Studiesof Dugongs at Shark Bay,Western Australia. II.* Surface and Subsurface Observations

Abstract

Observational techniques were tested and observations made along the eastern shore of Dirk Hartog I. in June and July of 1979. Dugongs could be closely approached by boats and divers in water with 3-5 m subsurface visibility. Dugongs detected objects in the water from much greater distances and approached and investigated boats and divers visually. This behaviour was highly stereotyped, and because other behaviours did not occur in proximity to boats or divers the effectiveness of surface and subsurface behavioural observation, photography, and attempts to catalogue individuals on the basis of distinguishing scars and other features was limited. Observation from shoreside cliffs was more productive. Vocalizations attributable to dugongs were not detected by hydrophones. Dugongs may vocalize only under exceptional circumstances. When feeding on the seagrass Amphibolis antarctica dugongs strip leaves from stalks, but do not dig into the substrate. Calves may ride on the backs of attending cows. Calves suckle beneath the surface, lying beside the cow and behind her axilla. Dugongs and dolphins interact, and harassment of a dugong cow and calf by a dolphin terminated when other dugongs interposed. Dugongs avoided a boat approaching at moderate speed, but did not take evasive action when the same boat approached at high speed.