Dugong
Dugong (Dugong dugon) Photo: C.J. Limpus
Common name: Dugong
Scientific name: Dugong dugon
Family: Dugongidae
Conservation status: Dugongs are listed as 'Vulnerable' in Queensland (Nature Conservation Act 1992) and nationally (Environment Protection and Biodiversity Conservation Act 1999). It is ranked as a high priority under the Department of Environment and Resource Management Back on Track species prioritisation framework.
Description
The dugong is a large herbivorous marine mammal that has a long rotund body and a tail or fluke for propulsion. Adult dugongs 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
Dugong density: Alana Grech and Helene Marsh, School of Earth and Environmental Sciences, James Cook University, Townsville Qld
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. Dugongs have also been found in New South Wales waters.
Diet
Dugongs 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', dugongs 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
Dugongs 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
Dugongs 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, dugongs 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 dugongs include incidental mortality in gill fishing nets and shark meshing.
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 land use 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 dugongs 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 Department of Environment and Resource Management 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.
Dugong mortality monitoring
The Department of Environment and Resource Management (DERM) monitors dugong mortality along the Queensland coast via StrandNet, the marine wildlife stranding and mortality database. StrandNet summarises the temporal and spatial distribution of injured, dying and dead marine wildlife in Queensland from records received by DERM, the Great Barrier Reef Marine Park Authority, other government agencies, and those received directly from the public and rehabilitation facilities.
The fourteenth annual Marine wildlife stranding and mortality database report (PDF, 2.0M)* summarises dugong strandings and mortalities in Queensland reported during 2009 and 2010. A total of 90 stranded or dead dugong (including five unconfirmed reports) were recorded in 2010 and 69 (with six unconfirmed reports) in 2009.
Marine wildlife stranding annual reports from 1998 onwards are also available.
Related information
Boat strike impact on turtles and dugong in Moreton Bay
Conservation and management of dugongs in Queensland (PDF, 118K)*
Maps defining 'turtle and dugong go-slow areas' can be found online (PDF, 235K)* or copies of the map and the Moreton Bay Marine Park Introductory Guides can be obtained from the Moreton Bay District office at 1st Floor, 34 Trafalgar Street Manly Queensland 4163, telephone (07) 3131 2888.
* Requires Adobe Reader
Last updated 26 August 2011