Dugongs

Dugong / Dugong dugon / Family - Dugongidae

Dugong / Dugong dugon / Family - Dugongidae

Dugongs, like whales and dolphins spend their lives at sea. But unlike whales and dolphins, dugongs are not predators, they are strictly herbivorous marine mammals.

Australian dugongs range from Shark Bay in Western Australia around the north to Moreton Bay in Queensland. It is believed there are 80,000 dugongs in Australian waters, approximately 14,000 of them on the Great Barrier Reef.

Dugongs have similar adaptations to whales and dolphins. They look rather like rotund grey-brown dolphins with a fish-like shape, flippers and tails and grow up to three metres long and weigh up to 400kg. Their skin is thick and smooth, with the nostrils near the front of the head enabling them to breath with most of the body beneath the surface. Unlike other mammals, dugongs cannot hold their breath under water for very long.

Dugongs have no dorsal fin. Their mouths are large, and the upper lip is covered in bristles which are used to find and grasp seagrass. Dugongs' ears and eyes are found on the side of the head, and their movements slow and graceful.

Reproduction

Dugongs live for approximately 70 years. Female dugongs first breed between the ages of six and 17 years old. They produce calves every 2.5-5 years.

Breeding commences when the female is in oestrous (on heat). Groups of male dugongs will follow the female around and will mate only with her. The mating process is quite violent, and inflicts scars to the females back and to the other males.

The female will produce a single calf  after a 14-month pregnancy. Most calves are born between September and October, and remain with, and suckle, their mother for around 18 months.

Feeding

Dugongs feed mainly on seagrass, but can also supplement their vegetarian diet with invertebrate animals.

Dugongs along the Great Barrier Reef feed mostly on small, delicate seagrasses, which are low in fibre, high in nitrogen and easily digestible. A dugong can dig up and entire seagrass plant including the roots.

Dugongs do not have a chambered stomach, but rely on an extremely long intestine and bacteria living within it to digest the cellulose of seagrass.

Threats

Dugongs have played an important part in traditions and cultures of Aboriginal and Torres Strait Island people for thousands of years, and are still harvested under controlled conditions today.

The greatest threat to dugongs is the loss of their habitat. As a result of increased siltation and nutrients from human activities such as dumping of dredge wastes, urbanisation, industry and agriculture, seagrass beds are diminishing.

Nets are also a threat to dugongs who can become entangled and drown in certain types of fishing mesh nets. In some locations fishing nets are restricted in order to establish dugong protection areas.

Shark nets erected to protect beaches have also been responsible for drowning dugongs in Queensland. Many of these nets have been replaced with drum lines.