Dugong

 

Classification:

Global Population

Description and Natural History

Physical Description
Dugongs, along with manatees, make up the mammalian Order, Sirenia. Dugongs are more streamlined and spindle-shaped than manatees and have a split tail fluke. Adult dugongs average about 2.7 meters (8.9 feet) in length and weigh between 250 and 300 kilograms (551 - 661 pounds). Males and females are similar in size. Adult males can usually be distinguished by short tusks which erupt at 12 - 15 years of age, although tusks are occasionally found in old females as well. The dugong's paddle-like flippers do not have nails.

Calves are about one meter (3.3 feet) long and weigh 20 - 35 kilograms (44 - 77 pounds) at birth. Dugongs are a pale cream color at birth, darkening to gray-brown as they age. Their skin is smooth, with hairs scattered over its surface. Dugongs are difficult to observe in the wild because the valve-like nostrils are located at the tip of the nose. This means that only a small portion of the head is visible above the surface of the water when they breathe. The muzzle is turned downward, an adaptation to bottom feeding, and the upper lip is covered with bristles which are used to locate and manipulate seagrasses, the mainstay of their diet.

Natural History
Dugongs are widely distributed in the waters of 43 countries along the western Pacific and Indian Oceans. They formerly inhabited most of the coastal areas of the tropical and subtropical Indo-West Pacific, where shallow seagrass meadows were found. Dugongs are the only sirenian found in the Indo-Pacific region. They are the most marine of the sirenians in their habitat preference, and almost never enter fresh water. Dugongs are most often found in shallow water, less than five meters deep.

Generally elusive and shy, dugongs have been observed feeding in groups of 100 animals or more. They swim slowly (less than 5.4 knots) using the tail fluke for propulsion and their flippers for braking, turning or sculling to keep their nostrils above the waves in rough water. Speeds of up to 13.5 knots can be maintained for short distances.

Dugongs breed in all seasons, with seasonal peaks varying in different areas of their range. Sexual maturity is reached at nine to ten years of age, although some females do not reproduce until they are 15 - 17 years old. Because calves are nursed for more than a year and may stay with their mothers for two years or more, females give birth to a single calf only every three to five years. Dugongs may live up to 70 years.

Dugongs, like manatees, have been reported to feed almost exclusively on a variety of seagrasses. However, there is growing evidence that dugongs in subtropical areas also feed on sessile benthic invertebrates (ascidians and polychaetes), perhaps as a result of nutritional stress caused by seasonal fluctuations in the abundance of seagrasses in the southern edge of their range.

The dugong population is estimated to number approximately 100,000 animals. The largest population is found in Australia.

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Status of the Species

Status
The International Union for the Conservation of Nature and Natural Resources (IUCN) lists the dugong as Vulnerable on a global scale, based on an inferred or suspected reduction of at least 20 percent over the last three generations (90 years).

The Australian dugong population is listed on Appendix II of CITES; all other populations are listed on Appendix I. Dugongs are listed as Vulnerable in Queensland, Australia, under that state’s Nature Conservation Act.

Threats to the Species
In the past, dugongs were widely hunted for their meat and oil. Today, they are hunted locally for subsistence purposes in many countries.

Dugongs often become entangled and die in gill nets set by fishers, and in shark nets set off beaches to protect swimmers. Loss of coastal habitat and increasing boat traffic are also blamed for declines in the dugong population.

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Authors and Sources

Sources
CITES. 2001. Dugong. http://www.cites.org.

CRC Reef Research Centre. 2001. Dugongs. http://www.reef.crc.org.au/discover/plantsanimals/dugong/index.html.

IUCN. 2001. The 2000 IUCN Red List of Threatened Species. http://www.redlist.org.

Reeves, R., B.S. Stewart, and S. Leatherwood. 1992. The Sierra Club Handbook of Seals and Sirenians. Sierra Club, San Francisco. 359 pp.

Reynolds III, J.E. and D.K. Odell. 1991. Manatees and dugongs. Facts on File, Inc., New York. 192 pp.

Rice, D.W. 1998. Marine mammals of the world: systematics and distribution. The Society for Marine Mammalogy Special Publication Number 4. 231 pp.

Ripple, J. 1999. Manatees and dugongs of the world. Voyageur Press Inc., Stillwater, MN. 131 pp.

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