2.4.16 Dugongs

The Region is home to a globally significant population of dugongs recognised as contributing to the Reef’s outstanding universal value. Dugongs are also culturally significant to many coastal First Nations people in Australia 141 and are a cultural keystone species.

Dugongs are the only herbivorous mammals that are strictly marine, and they play a fundamentally important ecological role in the functioning of coastal marine habitats, particularly seagrass systems. As large herbivores, dugongs consume a considerable amount of seagrass (approximately 7 per cent of their body weight per day). They are also highly selective eaters.516 This consumption strongly influences the productivity of seagrass ecosystems, which in turn provides other ecosystem services, such as carbon sequestration and storage.133 Individual dugongs have been recorded to move up to 560 kilometres within the Reef region 517 in a few days, potentially supporting large-scale seagrass dispersal and thereby contributing to connectivity between seagrass meadows and aiding seagrass resilience and recovery.

Two people on each side are looking out of the windows of a small aircraft.
Dugong surveyors. © Christophe Cleguer 2023

Australian dugongs are genetically diverse with an abrupt genetic break in the Whitsunday Island region (20.3 degrees south),518 suggesting that there are distinct breeding units north and south of the Whitsundays.447 Although individual dugongs can be highly mobile,517 movements across the genetic break in the Whitsundays have not been confirmed. However, such movements are inferred from aerial surveys, which recorded differences in population sizes between surveys that were too large to be explained solely by dugong births and deaths.519

Standardised aerial surveys of dugong distribution and population size along the Queensland coast have been conducted since the 1980s.520 Since dugongs are highly mobile, immigration and emigration between survey areas between years can confound any population estimates. Differences in total number of animals between years should, therefore, be regarded with caution. Trend analyses that consider long-term datasets provide more reliable indication of the status of a population.

Aerial surveys conducted in late 2023 indicate that the Great Barrier Reef region north of Cape Bedford (between 15.2 degrees south and 10.8 degrees south) continues to support globally significant populations of dugongs, with densities remaining stable, if not increasing, since 2006.521 As of the 2023 surveys, the dugong population density in this region was estimated to be 0.250 dugongs per square kilometre, very similar to the 2018–19 dugong density of 0.249 dugongs per square kilometre (Figure 2.11). Since 2006, the dugong densities in the northern Great Barrier Reef show an upward trajectory with an annual increase of 1.8 per cent per year (95 per cent confidence interval -0.012 to 0.047) and a probability of increase of 88 per cent. 

Dugong populations in the region south from Mission Beach to Bundaberg show evidence of a long-term decline between 2005 and 2022 of –2.3 per cent per year 522 (Figure 2.11). Poor condition of seagrass beds within the Fitzroy and Burnett Mary areas 149 may affect southern dugong populations.

Figure 2.11
Estimated dugong densities in the Great Barrier Reef since 2005

Estimated trend and per-survey densities of dugongs in northern areas of the Great Barrier Reef between 2006 and 2023 (left), and in central and southern areas of the Great Barrier Reef between 2005 and 2022 (right). The 2023 density and trend analyses for the northern area were performed on survey blocks between Cape York and Cape Bedford. The central and southern surveys occur from Mission Beach south to Bundaberg. Average population densities (circles) are shown ± 1 standard error (box), with 95 per cent confidence intervals (whiskers). Estimated trends (thick dashed lines) are shown with a 95 per cent confidence interval (fine dashed lines). Source: Cleguer et al. (2024)521 and Cleguer et al. (2023)522

The left figure shows that dugong density in the northern Great Barrier Reef has increased slightly since 2006, with the last two surveys in 2019 and 2023 showing very similar densities with 0.249 and 0.250 dugongs per square kilometer, respectively. The right figure show that the dugong density in the southern Great Barrier Reef has declined since 2005, with the lowest densities being recorded in 2011. The value for 2022 was less than 0.14 dugongs per square kilometer.

