An Experimental Evaluation of Dugong and Sea Turtle Aerial Survey Techniques

Some factors which affect the aerial counts of dugongs and sea turtles were examined experimentally. There was no significant difference in the observed density of dugongs when survey height was doubled from 137 m to 274 m with an accompanying doubling of transect width on either side of the aircraft from 200 m to 400 m. In contrast, a significantly higher density of turtles was observed at the lower heightharrower transect width. The higher level of glare on the exposed side of the aircraft, the time of day and the time from high tide made no significant difference to the observed densities of dugongs or turtles. The survey crew included a tandem team of two observers on each side of the aircraft, who reported their uncolluded observations into separate tracks of a two-track tape recording system. This allowed the reports of tandem observers to be compared in order to assess observer reliability. Overall, observers missed over 40% of dugong groups and over 80% of turtles visible within the transect including groups of more than 10 dugongs. The chance of observers missing a group of dugongs was independent of group size. There was little disagreement between tandem observers about the identification of animals, or the position of animals in the water column. However, observers differed markedly in their categorisation of dugong behaviour and in their counts of animals (particularly dugong calves) in larger groups.