Australian Wildlife: Dugong





Dugong (Dugong dugon)

The dugong is a large marine mammal which, together with the manatees, is one of four living species of the order Sirenia. Its closest modern relative, Steller's Sea Cow (Hydrodamalis gigas), was hunted to extinction in the 18th century. The dugong is found in warm, shallow waters between 26 north and south of the Equator, from east Africa to Vanuatu.

In The Wild

The world s most significant population  10,000 to 12,000 animals, or 10% of the species  lives in Shark Bay, WA. The bay s protected waters and plentiful seagrass meadows are perfect for the dugong, the world s only marine mammal herbivore. It feeds almost exclusively on seagrass  up to 50 kg (wet weight) per day! However, it does occasionally snack on molluscs and crustaceans. As it feeds it stirs up plumes of sand, leaving meandering trails that can be seen from the air.

The location of dugongs in Shark Bay varies during the year, depending on water temperature and the abundance of specific seagrass species. In winter they take refuge in warmer waters off Dirk Hartog Island and Bernier and Dorre Islands; in summer they gather to feed on nutritious seagrasses in the Gladstone Special Purpose Zone in the eastern bay, and at the southern end of Henri Freycinet Harbour. Commercial operators offer boat tours from Monkey Mia and Denham to view the vast array of marine life in the area. Dugongs, dolphins, turtles and rays are all on the agenda and the protected waters make for stable and exceptional wildlife viewing.

Moreton Bay off the coast of Brisbane, Qld, is another major home of the dugong because it also contains clean, clear water at the appropriate depth ranges; suitable food; and access to the sea for warmth. Although strong tidal currents affect the exact times and durations of each visit to the bay, the dugong return for protection from large sharks. Important to the future of the dugong, the area is a 200 km stretch of high-density human habitation and recreation, with easy access to study and learn how to best protect the remaining herds.

Moreton Bay's dugong herd used to number in the tens of thousands. Today only 600 to 800 individual dugongs now remain. In 2005, 84% of the 41 reported deaths in Queensland were human related. Boat strike, pollution, marine debris and fishing gear entanglement are the main culprits. Moreton Bay s dugong population is not recovering. Moreton Bay's dugongs can often been seen during ferry journeys across to bay to islands like North Stradbroke.



In Captivity

Worldwide, only six dugongs are held in captivity. Pig, a 10-year-old male, and Wuru, a four-year-old female, formerly lived at Sea World on the Gold Coast, Queensland, Australia, but in December, 2008, they were relocated to Sydney Aquarium. These are the only dugong in captivity in Australia. Elsewhere, two others are the featured attraction of Toba Aquarium in Toba, Mie, Japan; another, named Gracie, is at Underwater World, Singapore; a fourth is in Sea World Indonesia, which was saved after being caught by a local fisherman.

Geographic Range

Dugongs are found discontinuously in coastal waters of east Africa from the Red Sea to northernmost South, northeastern Indian, along the Malay peninsula, around the northern coast of Australia to New Guinea and many of the island groups of the South Pacific. Dugongs inhabit shallow, tropical marine coastal water and are more strictly marine than manatees. The majority of dugongs live in the northern waters of Australia between Shark Bay and Moreton Bay.

Description

Dugongs are born a pale cream colour, but they darken with age to a deep slate gray dorsally and laterally. The short hair is sparsely distributed over the body, save the bristles on the muzzle. The skin is thick, tough and smooth. The front-limbs have evolved into flippers that are 35-45 cm long. These are used for propulsion by young, but the adults use the fluke-like tail for locomotion, using the flippers for steering.

Despite its diet, the dugong has a relatively simple stomach. The muscular upper lip is cleft and protrudes over the down turned mouth. The lower lip and distal parts of the palate have horny pads used to grasp vegetation, which is then uprooted with the strong upper lip. Adults range in length from 2.4 to 4 m. Sexual dimorphism is either absent or females may slightly outsize males.

Breeding occurs throughout the year and peak months for birth vary geographically. The exact length of gestation is unknown, but it is presumed to be about 1 year. Single calves are the norm and twins are rare.



Behaviour

Feeding is the principal activity of dugongs and typically occurs in water 1-5 m deep. Wear on the tusks and trails through grass beds suggest that some digging or rooting is part of the feeding behaviour. Calluses on the flippers are caused by "walking" on them or drifting across the bottom while feeding. Head shaking during feeding appears to be used to clean sediment from the food before ingesting as little sediment is reported in the stomach contents of animals examined.

Frightened animals make a whistling sound and calves have a bleat-like cry. Though now rare, herds of several hundred animals were formerly known. Calves formerly left the herd during the day to form nurseries in shallow water. Groups of 6 animals are most common now. Males are not known to stay with the stable mother-calf social units. Long distance migration is unknown, but some daily and seasonal movements do occur in some populations. Tides, water temperature and food abundance are probably the main factors involved in these movements. Average swimming speed is 10 km/hr, but this can be doubled in a pinch. Dives typically last 1-3 minutes.

Food Habits

Dugongs are aquatic herbivores and feed on the phangerogamous sea grasses of the families Potomogetonaceae and Hydrocharitaceae. Also reported to occasionally eat algae, and crabs have been found in the stomachs of dugongs.

Conservation Status

Dugongs are listed as an endangered species. The dugong has been hunted for thousands of years for its meat and oil, although dugong hunting also has great cultural significance throughout its range. The dugong's current distribution is reduced and disjunct, and many populations are close to extinction.

Despite being legally protected in many countries throughout their range, the main causes of population decline remain anthropogenic and include hunting, habitat degradation, and fishing-related fatalities. With its long lifespan of 70 years or more, and slow rate of reproduction, the dugong is especially vulnerable to these types of exploitation. Dugongs are also threatened by storms, parasites, and their natural predators, sharks, killer whales, and crocodiles.

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