Dugong

Dugongs, or sea cows as they are sometimes called, are marine animals which can grow to about three metres in length and weigh as much as 400 kilograms. They are the only marine mammals in Australia that live mainly on plants.

The name sea cow refers to the fact that they graze on seagrass, which forms meadows in sheltered coastal waters. As dugongs feed, whole plants are uprooted and a telltale feeding trail is left behind. 

Dugong play an important ecological role in coastal marine ecosystems, and the status of dugong populations in an area can be used as an indicator of general ecosystem health.

Dugong are more closely related to elephants than to other marine mammals such as whales and dolphins, but their closest living aquatic relatives are the manatees. Manatees are aquatic mammals that live in freshwater rivers and coastal waters of West Africa, the Caribbean, South America and the southern United States (Florida).

Another close relative was Steller’s sea cow, previously found in the northern Pacific. It was hunted to extinction in the 1700's by sealers for its meat. It grew almost three times as long as the dugong and fed on large algae (kelp).

Distribution

Dugong inhabit shallow, tropical waters throughout the Indo-Pacific region. Most of the world’s dugong population now occurs in northern Australian waters between Shark Bay in Western Australia and Moreton Bay in Queensland.

The Great Barrier Reef Region supports globally significant populations of dugong. This being one of the reasons the area was given World Heritage status.

Much of the information about dugong in the Great Barrier Reef World Heritage Area comes from research conducted over the past 30 years by Professor Helene Marsh and her colleagues and students at James Cook University and from the local knowledge of Indigenous people and fishers. The contributions of these people in acquiring and providing this information are gratefully acknowledged.

Life in the sea

Dugong swim using their whale-like fluked tail and they use their front flippers for balance and turning. Their movements are often slow and graceful. Early explorers and sailors believed that they were mermaids because of their streamlined bodies and the large teats at the base of the flippers of the female dugong.

They have a rounded head with small eyes and a large snout. The nostrils are at the top of the snout and can be closed diving to keep the water out. 

As with all other marine mammals, dugong must surface to breathe. However, unlike other marine mammals such as some whales and dolphins, dugong cannot hold their breath under water for very long. Dives generally last for only a few minutes, especially if they are swimming fast.

Dugong have poor eyesight but very good hearing. They find and grasp seagrass with the aid of coarse, sensitive bristles that cover the upper lip of their large and fleshy snout.

Small tusks can be seen in adult males and some old females. During the mating season, male dugongs use their tusks to fight each other.

Their slow breeding rate and long life span mean that dugongs are particularly susceptible to factors that threaten their survival. Throughout their worldwide range they are threatened by human impacts, particularly by the effects of habitat degradation.

Dugong facts

Life span (maximum longevity -
most dugong die at a younger age)
70 years
Age before breeding (females) 6-17 years
Age before breeding (males) 4-16 years
Gestation period 13-15 months
Number of young
1
Lactation length 14-18 months
Time between breeding
3-7 years
Maximum possible rate of increase
(e.g. low natural mortality and no human-induced mortality)   
5% per year
Estimated natural mortality rate 5% per year

Dugong conservation

Worldwide, the dugong is listed under the IUCN - the World Conservation Union - Red List of Threatened Animals as being vulnerable to extinction

In Australia, dugong are protected under various pieces of legislation. They listed as a marine and migratory species under the Commonwealth Environment Protection and Biodiversity Conservation Act 1999. They are also protected by other Commonwealth legislation such as the Great Barrier Reef Marine Park Act 1975 where they are a protected species.

The Queensland Government's Nature Conservation (Wildlife) Regulation 2006 lists dugong as 'vulnerable to extinction'.

While dugong are threatened on a worldwide scale, Australia has a large proportion of the remaining population. This makes Australia the largest, and globally most important, refuge for dugong. The sensitive ecological status of these animals globally highlights the need for effective management strategies to protect and conserve the Australian population.

There are concerns about the dugong population within the Great Barrier Reef World Heritage Area. Based on current research it is thought that dugong numbers have declined along the urban coast of Queensland, south of Cooktown. There are a number of human related threats to dugong including boatstrike, incidental capture in fishing nets and marine debris, and habitat degradation due to coastal development and declining water quality.

A greater number of dugong occur along the remote coast (northern third of the Great Barrier Reef north of Cooktown), and populations appear stable in that area.

Current combined levels of mortality from all threats, as reported in the marine strandings database maintained by the Queensland Department of Environment and Resource Management/Queensland Parks and Wildlife, are thought to be unsustainable.  As dugong are long-lived and slow breeding animals, recovery from any population decline can take many years.

Cultural connections

Dugongs are an essential element of Aboriginal and Torres Strait Islander people's living maritime culture along the Great Barrier Reef World Heritage Area. The use of marine food resources such as the dugong greatly strengthens Aboriginal and Torres Strait Islander culture and demonstrates connection with traditional sea country.

The activities associated with the hunting of dugong and preparing and sharing the meat has great significance and is an expression of the continuance of long cultural traditions. In remote coast areas, dugongs have a high social and economic value because they provide subsistence food to communities where a nourishing diet is essential but often expensive to attain.