Acoustic Phenomena Explain Why Boats and Animals Collide

Researchers at Florida Atlantic University (FAU) 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, Edmund Gerstein, Ph.D., 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.

“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 instant.”

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 ships.

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