The distribution and abundance of the dugong in Shark Bay, Western Australia

Abstract

In July 1989, dugongs were counted from the air at an overall sampling intensity of 7.9% over 14 239 km2 in Shark Bay, Western Australia. Counts were corrected for perception bias (the proportion of dugongs visible in the transect that are missed by observers), and standardised for availability bias (the proportion of animals invisible because of water turbidity) with survey-specific correction factors. The resultant minimum population estimate was 10146 ± 1665 (s.e.) dugongs at an overall density of 0.71 ± 0.12 (s.e.) dugongs km-2, the highest density ever recorded on a large-scale dugong survey. The proportion of calves (19%) was higher than for most other dugong surveys conducted in Shark Bay and elsewhere, suggesting an exceptionally high calving rate in 1988. Dugong density was highest (>5 km-2) in relatively deep water (12-16 m) in the eastern half of Shark Bay opposite the tip of Peron Peninsula and in the western Bay opposite the northern half of Dirk Hartog Island. Fewer than 4% of dugongs sighted in Shark Bay during the survey were in waters colder than 18°C. Results of aerial surveys over 906 km2 in the Faure Sill region of the eastern Bay in November 1990 and January 1991 suggest that between a third and a half of the dugongs in Shark Bay are located in this region during the summer. In contrast, only one dugong was sighted there during the winter survey. The survey confirms that Shark Bay is an internationally significant dugong habitat.