Dugong Information Sheet

Dugong dugon

Size	3 metres in length
Weight	400 kilograms
Colour	light brown to light grey on upper back, paler underneath
Features	no dorsal fin, fluked tail, males have two tusks, large head with specially modified mouth - has sensitive bristles on lips to compensate for poor eye sight but seem to have acute hearing. Their eyes and ears have no flaps or lobes, and are on each side of the head. Their paired nostrils are on the top of the head and have valve like devices that prevents water entering when they dive. Dugongs surface for air for a few seconds, every few minutes.
Relatives	Dugongs are more closely related to the elephant than other marine mammals. Belong to Order Sirenia and closely related to Steller's sea cow.
Life cycle	Lives to 70 years, sexually mature at 10 years but are very slow to reproduce. Having their first calf between 9 - 17 years, usually calving once every four years. Gestation is 13 months and calves stay with the mother for 18 months and before being weaned. The age of a Dugong is estimated by counting the growth layers in its tusks, which erupt after puberty in males and in a small proportion of older females. Groups of males will follow a female Dugong in oestrous and many mate with her, inflicting scars on the females back, and on each other.
Food	Dugongs are herbivorous marine mammals, feeding mainly on sea grass. In Moreton Bay they are known to supplement their winter diet by consuming ascidians, such as Sea Squirts, Sycozoa pulchra. This maybe due to reduced sea grass productivity during winter in this most southerly extent of the Dugongs eastern seaboard population. Similarly, on the southern most extent of the Dugong population on the west coast, at Shark Bay, the Dugongs also eat animal material. Its not known for sure why Dugongs in sub tropical waters feed on invertebrates, whilst tropical Dugongs don't, but its suggested to relate to sea grass availability, and possible more so to do with nitrogen / nutrient availability. Dugongs seem to prefer Dugong Grass, Halophila ovalis, Spiny Dugong Grass, Halophila spinulosa and Halodule uninervis. Eelgrass, Zostera capricorni, is also eaten but least preferred, believed so because of its high fibre content and low nutrient value. Note, nitrogen fixation rates associated with sediments of seagrasses along Queensland Coast are between 5 and 100 times higher than those anywhere in the world. Nitrogen fixing bacteria are closely associated with sea grass roots. Sea grass roots exude dissolved organic carbon and pump oxygen into the adjacent sediment. This provides energy for the bacteria, which in turn fixate nitrogen and produce inorganic nutrients utilised by the sea grass. Sea grass are flowering plants.
Movements	While generally slow and graceful, they are capable of short fast bursts of speed. Unique to Moreton Bay is the herding aspect of Dugongs, sometimes herds up to 50 - 250 are seen on the sea grass beds on the eastern side of the Bay. This is unusual as Dugongs in most tropical areas are seen as solitary animals or in pairs. The reason for herding in Moreton Bay is unknown, it may relate to a combination of Dugong densities, availability of food and merging on tidal influences or some social aspect yet to be understood. In winter there is some migration into the South Passage, which is believed to be the Dugongs taking advantage of the warmer oceanic currents and avoiding the cold water of the Bay, which can be as low as 15 deg C. Dugongs have been found 12km out of South Passage and on some surveys, about 16% of Dugongs were sighted in South Passage during winter high tide. As there is no food in South Passage this would indicate Dugongs are fasting for prolonged periods, but this requires further research. Satellite tracking of Dugongs in the Great Barrier Reef Marine Park show they have a good memory, returning hundreds of kilometres to the same specific spot.