Protecting the Dugong
The dugong (Dugong dugon) is the only herbivorous mammal that is strictly marine and is the only extant species in Family Dugongidae. A characteristic dugong weighs about 400 kilograms and can grow up to three meters long (Stonehouse, 1992). It is easily distinguished from others in order Sirenia by its triangular whale-like tail. The manatee, the other member of order Sirenia, has a distinct paddle-shaped tail and spends its life in rivers and estuaries. Like the manatee, dugongs have a thick layer of blubber giving them a distinctly rotund posture, have small paddle-like flippers positioned far forward on the body, and a broad, flattened tail. The ochre brown ...view middle of the document...
Current theories suggest that the ancient dugong (Protosiren) was related to the ungulates (ancient hoofed mammals) and an ancestor of the elephants that fed on shallow sea grass meadows of the Caribbean during the warm Eocene period (54-38 MYA). The manatees appeared during the Miocene (26-7 MYA), when climate favored growth of plants in fresh water settings. Today there are only four species of siren: one dugong and three manatees. Up until 300 years ago, there also existed a 25-foot arctic Sirenian, the Stellar’s Sea Cow (Stonehouse, 1985).
As members of order Sirenia, dugongs are part of a unique group of non-ruminant herbivores; lacking a chambered or compartmentalized stomach. Expending little energy compared to other mammals of the same size, the dugong’s slow metabolic rate is attributed to a lack of natural predators and a constant warm environment. Few competitors for food in contrast to the complex division of resources found on terrestrial habitats also allow sirens to apply little energy in common behavior (Reynolds et al. 1991).
ECOLOGICAL POPULATION STATUS
There is a lack of information on the historical population status of dugong, except for observational sighting reports. A past account of dugong population size in Australia as quoted by Bertram and Bertram (1973) states that "In July 1883, a herd in Moreton Bay was reported as extending over a length of about three miles with a width of 300 yards." However, there is no certainty whether dugong populations are substantially reduced from pre-European times, or whether they have reached critical limits for continuity (Chase, 1981).
The current ecological status of dugongs includes a dugong population in the southern Great Barrier Reef that can only cope with a human-caused mortality of less than 1-2% each year. Therefore, a population of 100 dugongs can only cope with the loss of one to two animals per year. The current rate of mortality, believed to be between 6% and 10% annually, is totally and irrevocably unsustainable (GBRMPA).
The only marine mammals that feed primarily on plants, dugongs are the only vegetarian sea-dwelling mammals. Dugongs generally frequent coastal waters. Major concentrations tend to occur in wide protected bays, shallow mangrove channels and in lees of large inshore islands (Heinsohn et al. 1979). These areas are naturally synchronized with hefty seagrass beds. The dugong has wide range of distribution, from longitude 30°E to 170°E and between the latitude of 30°N to 30°S. However, the population is thinly distributed in scattered groups within this range. Major populations were reported in Australia, the Persian Gulf, parts of the Red Sea, northern and eastern coasts of east Africa, Sri Lanka, Indonesia and the Pacific Islands. Most of the world's dugongs are found in Australian waters, from Shark Bay in Western Australia, across the Northern Territory, and to Moreton Bay in Queensland. Of the 80,000...