Project Title: Distribution and abundance of the dugong in Gulf of Carpentaria waters: a basis for cross-jurisdictional conservation planning and management
Chief Investigators: Prof Helene Marsh and Mr Keith Saalfield
This survey provides the first synopsis of the distribution and abundance of the dugong in the Gulf of Carpentaria. The results of previous surveys of parts of this region in both Queensland and the Northern Territory have been difficult to interpret because of the potentially confounding influences of unpredictable dugong movements between areas within the region.
The results for the 2007 aerial survey of the whole survey region of 35592 km2 suggest a total population of 12438 + s.e.1951 dugongs; 7095 + s.e.1565 off Queensland, 5343 + s.e.1164 off the Northern Territory. Overall 30 percent of sightings were in Commonwealth waters.
This estimate is lower than previous comparable estimates for the Northern Territory and higher than the previous comparable estimate for Queensland. There are methodological reasons for these differences: (1) the methodology used to correct for availability bias was different for the Queensland surveys; (2) the methodology used to correct for both availability bias and perception bias was different for the Northern Territory surveys; (3) the previous surveys had been conducted at different times and so the results were likely confounded by movements between survey regions.
Standardised comparisons of the results for 2007 with the results of previous surveys suggest that the dugong density averaged over all regions in the Queensland waters of the Gulf of Carpentaria for which such comparisons could be made was not significantly different between 1997 and 2007. However, density varied substantially among regions. The average block differences resulted from the significantly higher density of dugongs in the Wellesley Islands region compared with the Northern Cape York Coast. In addition, too few dugongs were sighted along much of Cape York coast to obtain robust population estimates. There were also significant differences in dugong density among years in the Wellesley Island region where there was a significant increase in dugong density from 1997 to 2007. Dugong density in the Northern Territory did not differ among survey years, and there were no significant differences among survey blocks.
Thus the differences in dugong population estimates between surveys should not be used as evidence of a decline in abundance in the Northern Territory (or for an increase in abundance in Queensland); but as a starting point for future monitoring. We believe that the methodology adopted in 2007 is more accurate than the methodology used previously because of the improved capacity to correct for dugongs that are unavailable to observers because of water turbidity.
The between survey comparisons in Queensland suggest considerable movement of dugongs between survey blocks within that region, particularly between the Wellesley Island area and the waters off northern Cape York. A likely reason for the movement of dugongs within the region is the susceptibility of tropical seagrasses to episodic diebacks, the frequency of which may be exacerbated by climate change.
PBR modelling suggests annual sustainable anthropogenic mortality limits of 82-164 dugongs for the Gulf of Carpentaria as a whole; 33-67 for Northern Territory waters, 44-89 for Queensland waters, and 34-69 for the Wellesley Island area. We suggest that the upper limits of these values would be prudent interim management targets if the management objective for the region is dugong population maintenance.
The dugong population in the Gulf of Carpentaria region is substantial (~12,500 individuals), making it one of the most important regions for dugongs in Australia and the world. 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.
