Generalities on dugongs
The dugong has a large range that spans at least 37 countries and territories and includes tropical and subtropical coastal and island waters from east Africa to Vanuatu, between about 26° and 27° north and south of the equator (Nishiwaki & Marsh 1985). The dugong’s historic distribution is believed to have been broadly coincident with the tropical Indo–Pacific distribution of its food plants, the phanerogamous seagrasses of the families Potamogetonaceae and Hydrocharitaceae (Husar 1978).
The known range of the dugong (Marsh et al., 2002)
Dugongs are long-lived with a low reproductive rate, long generation time, and a high investment in each offspring. The oldest dugong whose tusks have been examined for age determination was estimated to be 73 years old when she died. Marsh’s research suggested that females do not bear their first calf until they are at least ten and up to 17 years old. Using the same age determination technique, Kwan (unpublished data) has recent evidence of dugongs producing their first calf as early as age 6 years. Gestation is in the region of 13-15 months. The usual litter size is one. The calf suckles for 14-18 months or so, and the period between successive calvings is spatially and temporally variable; estimates range from 2.4 (Kwan unpublished data) to seven years. Dugongs start eating seagrasses soon after birth, but they grow rapidly during the suckling period when they also receive milk from their mothers.
Dugongs are seagrass specialists, uprooting whole plants when they are accessible, but feeding only on leaves when the whole plant cannot be uprooted (Anderson 1982a; Marsh et al. 1982, 1999). Dugongs prefer seagrasses that are lower seral or ‘pioneer’ species (Preen
& Marsh 1995), especially species of the genera Halophila and Halodule. Diet selection is correlated with the chemical and structural composition of seagrass (Lanyon 1991; Aragones 1996). The most frequently selected species are lowest in fibre and highest in available nitrogen and digestibility (Lanyon 1991; Aragones 1996). Selection for the species that are highly digestible (Halophila) and have high nutrients (Halodule) means that dugongs maximize the intake of nutrients rather than bulk (Aragones 1996). Marine algae are also eaten (Marsh et al. 1982), but this is believed to occur only when seagrass is scarce (Spain & Heinsohn 1973).
Dugongs feeding on seagrass (Françoise Margherite)
Dugongs generally frequent coastal waters. Major concentrations tend to occur in wide shallow protected bays, wide shallow mangrove channels and in the lee of large inshore islands (Heinsohn et al. 1979). These areas are coincident with sizeable seagrass beds. Dugongs are also regularly observed in deeper water further offshore in areas where the continental shelf is wide, shallow and protected. For example, in Torres Strait between Australia and Papua New Guinea, significant numbers of dugongs are seen more than 10km from land (Marsh & Saalfeld 1989, 1991).
Information taken from Marsh, H., Penrose, H., Eros, C. & Hugues, J., 2002. Dugong: status reports and action plans for countries and territories. . In Early warning and assessment report series, (ed. UNEP).
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