Dugongs Draw Hungry Sharks to Australia Bay

Jennifer Vernon
for National Geographic News
January 23, 2004

This story is one of a series looking at National Geographic Crittercam research. Crittercam is a research instrument worn by wild animals and equipped with a video camera and other information-gathering equipment. Crittercam is used on animals both in the ocean and on land.

To learn more about the Crittercam's field test in Australia, tune in to the Crittercam: Dugongs episode on the National Geographic Channel in the United States on Saturday at 8:30 p.m. ET. Got a high-speed connection? Click here to watch previews of the Crittercam television documentaries on the National Geographic Channel Web site.

Shark Bay by its name alone may not sound like the most welcoming of habitats, but dugongs would beg to differ. Located on the western coast of Australia, Shark Bay contains vast seagrass meadows within its warm, shallow waters—just the right habitat for a myriad of marine animals, including the distinctive "sea cow," or dugong.

Belonging to the order Sirenia, dugongs are believed to have given rise to the mermaid myth. Up close, however, the resemblance may be somewhat of a stretch. Small eyes, a large, bristle-covered snout, and a stout body do not quite bring to mind the legendary beauty described by sea-weary sailors.

But a less than mythic appearance is balanced by the graceful way in which dugongs navigate their aquatic environment. While their stamina is not stellar, dugongs demonstrate great agility and surprising bursts of speed over short distances—up to 20 kilometers per hour—when fleeing or evading a predator.

The most startling fact about dugongs may well have to do with its ancestry. The dugong and its slightly larger cousin, the manatee, are both most closely related to the elephant. Perhaps in a nod to their ancient relations, male dugongs (and some very old females) sport tusks, and the fleshy upper lip used by both dugongs and manatees to grasp vegetation is vaguely reminiscent of the end of an elephant's trunk.

Population Decline

Dugongs can be found along coastlines throughout the Pacific and Indian Oceans, although their extensive range belies the fact that their numbers are dropping in most of these areas. Specifically, the International Union for Conservation of Nature and Natural Resources (IUCN) has classified dugongs as "vulnerable," meaning that a 20 percent population loss over either a ten-year period or three generations is suspected.

The biggest cause of this attrition is loss or degradation of their seagrass habitat. Other mortality risks include being hunted, a practice still prevalent throughout much of southeastern Asia, entanglement in fishing nets, and injury from boat propellers. Given that dugongs have a potentially long life span (up to 70 years) but a slow breeding rate, it only takes a few deaths to detrimentally affect a given population.

Because the herbivorous dugong depends primarily on a diet of seagrass—which is itself especially vulnerable to environmental factors—the species dugongs choose to eat are of particular importance in developing conservation measures to protect vital dugong habitats.

Conversely, seagrass forms the very foundation upon which an entire ecosystem is built. Being voracious but selective grazers, dugongs leave their mark on seagrass abundance and variety. Which types of seagrass are eaten, and how much is eaten, affects other creatures dependent on seagrass for food or shelter. So variations in a dugong population, due either to predation or human factors, directly affect the health of an entire seagrass ecosystem.

It is this critical interdependency that makes the dugong such a vital species, one whose own health is inextricably linked to the health of an entire marine community.

Continued on Next Page >>




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