Dugong
Dugong (Dugong dugon) Photo © C.J. Limpus.
Common name: Dugong
Scientific name: Dugong dugon
Family: Dugongidae
Conservation status: Dugong are listed as ‘Vulnerable’ in Queensland (Nature Conservation Act 1992)
Description: The dugong is a large herbivorous marine mammal that has a long rotund body and a tail or fluke for propulsion. Adult dugong can reach lengths of more than three metres and weigh up to 420kg.
Dugongs have relatively poor eyesight, so rely on the sensitive bristles covering the upper lip of their large snouts to find and grasp seagrass. Cows and calves communicate by producing 'chirps'.
Habitat and distribution: Major concentrations of dugongs along the Queensland coast occur in wide, shallow, protected bays and mangrove channels, and in the inside edge of large inshore islands. These areas coincide with significant seagrass beds. They also use deep-water habitats. Large numbers have been sighted in water more than 10m deep in several areas, including the Torres Strait, the northern Great Barrier Reef region, and Hervey Bay in southeast Queensland.
A large proportion of the world’s dugong population is found in northern Australian waters from Moreton Bay in the east to Shark Bay in the west, they have also been found down into New South Wales.
Diet: Dugong feed almost exclusively on seagrass, a flowering plant found in shallow water areas. An adult will eat about 30 kilograms of seagrass each day. As dugong feed, whole plants are uprooted leaving telltale tracks behind. They will also feed on macro-invertebrates.
Known as ‘cultivation grazers’, dugong feed in a way that promotes growth of Halophila ovalis – their preferred seagrass species. Pulling out the seagrass aerates the sea floor and increases the amount of organic matter in the area, therefore encouraging regrowth of the seagrass.
Behaviour and life history: Dugong may live for 70 years or more and are slow breeders. The female dugong does not begin breeding until she is 10-17 years old, and only calves once every three to five years, providing seagrass and other conditions are suitable. This slow breeding rate means that dugongs are particularly susceptible to factors that threaten their survival.
Threatening processes: Dugong are particularly vulnerable to boat strike as they come to the surface to breathe, putting them directly in the path of boats and other watercraft. Boats travelling at speed or in shallow waters over seagrass beds or coral reefs pose the greatest threats.
Dugongs are also under threat from diminishing food sources. Seagrass meadows, are being detrimentally affected by pollution (pollutants can include herbicide runoff, sewage, detergents, heavy metals, hypersaline water from desalination plants, and other waste products), algal blooms, high boat traffic and turbid waters. Today, dugong need to rely on smaller seagrass meadows for food and habitat. When the seagrass habitat becomes unsuitable for foraging, dugong populations are displaced and placed under greater threat.
Other direct threats to dugong include incidental mortality in gill fishing nets, shark meshing and from boat strike.
Recovery actions:
- Monitor the effects of boating traffic in areas used by dugongs and introduce controls in marine parks.
- Sixteen dugong protection areas are in place along the Queensland coast, within these areas setting fishing nets are either prohibited or have restrictions placed on them (see the Conservation and management plan).
- Develop management plans for catchments where landuse activities detrimentally affect dugong habitat. These plans will attempt to manage the level of freshwater flow, siltation and herbicide use.
What can be done to help this species?
- When boating especially in shallow waters, be on the lookout for dugong to avoid injuring them.
- Get the guide! Know where you need to go slow. The Introductory Guide to Moreton Bay Marine Park is available from your local EPA/QPWS office.
- Get the ‘Go Slow’ aerial map and always have the Maritime Safety Queensland Beacon to Beacon publication on board your vessel.
- Remember that rubbish you throw away, or chemicals you discard, may find their way into drainage systems that run into the ocean. Such pollutants can affect the health of coastal mammals directly or indirectly through accumulation of poisonous substances in their food.
Further information:
Boat strike impact on turtles and dugong in Moreton Bay
Conservation and management of dugongs in Queensland
Maps defining ‘turtle and dugong go-slow areas’ can be found online or copies of the map and the Moreton Bay Marine Park Introductory Guides can be obtained from the Moreton Bay District office at 127 Russell St, Cleveland QLD 4163, phone (07) 3821 9000.
Last updated: 12 December 2007
