Dugongs
Written by:
Bronwyn
Allan
Species –
Dugong dugon
Synonyms – Sea cow
Throughout history they have given rise to legends. They inspired the
tales of mermaids, sea creatures that were half-human and half-fish, and
sweet-singing sirens, who lured sailors to their doom. Their skin was
supposedly used to wrap the Ark of the Covenant. They are dugongs,
sea-dwelling mammals that inhabit coastal waters between 26 and 27 degrees
both north and south of the equator.
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Dugongs (Dugong dugon) belong to the Order Sirena, so-named
after the sirens whose myth they allegedly spawned. Within the Order they
are one of only two species of the Family Dugongidae. The other, Steller’s
sea cow, was hunted to extinction less than three decades after it was
discovered.
Today, dugongs can also be known as sea cows, probably due to both
their size (they can grow up to three metres in length and weigh up to 400
kilograms) and their diet of seagrass. In fact, dugongs are the only
herbivorous mammals that live exclusively in a marine environment; they
feed predominately on seagrasses, especially those of the genera
Halophila and Halodule.
Habitat
As a result of this diet, dugongs are mainly found in areas which are
conducive to the growth of these seagrasses; namely, wide, shallow bays
and mangrove channels, and in the sheltered waters of large inshore
islands. They can also occur further out to sea where the continental
shelf is wide, shallow and protected.
Life History
Dugongs begin feeding on seagrass not long after they are born;
however, they continue to suckle milk until they are between 14 and 18
months old. Dugongs are not sexually mature before the age of six and may
in fact not reach sexual maturity for up to 10 or so years after this age.
The gestation period of a dugong is around 13 – 15 months and the females
usually give birth to a single calf. The time period between calving
stretches for years.
Status
According to the World Conservation Union, globally, dugongs are listed
as vulnerable. In other words, they "face a high risk of extinction in the
wild in the medium-term future". According to the American Humane Society,
dugongs currently inhabit the tropical and sub-tropical waters of 43
countries stretching from East Africa and the Persian Gulf across to
Australia.
Australian Status
Australia, with around 85 000 dugongs, has the largest share of the
global population. However the dugong is still under threat in Australia;
according to the Great Barrier Reef Marine Park Authority, dugong numbers
have decreased by more than 90% along the urban coast of Queensland over
the last 30 years. In the eight years between surveys undertaken in the
Great Barrier Reef Marine Park in 1986 and 1994, dugong numbers halved.
Cause of decline
The cause of these declines, both global and more locally in Australia,
is probably largely due to human activities. Humans can impact directly
upon dugongs when the animals get tangled in fishing nets and get hit or
are driven from their feeding grounds by boats. Changes to their
environment that are caused by humans also affect dugongs: the seagrass
beds on which they rely for food are very sensitive to human interference.
Processes, such as dredging, land clearing, land reclamation and trawling,
stir up the sediment in the water, which decreases the amount of light
that seagrass beds receive.
Protection
The Great Barrier Reef Marine Park Authority introduced Dugong
Protection Areas (DPAs) in 1997. Netting is restricted to some degree, or
banned entirely, within these areas. They are chosen due to some
combination of their high abundance of dugongs, presence of seagrass beds,
size and other factors such as the ease of gene flow between areas.
Protection of dugongs is very important as their long lives and low
reproduction rate mean that even a small drop in adult numbers can affect
the population. In fact, even under ideal conditions, simulations have
predicted that dugong numbers will not rise by more than about 5% per
year.
References
- Schaffelke, B.; Waterhouse, J & Christie, C. (2002) A Review of
Water Quality Issues Influencing the Habitat Quality in Dugong Protection
Areas. Research Publication No. 66. Townsville, Great Barrier Reef
Marine Park Authority
- Oliver, J & Berkelmans, R. (1999) A Dugong Research Strategy for the
Great Barrier Reef World Heritage Area and Hervey Bay Research
Publication No. 58. Townsville, Great Barrier Reef Marine Park Authority
- Marsh, H. (2002) Dugong: Status Report and Actions Plans for
Countries and Territories. Cambridge, United Nations Publications.
- Great Barrier Reef Marine Park Authority.
Facts About Dugongs
- The IUCN Species Survival Commission.
IUCN Red List of
Threatened Species
- Vine, P. Mermaids: Fact or Fiction? Arabian Wildlife
[Internet] Volume 1(2). Available from:
Arabian Wildlife [Accessed
August 13th, 2004]
