Neither slow nor stupid, manatees are killed by boats because they can't hear them

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Neither slow nor stupid, manatees are killed by boats because they can't hear them
Jeremy Hance, mongabay.com
December 12, 2008

Last year 73 manatees were killed by boats in Florida, despite two decades of manatee-related protections. In fact, manatee deaths due to boat collisions have only increased since protections were implanted. A recent study at the Florida Atlantic University has finally revealed why boats are so dangerous to manatees: the manatee cannot run from what it does not hear.

Biologists had assumed that manatees could hear the boats, but were just too slow to avoid them and too dull to learn to avoid boats after being hit. Dr. Edmund Gerstein, director of marine mammal research at FAU’s Charles E. Schmidt College of Science, was not convinced by this generally accepted argument.

“Manatees have the cognitive prowess to learn and remember as well as dolphins and killer whales,” Gerstein said. “Furthermore, when startled or frightened, manatees explode with a burst of power and can reach swimming speeds of up to 6.4 meters per second in an instant.”

Photo by R.K. Bonde, U.S. Geological Survey, Sirenia Project"

Considering that the manatee possesses an advanced ability to learn and the necessary speed to avoid collisions, Gerstein and colleagues began to investigate other possibilities for the manatee’s vulnerability to boats. Beginning in 1991, Gerstein and his team conducted underwater studies to test what sounds the manatee could hear, and in what situations. Gerstein discovered that manatees cannot hear the low frequency sound produced by boats. Unlike other sea mammals, such as dolphins and whales, manatees do not use sonar. They are entirely dependent on listening to avoid danger. Therefore, one of the major policies put in place to protect manatees—requiring boats to slow down in manatee habitat—may have actually contributed to the rise in manatee mortalities.

"It is ironic that slow speed zones result in quieter and lower frequency sounds which manatees can’t hear or locate in Florida’s murky waters,” Gerstein explains. “Slow speed zones make sense in clear water where the boater and the manatee can see each other and therefore actively avoid encounters. However, in turbid waters where there is no visibility, slow speeds actually exacerbate the risks of collisions by making these boats inaudible to manatees and increasing the time it takes for a boat to now travel through manatee habitats thereby increasing the risk and opportunities for collisions to occur.”

The new findings spurred Gerstein and his team to find a solution. They created an acoustic device specifically made to alert manatees. “The alarm emits a high-frequency signal which isn’t loud, doesn’t scare or harm manatees and doesn’t disturb the marine environment,” said Gerstein.

Testing the device in a NASA wildlife refuge, Gerstein found that when the device was sounded 100 percent of manatees avoided the boat. In trials without the device, only 3 percent of manatees moved out of the boat’s way upon approach.

Gerstein believes the same type of device can be used to save other marine mammals. For example, right whales are known to be injured or killed from being struck by large ships in shallow seas.

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