Thursday, December 11, 2008

ENV: Sonar and Manatees

Acoustic Phenomena Explain Why Boats And Animals Collide

Researchers at Florida Atlantic University have laid the groundwork

for a sensory explanation for why manatees and other animals are hit

repeatedly by boats. Last year, 73 manatees were killed by boats in

Florida's bays and inland waterways. Marine authorities have

responded to deaths from boat collisions by imposing low speed

limits on boats.

In spite of manatee protection policies that have been in effect for

nearly two decades to slow down boats passing through manatee-

protection habitats, the number of injuries and deaths associated

with collisions has increased and reached record highs.

In an effort to reduce manatee deaths and injuries from boats, Dr.

Edmund Gerstein, director of marine mammal research and behavior in

FAU's Charles E. Schmidt College of Science, set out in 1991 to

investigate what might be the underlying cause for these collisions.

Gerstein disagreed with the unsubstantiated assumptions, which

wildlife officials had relied upon, that manatees could hear boats,

but they were just too slow and could not learn to avoid boats.

"Manatees have the cognitive prowess to learn and remember as well

as dolphins and killer whales," said Gerstein. "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


Given that manatees have the cognitive ability to recognize danger

and the physical prowess to evade boats, Gerstein sought to explore

the answers to some simple questions. "After a manatee has been hit

more than once (some have been hit up to 50 different times) why

doesn't the animal learn to get out of the way?" "Is it possible

that manatees are not aware or cannot hear the sounds of an

approaching boat?"

Gerstein and his colleagues conducted rigorous, controlled

underwater psychoacoustic (audiometric) studies to understand what

sounds manatees can hear in their environment. After a comprehensive

series of hearing studies, his research revealed that manatees

cannot hear the dominant low frequency sounds of boats and that

those sounds do not transmit well in shallow water. Furthermore,

ambient noise in manatee habitats can conceivably mask the

perception of many kinds of signals. Unlike dolphins, which can use

active sonar to navigate and detect objects in the environment,

manatees are passive listeners restricted to listening to their

auditory landscape.

"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," said Gerstein. "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."

With these issues in mind, Gerstein and his colleagues developed an

acoustic alerting device specifically tailored to exploit the

manatees' hearing ability. The environmentally friendly device is

narrowly focused in front of the boat so that only manatees in its

direct path are alerted.

"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.

Gerstein has been testing this alarm in a NASA wildlife refuge where

controlled studies are possible. He has reported that 100 percent of

the controlled approaches toward manatees by a boat with the alarm

have resulted in the manatees avoiding the boat up to 30 yards away.

Without this alarm, only three percent of the manatees approached by

the same boat moved to avoid the boat.

Manatees aren't the only animals that collide with boats. Other

passive-listening marine mammals, including great whales, are

vulnerable to collisions when near the surface, where the risk of

collisions with ships and boats is greatest or in shallow water.

Gerstein and his colleagues are using the findings from their

studies to help understand and reduce collisions in the open seas

where great whales are regularly injured and often killed by large


Source: Florida Atlantic University.

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