2023 Dugong Aerial Survey: Mission Beach to Cape York

Overview

Project objectives

• The objectives of our study were to:

1. Continue the time series of surveys for dugongs across the inshore waters of the northern section of the Great Barrier Reef World Heritage Area.
2. Continue to explore the reduction uncertainty in the results for the surveys.
3. Provide advice to relevant management partners (GBRMPA, DCCEEW, and the Queensland Government) and Traditional Owners about the implications of the findings for the conservation, management, and monitoring of dugongs in the northern Great Barrier Reef.

Methods

 We surveyed the inshore waters of the Great Barrier Reef (GBR) between Mission Beach and Cape York in October–November 2023.

• The survey design was based on the aerial surveys conducted by researchers at James Cook University since the 1980s as optimised during the RIMReP process.
• The aerial survey methodology followed the strip transect aerial survey technique used in earlier surveys along the Queensland coast.
• Imagery experiments were undertaken as part of this survey but will be synthesised in a separate report.
• Dugong abundance was estimated using the Hagihara method, which corrects for detection biases.
• N-mixture Bayesian models were used to assess trends in the dugong population across the surveyed region between 2006 and 2023.
• We developed spatially explicit models of dugong density distribution for survey years 2006, 2013, 2018-19, and 2023 using the Inverse Distance Weighting (IDW) technique.

Key findings

• The dugong population in the northern Great Barrier Reef (nGBR) in late 2023 was estimated at 6,838 dugongs (±se 968) using the Hagihara method. This compares with the (revised) population estimate
  of 6942 (±se 1618) in 2018–19.
• 9.1% of dugong sightings in the nGBR were calves (43 calves out of 471 dugongs), the highest
  percentage recorded since 2006 and within the ‘normal’ range for dugong surveys
• Over 17 years, the population has grown at approximately 2% per year, indicating that the dugongs in the nGBR are in good condition.
• The population density in 2023 was the highest among all survey years (2006, 2013, 2018–19, 2023).
• Most population growth occurred between 2006 and 2019, with no statistical difference between 2019 and 2023.
• This population trend confirms the status of the dugong in the nGBR, an attribute of the Outstanding Universal Value of the region’s Outstanding Universal Value.
• The increasing population in the nGBR contrasts with a significant population decline in the southern Great Barrier Reef (sGBR), and concerns about their status in Hervey Bay and Moreton Bay.
• From 2006 to 2023, high and very high dugong densities were consistently found inshore and offshore between north of Cape Flattery (14° 53′ S) and Cape Melville (14° 10′ S), and in sheltered bays like Lloyd Bay(12° 51′ S), Temple Bay (12° 20′ S), and Shelburne Bay (11° 55′ S).
• Consistently low dugong densities were found inshore between north of Shelburne Bay and the northern boundary of the survey region.
• Spatial models indicate spatial variations in dugong density ‘hotspots’, likely reflecting temporary emigration of dugongs within and possibly out of the survey area between survey events.
• Spatial models from the 2018 and 2023 surveys in the northern Central GBR (Mission Beach to Cape Bedford, north of Cooktown) show overall low dugong densities, but an increase in dugong density around Innisfail and fewer dugongs in the Cape Tribulation area in 2023 compared to 2018. These differences may be due to temporary emigration.
• We repeated two survey transects where a high number of dugongs were present to test the increase in the precision of estimates and decrease the statistical variance of transect densities. Our analysis also revealed that the gains in repeat surveys was only slightly better than the gains from expanding the overall number of transects. Therefore, it is an open question whether resources would be better spent on surveying more transects, or whether it is better to do repeat surveys on the same transects where high number of animals are observed. This conclusion is tentative given that only two transects were repeated but suggest an area for further research to reduce the uncertainty surrounding dugong population estimates.

Key findings for policy makers

• The status of the dugong population in the remote coast of the nGBR is good and in much better condition than the populations along the more urbanised coast of the sGBR.
• The first priority of the survey team/dugong scientists must be to consult with the Traditional Owners of the Indigenous communities in the nGBR about the results of this survey and how they would like to share these results with their communities.
• Despite considerable attempts to reduce impacts in the GBRWHA, threats to dugongs and their habitats remain. An important next step might be to work with key Traditional Owners of: 1) the Sea Country bordering the dugong ‘hotspots’ in the sGBR (e.g., Hinchinbrook, Townsville and Shoalwater Bay), and 2) in the Sea Country of the major communities in the nGBR: to consider the risks, including the climate risks, to dugongs and their habitats in their region.
• This consultation could be a key first step in developing a Wildlife Conservation Plan for Dugongs (or Coastal Megafauna more generically) in the coastal waters of the GBRWHA.
• The development of such a Plan would provide a vehicle for exploring opportunities for increased Traditional Owners participation in the research and management of dugongs in the GBRWHA.
• The 5-yearly aerial surveys of dugongs across the GBRWHA are clearly an effective method of monitoring the status of dugongs in the region.