Florida Manatees: Silent Danger

The endangered Florida manatee, Florida’s state marine mammal, is a large aquatic relative of the elephant. Manatees can be found in the warm waters of shallow rivers, bays, estuaries and coastal waters. Rarely do individuals venture into waters that are below 68 degrees Fahrenheit. Well known for their gentle, slow-moving nature, manatees have also been known to body surf or barrel roll when playing.

The current main threat to manatees in the United States is being struck with boats or slashed with propellers. Manatees have enjoyed nearly three decades of protection policies that have focused on requiring boats to slow their speed in known manatee habitats (“no wake” zones). Despite this protection, however, manatee deaths from watercraft collisions have continued to rise, peaking at 96 in 2006.

Although manatees are normally very slow swimmers, they are capable of brief bursts of power when frightened. This fact has caused researchers to question why manatees cannot learn to avoid boats, even after repeated injuries. This led to their hypothesis that manatees may be unaware of impending danger because they are unable to effectively hear approaching boats.

Dr. Edmund Gerstein, PhD is one of the leading experts on manatee behavior and is director of marine mammal research at the Charles E. Schmidt College of Science at Florida Atlantic University. His research suggests that the “no wake” zones designed to protect the manatee may be contributing to the increased number of propeller strikes because the animals cannot hear the propellers at idle speeds.

Peak hearing sensitivity for manatees lies between 16-18 kHz. Below 16 kHz, sensitivity decreases approximately 10 dB per octave. Below 2 kHz, hearing sensitivity drops precipitously and the low-frequency cut-off is approximately 400 Hz. The idle speed on boats is often around 600 Hz. These hearing thresholds were tested in quiet, captive pools but the manatee’s natural habitat is noisier and therefore manatees’ hearing thresholds are even higher in the wild to hear over the masking noise. Also, many of the lower frequency sounds do not propagate far enough to reach the manatees’ ears. Typical ambient sound levels of only 70-90 dB SPL can significantly interfere or mask the sound of boats.

Researchers are working on a sound-emitting device that can be placed on the front of a boat. The device incorporates a parametric design to produce multiple frequencies. The different frequencies produced fall within the animal’s best range of hearing. The device incorporates higher frequency elements of boat motor noise as a “carrier” signal to provide a sound that “ramps” up or down as the boat changes speed, which manatees can recognize and detect.

In field tests, the manatees are avoiding the alarm without any prior experience. If an animal cannot detect the danger they cannot learn to avoid it. Since speed laws have been enacted, there has been an increase in manatees with multiple boat scars, and the deaths from boat collisions have increased to record highs. In controlled tests, manatees have successfully avoided boat approaches with prototypes of the warning system. Without the warning device, it was necessary to veer away from the animals 97 percent of the time when approached with the same boat at the same speed.

From: Audiology Today Mar/Apr 2009

by Carol Hawkins