barrow Island experience natural sediment resuspension, there are not the huge quantities of soft riverine mud in the vicinity as is the case close to many parts of the Queensland coast. Most of Western Australia is a desert with very few rivers. For this reason, the ecosystems are likely to be less tolerant of high sediment loads.
Ironically, in assessing damage to corals, the Barrow Island dredging operation was an ideal experiment to see the change from complete mortality within a few hundred metres of the dredge to unmeasurable effects 30 km away. Previously, every project in which I had been involved had caused such little damage to corals that it was difficult to say accurately what was a safe distance from a dredge to the coral. All we could say was that it was less than about five kilometres.
The monitoring occurred from 2007 to 2011 including the 18 months of dredging which moved about eight million cubic meters of sediment (Sun et al., 2016).[5] The scale of environmental monitoring was enormous and was estimated to cost at least $130 million alone.[6] The results can be summarised as follows:
- Occasional short-lived plumes of slightly turbid (muddy) water may travel as far as 20 km. This staining of the water could be seen in satellite pictures.
- Over longer periods there was an increase in water turbidity above natural levels within three kilometres from dredging, with an occasional measurable effect to 10 km (Fisher et al., 2015).[7]
- Measurable reduction in light occurred occasionally up to five kilometres. (Fisher et al 2015).
- Signs of stress on coral, occasional production of a mucous layer on the coral, reached 10 km (Bessell-Browne et al., 2017).[8]
- Mortality of many species was greater than 50 percent at one km, but by 4 km was unmeasurable higher than usual. (Fisher et al., 2018).[9]
A different group of scientists under the aegis of the Australian Research Council Centre for Coral Reef Studies, based at James Cook University, measured rates of coral disease to be seven percent of the corals within a few kilometres of the dredge compared with three percent far from the dredge (Pollock et al., 2014).[10]
The data of Pollack et al. (2014) showed only a modest increase in disease and only for very short distances from a huge dredge operation. Their peer-reviewed study, however, ignored the very small spatial scale of the diseased coral and they grandly proclaimed that their “results may help to explain observed increases in global coral disease prevalence in recent decades.”
Remarkably, a study showing a small increase in disease very close to a huge dredge was being extrapolated to a global scale. A predictable headline by the Australian Broadcasting Corporation (ABC) stated that
a world-first study has found that dredging can more than double the level of coral disease in reefs.[11]
This is only true if the coral is right next to an enormous dredge for 18 months. The rest of the coral in the world will be fine.
The dredge-coral disease story did not end with these grand headlines. In 2016, Pollock and his team published a correction to their work acknowledging a fundamental statistical mistake.[12] The corrected work yielded a lower level of certainty of their results. But the coupe de grace was finally delivered by scientists with far greater experience in dredge monitoring (Stoddart et al 2019).[13] They demonstrated not only that Pollock’s statistics were wrong, but that their entire observational method was wrong. They consistently over-estimated the rate of disease because their observers were often mis-identifying disease. Stoddart concluded “the frequency of occurrence of coral disease (usually <5% of corals) was not significantly altered by dredging.”
The ABC, for its part, did not report that the original findings of coral disease were wrong, nor did it retract the ludicrous extrapolation of observations within a couple of kilometres from an enormous dredge to the rest-of-the world. The Australian Research Council Centre for Coral Reef Studies did not correct the findings in any public way, which may explain why the ABC did not take action.
In summary, on the question of how far does the effect of dredging extend, the answer is just a few kilometres. The GBR, however, is invariably much further away – usually ten times further, most is more than a hundred times further away.
[1] See Endnote 6
[2] Kaly, U.L., Mapstone, B.D., Ayling, A.M. and Choat, J.H. (1994). Coral communities. In: L.J. Benson, P.M. Goldsworthy and I.R. Butler, eds., Townsville Port Authority capital dredging works 1993: environmental monitoring program. Townsville, Qld: Townsville Port Authority.
[3] SKM (2013). Improved dredge material management for the Great Barrier Reef Region. Townsville: Great Barrier Reef Marine Park Authority.
[4] Islam, M.A. (2007). Evaluation of satellite remote sensing for operational monitoring of sediment plumes produced by dredging at Hay Point, Queensland, Australia. Journal of Applied Remote Sensing, 1(1), p.011506.
[5] Sun, C., Shimizu, K. and Symonds, G. (2016). Numerical modelling of dredge plumes: a review. Report of Theme 3 – Project 3.1.3. Perth, Western Australia: Dredging Science Node, Western Australian Marine Science Institution.
[6] Stoddart, J. and Poiner, I. (2014). Some suggestions for the environmental management of harbours during and after dredging programs. Ports Australia Conference.
[7] Fisher, R., Stark, C., Ridd, P. and Jones, R. (2015). Spatial Patterns in Water Quality Changes during Dredging in Tropical Environments. PLOS ONE, 10(12), p.e0143309.
[8] Bessell-Browne, P., Fisher, R. and Jones, R. (2017). Mucous sheet production in Porites spp. and links to sediment. Report of Theme 4 – Project 4.8. Perth, Western Australia: Dredging Science Node, Western Australian Marine Science Institution.
[9] Fisher, R., Walshe, T., Bessell-Browne, P. and Jones, R. (2017). Accounting for environmental uncertainty in the management of dredging impacts using probabilistic dose-response relationships and thresholds. Journal of Applied Ecology, 55(1), pp.415–425.
[10] Pollock, F.J., Lamb, J.B., Field, S.N., Heron, S.F., Schaffelke, B., Shedrawi, G., Bourne, D.G. and Willis, B.L. (2014). Sediment and Turbidity Associated with Offshore Dredging Increase Coral Disease Prevalence on Nearby Reefs. PLoS ONE, [online] 9(7), p.e102498. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4100925/.
[11] environment and science reporter Jake Sturmer (2014). Dredging can double coral disease in reefs: study. [online] ABC News. Available at: https://www.abc.net.au/news/2014-07-17/dredging-can-double-coral-disease-in-reefs:-study/5602782?nw=0.
[12] Pollock, F.J., Lamb, J.B., Field, S.N., Heron, S.F., Schaffelke, B., Shedrawi, G., Bourne, D.G. and Willis, B.L. (2016). Correction: Sediment and Turbidity Associated with Offshore Dredging Increase Coral Disease Prevalence on Nearby Reefs. PLOS ONE, 11(11), p.e0165541.
[13] Stoddart, J., Jones, R., Page, C., Marnane, M., De Lestang, P. and Elsdon, T. (2019). No effect of dredging on the prevalence of coral disease detected during a large dredging program. Marine Pollution Bulletin, 140, pp.353–363.
Great Barrier Reef Science Commentary 