There is evidence of a long-term decline in dugong populations in the south

Key impacts on dugongs include direct pressures causing mortality, such as incidental catch, vessel strikes, marine debris, traditional hunting and illegal fishing. Indirect pressures, such as loss or degradation of seagrass habitats (Section 2.3.4), due to prolonged periods of rain, floods or cyclones, can severely and acutely deplete the populations, because the reduced food availability delays reproduction and ultimately causes starvation. Climate change is a threat to dugong populations since it affects coastal environmental conditions, flood events and severity and frequency of cyclones, all of which affect dugongs and the seagrass habitats they rely upon. There is potential for pollutants to accumulate in dugongs due to their longevity, slow metabolism and high fat deposits,523 but this risk is considered low.524

Dugong populations from the Great Barrier Reef region north of Cape Bedford are considered stable, if not increasing. However, between 2005 and 2022 there is evidence of a long-term decline in dugong populations in the south.

  • 133. York, P.H., Macreadie, P.I. and Rasheed, M.A. 2018, Blue Carbon stocks of Great Barrier Reef deep-water seagrasses, Biology Letters 14(12): 20180529.
  • 141. Marsh, H., O'Shea, T.J. and Reynolds III, J.E. 2011, Ecology and conservation of the Sirenia: dugongs and manatees, Cambridge University Press, Cambridge.
  • 149. McKenzie, L.J., Collier, C.J., Langlois, L.A., Yoshida, R.L. 2024, Marine Monitoring Program: Annual report for inshore seagrass monitoring 2022–23. Report for the Great Barrier Reef Marine Park Authority, Great Barrier Reef Marine Park Authority, Townsville.
  • 447. Miller, J.D. and Limpus, C.J. 1981, Incubation period and sexual differentiation in the green turtle, Chelonia mydas, in eds. C. B. Banks and A. A. Martin, The Zoological Board of Victoria, Melbourne, pp. 66-73.
  • 516. Scott, A.L., York, P.H., Macreadie, P.I. and Rasheed, M.A. 2021, Spatial and temporal variability of green turtle and dugong herbivory in seagrass meadows of the southern Great Barrier Reef (GBR), Marine Ecology Progress Series 667: 225-231.
  • 517. Sheppard, J.K., Preen, A.R., Marsh, H., Lawler, I.R., Whiting, S.D., et al. 2006, Movement heterogeneity of dugongs, Dugong dugon (Müller), over large spatial scales, Journal of Experimental Marine Biology and Ecology 334(1): 64-83.
  • 518. McGowan, A.M., Lanyon, J.M., Clark, N., Blair, D., Marsh, H., et al. 2023, Cryptic marine barriers to gene flow in a vulnerable coastal species, the dugong (Dugong dugon), Marine Mammal Science 39(3): 918-939.
  • 519. Sobtzick, S., Cleguer, C., Hagihara, R. and Marsh, H. 2017, Distribution and abundance of dugong and large marine turtles in Moreton Bay, Hervey Bay and the southern Great Barrier Reef. Report to the Great Barrier Reef Marine Park Authority. Publication 17/21, Centre for Tropical Water and Aquatic Ecosystem Research (TropWATER), James Cook University, Townsville.
  • 520. Cleguer, C. and Marsh, H. 2023, An inventory of dugong aerial surveys in Australia. Report to the National Environmental Science Program. Report 23/15, Centre for Tropical Water and Aquatic Ecosystem Research (TropWATER), Townsville, Centre for Tropical Water and Aquatic Ecosystem Research (TropWATER), Townsville.
  • 521. Cleguer, C., Hamel, M., Rankin, R., Edwards, C. and Marsh, H. 2024, 2023 Dugong Aerial Survey: Mission Beach to Cape York, JCU Centre for Tropical Water & Aquatic Ecosystem, Townsville.
  • 522. Cleguer, C., Hamel, M., Rankin, R., Genson, A., Edwards, C., et al. 2023, 2022 Dugong aerial survey: Mission Beach to Moreton Bay, Publication 23/44, JCU Centre for Tropical Water & Aquatic Ecosystem Research, Townsville.
  • 523. Lanyon, J.M. and Marsh, H. 1995, Digesta passage times in the dugong, Australian Journal of Zoology 43(2): 119-127.
  • 524. Weijs, L., Leusch, F. and Covaci, A. 2019, Concentrations of legacy persistent organic pollutants and naturally produced MeO-PBDEs in dugongs (Dugong dugon) from Moreton Bay, Australia, Chemosphere 229: 500-508.