Contributions to Zoology, 67 (1) 71-77 (1997)H.H. de Iongh; B. Bierhuizen; B. van Orden: Observations on the behaviour of the Dugong (Dugong dugon Müller, 1776) from waters of the Lease Islands, eastern Indonesia

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Discussion

In our study there was no indication of the fact that dugongs dislike rough water, in contrast with earlier observations (Allen et al., 1976). Even during very rough seas, dugongs would still come to forage. This could be explained either by the importance of the nutritional value of the seagrasses or by the depth of the meadow (7-9 m). We spotted dugongs both in the morning and in the afternoon, during both low tide and high tide. These observations do not indicate regular feeding times.

Anderson & Birtles (1978) state that feeding tracks are only found in Zostera and Halophila and thought that each trench represented the effort of a single dive. During our study this statement was not confirmed; a dugong was observed feeding, producing multiple feeding tracks. The dugong’s behaviour may be adjusted in order to minimize effort for a maximum of energy intake (De Iongh et al., 1995b). Multiple feeding tracks during one dive support this assumption.

In our study the mean submergence time of the dugong is 4.6 minutes. This is longer than the time lengths found by Allen et al. (1976), Anderson (1982), and Anderson & Birtles (1978), who observed mean submergence times of 2.5, 2.7, and 1.2 minutes, respectively (Table II). Those studies were done in waters up to 3 meters of depth. Our studies were carried out in seagrass beds up to 9 m depth. This suggests that the submergence time may be correlated with the depth of the bed, coinciding with Anderson (1982) who concluded that the interval between appearances at the surface varies with locality (habitat), foraging mode and foraging species, activity, and reproductive status. His data, obtained from waters of varying depths, suggest a trend for dugongs to remain submerged longer in deeper water. In the present study, the time spent at the surface was minimum 3 seconds for one breath, which agrees with Anderson & Birtles (1978).

With respect to the feeding ecology of the dugong, some of our observations do not correspond with the findings of other authors. Anderson (1981) and Anderson & Birtles (1978), for example, report that dugongs forage at depths of 3 to 4 m in both sublittoral and intertidal areas. Most of the feeding tracks we observed at the seagrass bed in Saparua Bay were located at a greater depth (7-9 m). Marsh et al. (1994) and Marsh (1993) reported on the importance of deepwater seagrass meadows to dugongs and found evidence of dugong feeding up to 40 m depth. Favoured feeding areas appeared to be characterized by relatively sparse seagrass and ready access to deeper water. The regular recropping by dugongs of restricted grazing swards in a monospecific Halophila ovalis meadow may indicate a pattern of cultivation grazing as described by Preen (1993, 1995) for dugongs in Moreton Bay, but more research data are needed to confirm this.