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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.

Dugongs in Shark Bay

One major exception to the declining population trend is Shark Bay, where the dugong population is estimated at between 10,000 and 15,000 individuals. Unfortunately for dugongs, however, it is their very presence that inadvertently gives the bay its name: tiger sharks are attracted to the area in large numbers, lured by the prospect of dining on dugong. This concentration of apex, or top, predators, in turn, affects other bay residents, such as dolphins, sea turtles, and sea snakes—all of whom can quickly become part of a hungry tiger shark's diet.

The correlation between dugongs and tiger sharks was not discovered until the mid-1990s, when a study was initiated to evaluate the predator-prey interaction between dolphins and tiger sharks in Shark Bay. The study, conducted by then-Ph.D. student Michael Heithaus, highlighted a curious pattern: the sharks tended to prefer dugongs to dolphins when shopping for a meal.

According to Lawrence Dill, head of the Behavioral Ecology Research Group at Simon Fraser University and Heithaus's advisor, a hypothesis emerged, linking the observed migration of tiger sharks into and out of Shark Bay with the movements of dugongs on their seasonal searches for warmer waters.

A new study was necessary to focus on the importance of the dugong-tiger shark relationship, and its subsequent effects on the seagrass community as a whole. The challenge fell to Aaron Wirsing, one of Dill's current Ph.D. students, to learn more about this relationship and to collect additional data on the shy and reclusive dugong's habits.

Wirsing most recently determined through use of Crittercam that an abundance of seagrass exists in some of the deeper waters of Shark Bay where dugongs are found. He hypothesizes that dugongs may spend more time than previously thought grazing on seagrass in these areas because the added depth allows for increased maneuverability in escaping from prowling tiger sharks.

In the future, Wirsing hopes to use Crittercam to study the specifics of how dugong foraging habits affect the composition of the Shark Bay seagrass meadows. The dugongs, however, may have something else in mind.

"They were much savvier customers than we thought," says Wirsing. "They'd tolerate the presence of the boat. You could certainly get close enough to deploy Crittercam [by pole]. But they were almost playing games with us, in that as they would come to the surface, giving us a target, they would sort of casually move just out of our range."

"So it would seem to me that they were quite well aware of what was going on and were sort of having their way with our operation … so we need to learn better techniques, or develop a longer pole, or something."

Perhaps dugongs have more in common with their siren namesake after all.
 

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