Dugongs Draw Hungry Sharks to Australia Bay

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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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