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
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
"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,"
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.
Labels: bioacoustics, cetaceans, endangered species, environment