Abstract:

A comprehensive series of underwater psychoacoustic tests was conducted to measure the hearing abilities of West Indian manatees. Pure tones, complex and real world sounds were presented to manatees under controlled acoustical conditions. The results from 30 000 threshold trials measured their audiogram, temporal integration, critical ratios, MMAs and directional hearing. Complementing these investigations, underwater acoustical measurements of manatee habitats and vessel noise propagation were conducted to evaluate acoustical factors that render Florida manatees vulnerable to repeated collisions with vessels. Both low-frequency cutoffs in shallow water and near surface boundary effects limit the propagation of the dominant low-frequency spectra from slow moving boats. Slow speed zones implemented to protect manatees do not address this underlying acoustical challenge. Ironically, the strategy can be counter-productive in turbid waters and can exacerbate the problem, making vessels more difficult or impossible for manatees to detect while increasing transect times and the opportunities for collisions. While manatees are not adapted for hearing the dominant low-frequency spectra from a slow watercraft, they are well equipped to detect and locate higher-frequency modulated sounds. This provides a narrow sensory window through which to alert manatees of approaching vessels. A specially designed alarm to alert manatees is being tested. [Work funded by the U.S. Department of Defence Legacy Resource Management Program, Navy, Florida Inland Navigation District, and Florida Fish and Wildlife Conservation Commission.]