Dugong

Dugong

Dugong dugon

Conservation Status

Queensland State: Vulnerable
Australian Government: Vulnerable
Listed Marine
Listed Migratory

Species and status overview
Dugongs primarily inhabit coastal waters but have been known to utilise estuarine creeks and have been tracked for several kilometers upstream. Dugongs aggregate in feeding groups in wide, shallow, protected bays, mangrove channels and among large inshore islands. They can also be seen further offshore where the continental shelf is wide, protected and shallow. Dugong feeding trails indicate that they also utilise deep water sea-grass species with one feeding trail found in 33 m of water in northeast Queensland.

Dugongs are highly migratory animals and are known to move seasonally to warmer waters over short distances, trips between 100-400 km are commonly observed. Long distance movements are also occasionally known to occur, with an adult female tracked for 600 km over 5 days between two locations in the Gulf of Carpentaria. It is believed that this movement may be due to the ephemeral nature of sea-grass beds on which the species rely.

Dugongs are long-lived animals, with the oldest Dugong age-assessed (through tusk examination) estimated to be 73 when she died. They are also slow to breed with most females having their first baby between 10 and 17 years of age. Following a gestation period of 13 months, the young are born, usually with only one young in a litter. Mothers give birth in shallow waters, such as tidal sandbanks and estuaries as a strategy to avoid predators. The calf will suckle for 18 months and supplement the mothers milk with sea-grass from shortly after birth, assisting their rapid growth from an early age. The mother will generally not breed again for at least 3-7 years, thus they have a very low reproduction rate. Dugongs have the ability to delay breeding if insufficient food sources are available.
Dugong are members of the order Sirenia that also includes three species of manatees and the Steller’s sea cow that has been extinct since the 1800’s. The name Sirenia derives from the Greek ‘sirens’ and comes from a legend involving lonely sailors at sea mistaking Dugong for mermaids.
Dugong feed primarily on sea-grass, which they remove entirely from the seabed including roots. However they are quite selective about their diet, targeting species that are high in nitrogen and low in fibre. This means they maximise their intake of nutrients, whilst reducing the amount of bulk.  Dugongs also feed on marine algae and are known to search for macro-invertebrates at the southern limits of their range in both Western Australia and on the East Coast. The Dugong is the only marine mammal in Australia that feeds primarily on plants.

A Dugong diet is very specialised eating only certain suitable sea-grass meadows. When referring to the total area of sea-grass, this is a poor indication of its value to the species and localised impacts (e.g. high rates of aquatic sedimentation) can have a huge impact on a Dugong population within an area.

Sedimentation can be caused by many forms of human activities and natural disasters (see threats below). Sedimentation causes increased levels of turbidity that decrease the amount of available light and smother sea-grasses. Other threats include increased epiphytic growth caused by high levels of nutrients.

Dugongs are recognised as one of the values for which the Great Barrier Reef was declared a World Heritage Area.

Where have they been recorded in the area?:

  • Cape Gloucester
  • Edgecumbe Bay
  • Pioneer Bay
  • Shute Harbour
  • Offshore Islands
  • Repulse bay
  • Sand Bay
  • Glendowner Bay
  • Coconut Point

Description
The Dugong can grow to around 2.5-3.5 m in length and can weigh an enormous 230-420 kg. The Dugong is a large, rotund, herbivorous mammal that spends its entire life in the sea. They have a broad horizontal tail that provides propulsion, and paddle-like forelimbs. Like all mammals, Dugongs breathe air through their nostrils located near the top of their snouts and the only reason these animals ever surface from the ocean is to breathe. Sensitive bristles or hairs cover the upper lip of their large snouts which allow them to find sea grass despite their limited vision.

Dugong mothers and calves communicate through a series of ‘chirps’ and although they are predominantly solitary animals they can be seen travelling in pairs or groups of 3 – 6 animals.  Dugongs are more closely related to elephants than any other marine mammal including whales and dolphins.
Conservation concerns
Despite the fact that commercial dugong hunting no longer occurs, there are a wide range of threats that continue to put pressure on their global population.

Threats:

  • Accidental catch in gill and mesh fishing nets, including during commercial Barramundi fishing.
  • Accidental catch in shark control nets.
  • Habitat loss and degradation – seagrass communities are very sensitive to human influence including sedimentation from runoff, dredging activities, mining, trawling and natural disasters such as cyclones. They are also heavily affected by increased nutrients from sewage runoff, detergents, heavy metals, water discharge from desalination plants, fertiliser and other waste products.
  • Indigenous harvest
  • Boat strike and boating activities, Dugongs travel slowly, only around 10 km/h and must surface every 1-3 minutes to breathe making them extremely susceptible to boat strike.
  • Tourism – Dugong watching and harassment
  • Acoustic pollution
  • Chemical pollution
  • Disease and parasites
  • Capture stress
  • Aquaculture
  • Tidal surges
  • Starvation/Loss of seagrass

One method used to mitigate the effects of commercial fishing in the Great Barrier Reef Marine Park is the implementation of Dugong Protection Areas (DPA’s).

DPA – A Zones contain over 50% of dugong numbers in the southern GBR and prohibit offshore, foreshore and drift nets. Within the wider Mackay/Whitsunday region, Upstart Bay and the Newry region are DPA – A zone declared areas.

DPA – B Zones contain about 22% of the Dugong population in the southern GBR. Here, mesh netting is allowed to continue but with restrictions in place. Within the region DPA – B zones include Edgecumbe Bay, Repulse Bay, Llewellyn bay and the Clairview region.

To report sick, injured or orphaned wildlife, marine animal strikes or marine animal strandings, please call RSPCA Qld. on 1300 ANIMAL (1300 264 625).
What can I do?

  • Attend fishing nets at all times
  • Reduce boat speed in warm shallow waters
  • Ensure your activities in and around wetland and riparian vegetation do not adversely affect water quality or hydrology

More Information

Photo credit: Commonwealth of Australia

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