Results of Dugong Research

Dugong belong to the Order Sirenia, which dates back 50 million years and share a common ancestor with the elephant. The Order was named Sirenia from the Greek myth of the Sirens, those mermaids whose singing lured ancient mariners to their doom on dangerous rocks.

There are currently four living type of sea cow, three are Manatee species (Trichechiadae) found in the Atlantic ocean and one Dugong species (Dugongdiaade) found in Australian tropical and sub-tropical waters.

The Dugong's closest relative, Steller's sea cow, weighed more than 6000 kg and was over 8 metres long. This huge, slow moving animal grazed the kelp beds of the Northern Pacific for 2 million years. This large sea cow must have been widely hunted. When a remnant population was found by Mr Steller off Siberia in 1741, it was hunted to extinction in 2 decades. The first marine mammal to become extinct in modern history.

Manatee are slightly larger and more rotund than their cousins, the Dugong. Manatee have paddle shaped tails and nails on their flippers, they spend more time in rivers and some never go to sea. Manatee populations are also threatened, but their rate of increase is greater, as they breed more easily and frequently than dugong.

The Dugong live to about 70 years or more. Their age can be calculated by measuring the growth layers in their tusks. In males the tusks erupt after puberty and a small proportion of older females. Females don't have young till they are at least ten to seventeen years old, having a pregnancy lasting a year they have only have one calf, which will suckle for eighteen months or more. The calves are never far from their mothers and often ride on their backs. The Dugong will only produce once every three to five years and therefore the potential rate of population growth is slow.

Interestingly, research in the Great Barrier Reef Marine Park shows Dugongs have a good memory of place, as satellite tracking shows them returning hundreds of kilometres to specific spots. Genetic research in the same area shows Dugong populations around the Australian coast appear to have been influenced by an ancient land bridge that once connected Papua New Guinea with Australia. It was found there was no one large inter-breeding population, but rather some indications that Dugongs tended to breed within a region. That is, Dugongs bred with others in a neighbouring Bay rather than those from some distance away.

Research has also shown Dugong predominantly feed on small delicate sea grass, such as Halophila and Holodule, which are the lowest in fibre but high in nitrogen and digestibility. Halodule uninervis, Halophila ovalis (Dugong grass) and Halophila spinulosa (Spiny Dugong grass) are found in Moreton Bay. While Dugongs primarily eat sea grass, in Moreton Bay, which is the southern most extent of the distribution of the Dugong, they are known to eat ascidians (Sea squirts), such as the species Sycozoa pulchra. This maybe the consequence of lack of available preferred food.

Boats Pose a Problem

One major negative impact upon dugongs is boats, through noise and boat strikes (GBRMPA, 1997). Research on manatees, a relative of the dugong (order Sirenia), show their most acute hearing was at about 17 kilohertz, with a threshold intensity of 50 decibels. In contrast, at 0.5 kilohertz the threshold was about 50 decibels higher, meaning that a sound had to be 100,000 times as intense before the manatee could hear it. The frequency of a boat motor is about two kilohertz, which is at the poor end of the manatee's hearing range. Boats slowing down to avoid manatees produce even lower frequency sounds and increase the difficulty for manatees. Near the surface of the water, due to the phenomenon known as Lloyd's mirror effect, low tones also propagate poorly, because sound waves bounce of each other (Newscientist, July 1999).

Scientific Report on Dugong Stranding and Mortality in Queensland

The Queensland Marine Wildlife Stranding and Mortality Database summarises all records of sick, injured or dead marine wildlife reported to the Queensland Parks and Wildlife (QPWS).

There were seventy two records of stranded or dead dugong for all of Queensland for the calendar 1999 year.

This number was approximately double the number reported in each of the previous three calendar years.

The greatest concentration of stranding occurred in the Hervey Bay-Sandy Strait Dugong Protection Area and adjacent areas. The February 1999 Mary River flooding is suggested as the cause of many of these. This flooding event caused a large local die-off of seagrass.

There is expected to be a continued elevated mortality of dugong in areas where seagrass pastures have been depleted by flooding and other coastal processes.

Of the 72 records, 37 dugong mortality and stranding reports were from within dugong Protection Areas and one was from within the Moreton Bay Marine Park. Many of the carcasses within the dugong Protection Areas were so badly decomposed it made identification of the cause difficult. However disease was implicated in four deaths.

Two dugongs drowned in Department of Primary Industries Queensland Shark Control Program nets: one at Magnetic Island and one on the Sunshine Coast.

There were nine reported dugong deaths in the Yarrabah Community fishery and three dugong were reported as taken during non-permitted hunting activities in the Great Barrier Reef Marine Park area.

There was one dugong death reported for Moreton Bay in 1999. However, we have already been advised of at least two dugong deaths in the last 12 months.