The dugong (Dugong dugon) is the only plant-eating mammal that lives its entire life in the marine environment. Often called sea cows, these shy, slow moving creatures spend most of their day feeding on seagrass. Interestingly, dugongs and their relatives are more closely related to elephants than any marine mammal.
Dugong feed almost exclusively on seagrass, a flowering plant found throughout the shallow banks of Moreton Bay Marine Park. An adult will eat about 30 kilograms of seagrass each day, which is equivalent to approximately 60 lettuces. Dugong have relatively poor eyesight, so rely on the sensitive bristles covering the upper lip of their large snouts to find and grasp seagrass.
As dugong feed, whole plants are uprooted leaving telltale tracks behind. 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 to the area, therefore encouraging regrowth of the seagrass. Halophila ovalis is a fast growing seagrass that is high in nitrogen and low in fibre. It is very important to the survival of the Moreton Bay dugong.
Growing to about three metres in length and weighing as much as 500 kilograms, dugong may live for 70 years or more. Dugong 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.
Moreton Bay dugongs are different
In Australia, dugongs are found in the northern coastal waters. Their distribution stretches from Shark Bay in Western Australia, over the northern coastline and down the Queensland coast. Moreton Bay Marine Park is the southern limit of dugong along the east coast of Australia.
The marine park is home to about 600-800 of these gentle sea creatures. Moreton Bay's dugongs are not completely herbivorous, as was commonly thought. Research shows that they supplement their seagrass diet with macro-invertebrates, such as ascidians (or sea squirts).
Once found throughout the marine park, dugongs are now mostly found on the Moreton and Amity banks, however some are found in Pumicestone Passage and the southern bay. Usually seen singly or in pairs elsewhere, Moreton Bay Marine Park's dugong are commonly found in herds of about 100 animals. Dugong can travel long distances, with Moreton Bay's dugong travelling as far as Hervey Bay, 200km north.
Exploitation of dugong
Herds of dugongs were once observed in incredible numbers in the mid 1800s in the Moreton Bay area. In the 1850s, Europeans began to hunt dugongs for their flesh and oil to export to Europe for medicinal purposes. The oil soon became so popular that demand from England and Europe far exceeded the supply. This demand pressure affected quality control causing irregularities in preparation, inferior quality of the product and uncertain supply. As a result, trade with Europe ceased and fishing stations were shut down in the latter half of the 1800s.
Over the years, various regulations have been implemented to manage dugong hunting. There have been licence requirements to hunt dugong, introduction of short-term closures, rules on what fishing equipment could be used, when they could be fished, and requirements for disposal of flesh and offal. On 20 March 1969, the fisheries department introduced a total prohibition on the taking of dugong in Moreton Bay. Under the Native Title Act 1993 traditional hunting of dugong is allowed for non-commercial purposes. Guidelines for administering traditional hunting activities in Moreton Bay Marine Park have been implemented.
Threats to dugong
Dugong are particularly susceptible to boat strike in Moreton Bay Marine Park. Dugong 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. With south-east Queensland's rapid population increase, and therefore more visitors to the marine park, the risk of boat strike is also increasing.
The dugongs in Moreton Bay Marine Park also face the threat of diminishing food sources. Seagrass meadows, the primary food source for dugong, once occurred close to Brisbane's shoreline but pollution, algal blooms, high boat traffic and turbid waters have reduced their distribution. 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.
Moreton Bay Marine Park is the only place in the world where dugong are found close to a major capital city.