Dugong - Distribution and Habitat

Distribution and Habitat

Dugongs are found in warm coastal waters from the western Pacific Ocean to the eastern coast of Africa, along an estimated 140,000 kilometres (86,992 mi) of coastline between 26° and 27° degrees to the north and south of the equator. Their historic range is believed to correspond to that of seagrasses from the Potamogetonaceae and Hydrocharitaceae families. The full size of the former range is unknown, although it is believed that the current populations represent the historical limits of the range, which is highly fractured. The dugong is the only strictly-marine herbivorous mammal, as all species of manatee utilise fresh water to some degree. Recorded numbers of dugongs are generally believed to be lower than actual numbers, due to a lack of accurate surveys. Despite this, the dugong population is thought to be shrinking, with a worldwide decline of 20 per cent in the last 90 years. They have disappeared from the waters of Hong Kong, Mauritius, and Taiwan, as well as parts of Cambodia, Japan, the Philippines and Vietnam. Further disappearances are likely.

Populations of dugongs exist in the water of 37 countries and territories. In the late 1960s, herds of up to 500 dugongs were observed off the coast of East Africa and nearby islands. However, current populations in this area are extremely small, numbering 50 and below, and it is thought likely they will become extinct. The eastern side of the Red Sea is the home of large populations numbering in the hundreds, and similar populations are thought to exist on the western side. In the 1980s, it was estimated there could be as many as 4,000 dugongs in the Red Sea. The Persian Gulf has the second-largest dugong population in the world, inhabiting most of the southern coast, and the current population is believed to be around 7,500.

A highly isolated breeding population exists in the Gulf of Kutch, the only population remaining in western India. It is 1,500 kilometres (932 mi) from the population in the Persian Gulf, and 1,700 kilometres (1,056 mi) from the nearest population in India. Former populations in this area, centred on the Maldives and the Laccadive Islands, are presumed to be extinct. A population exists in the Gulf of Mannar and the Palk Strait between India and Sri Lanka, but it is seriously depleted. A small population exists around the Nansei Shoto islands, and a population formerly existed off Taiwan. An endangered population of 50 or fewer dugongs survives around Okinawa.

A small population exists off Southern China, centred around the island of Hainan. The population around the Andaman and Nicobar Islands are known only from a few records, and although the population was large during British rule, it is now believed to be small and scattered. All the islands of the Philippines are believed to have once provided habitat for dugongs, which were common until the 1970s. The Gulf of Thailand also used to have a large population, but they have not recently been sighted in the west of the gulf, and the population in the east is thought to be very small. Dugongs are believed to exist in the Straits of Johor in very small numbers. The waters around Borneo support a small population, with more scattered throughout the Malay archipelago. Populations exists around the Solomon Islands archipelago and New Caledonia, stretching to a westernmost population in Vanuatu. A highly isolated population lives around the islands of Palau.

Australia is home to the largest population, stretching from Shark Bay in Western Australia to Moreton Bay in Queensland. The population of Shark Bay is thought to be stable with over 10,000 dugongs. Smaller populations exist up the coast, including one in Ashmore reef. Large numbers of dugongs live to the north of the Northern Territory, with a population of over 20,000 in the gulf of Carpentaria alone. A population of over 25,000 exists in the Torres Strait, although there is significant migration between the strait and the waters of New Guinea. The Great Barrier Reef provides important feeding areas for the species; this reef area houses a stable population of around 10,000, although the population concentration has shifted over time. Large bays facing north on the Queensland coast provide significant habitats for dugong, with the southernmost of these being Hervey Bay and Moreton Bay.

Dugongs are generally found in warm waters around the coast with large numbers concentrated in wide and shallow protected bays. Large numbers also exist in wide and shallow mangrove channels and around leeward sides of large inshore islands, where seagrass beds are common. They are usually located at a depth of around 10 m (33 ft), although in areas where the continental shelf remains shallow dugongs have been known to travel more than 10 kilometres (6 mi) from the shore, descending to as far as 37 metres (121 ft), where deepwater seagrasses such as Halophila spinulosa are found. Special habitats are used for different activities. It has been observed that shallow waters are used as sites for calving, minimising the risk of predation. Deep waters may provide a thermal refuge from cooler waters closer to the shore during winter.