This survey is the second of an integrated series of three aerial surveys which aim to survey the entire Queensland coast for dugongs over three field seasons. It provides the first synopsis of the distribution and abundance of the dugong on the remote coast of Queensland from Cooktown north including Torres Strait. The results of previous surveys of sections of this region have been difficult to interpret because of the potentially confounding influences of unpredictable dugong movements between areas within the region. The results of the 2006 survey of the whole region of almost 56,000 km2 suggest a total population of some 23500 ±2900 dugongs close to the estimate of some 23000 ±2600 for the combined 2000 survey of the Northern Great Barrier Reef (GBR) and 2001 surveys of Torres Strait. Both estimates are based on the method of Pollock et al. (2006). The time series of surveys since the mid 1980s suggests considerable movement of dugongs between survey blocks within the Northern GBR region. Population movement between the Northern and Southern GBR are also likely and may also explain some of the variation in the dugong population estimates of both regions. The overall result of the time series of aerial surveys for the whole Northern GBR/Torres Strait region suggests that the fluctuations in the estimates in the size of the Torres Strait dugong population are unlikely to result from significant movements at a population level between the Northern GBR and Torres Strait. This result accords with new genetic evidence. A likely reason for the movement of dugongs within the Torres Strait region is the susceptibility of the region to episodic seagrass diebacks, which are now believed to be largely natural events, the frequency of which may be exacerbated by climate change.

The aerial surveys of the Northern GBR and Torres Strait since the mid 1980s for dugongs have not demonstrated a significant decline in dugong numbers, despite concern about the sustainability of the traditional harvest of dugongs in this region and the limited arrangements to regulate this harvest to date. However, given the difficulty in detecting declines in marine mammal stocks, we caution about using this result as a reason for postponing community-based management initiatives, especially as: (1) the whole area of dugong habitat in Torres Strait has not been surveyed because the region to the west of the survey area is inaccessible from light aircraft based in Australia; (2) there is considerable uncertainty surrounding the impact of climate change on the frequency of seagrass diebacks; (3) there is evidence that the life history and reproductive rate of female dugongs are reduced by seagrass diebacks; and (4) the fact that dugongs in Torres Strait are breeding at younger ages, smaller sizes and more often than has been recorded elsewhere may be a density-dependent response to declining population size. The discrepancy between the estimated sustainable catch and the anecdotal catch estimates, particularly for Torres Strait, suggests that one of the following may be true: (1) the aerial surveys underestimate the actual dugong population size, probably because: (a) the availability correction factor is underestimated; and/or (b) the assumption of full independence between the two observers in a tandem team is violated; (2) dugongs are breeding faster than estimated, either because of a density-dependent response to declining population size and/or environmental conditions that have improved the food supply; and/or (3) the anecdotal estimates of the harvest rate are too high. The data generated using PBR modelling suggest annual sustainable anthropogenic mortality limits of 56-112 dugongs in the Northern GBR. We suggest that an annual sustainable anthropogenic mortality limit of 56 would be a prudent interim management target given the World Heritage Status of the region and the management objective of population recovery for dugongs in the Great Barrier Reef Marine Park. PBR modelling suggests annual sustainable anthropogenic mortality limits of about 100-200 dugongs per year for Torres Strait depending on the value given to the Recovery Factor. The latter is a policy decision linked to the objective of the management arrangements. The dugong population in the Northern GBR/Torres Strait region is substantial (>20,000 individuals) and is genetically healthy. We believe that there is time to work with local Traditional Owners and commercial fishers to develop appropriate management arrangements without dugongs becoming locally extinct within this region. This approach accords with the Torres Strait Treaty 1985 between Australia and Papua New Guinea. The Treaty recognizes the importance of: (1) ‘protecting the traditional way of life and livelihood of Australians who are Torres Strait Islanders and of Papua New Guineans who live in the coastal area of Papua New Guinea in and adjacent to the Torres Strait’; and (2) ‘the marine environment’ of the region. Index Blocks in Hervey Bay in southern Queensland were also surveyed to provide a context for the survey at a larger spatial scale. Despite the high correlation between the population estimates for the Index Blocks and their total region, the Index Blocks were not particularly robust indices of the dugong population in their region because they represent an unknown and probably variable proportion of the population. Index blocks external to a survey region add considerably to the expense and logistical problems associated with a regional survey and we consider that the practice of surveying Index Blocks should be discontinued.

Funding for these surveys was obtained from the following government agencies: Great Barrier Reef Marine Park Authority (GBRMPA), Department of Environment and Resource Management (DERM), Australian Marine Mammal Centre (AMMC), Torres Strait Regional Authority (TSRA), Australian Fisheries Management Authority (AFMA), Marine and Tropical Sciences Research Facility (MTSRF), National Environmental Research Program (NERP).