Of manatees and men: A future: Thanks to you, Joe

A comprehensive series of underwater psychoacoustic tests was conducted to measure the hearing abilities of the West Indian manatee under varying acoustic conditions. Pure tones, complex noise and real sounds, boat noise and species-specific calls, were presented to captive manatees under various acoustical conditions. The results from more than 30 000 threshold trials define the first behavioral audiograms, directional hearing, and critical masking ratios for any sirenian species. Psychoacoustic tests demonstrate the noise produced by slow-moving boats is more difficult for manatees to hear and locate than the high-energy cavitation noise from faster moving vessels. The dominant frequency spectra of slow-moving boats fall near the fringe of their low-frequency hearing limits and below critical masking ratios. Complementing underwater ambient noise and vessel noise measurements revealed physical propagation factors that further render manatees vulnerable to watercraft collisions in their shallow-water habitats. Ironically, slow speed zones implemented to protect manatees may exacerbate the problem, making vessels difficult to hear, while increasing transect times and opportunities for collisions. By exploiting the manatee’s best hearing capabilities, it may be possible to mitigate collision risks with a low-intensity, environmentally friendly acoustic warning device that alerts manatees of approaching vessels. [Research supported by DOD Navy Legacy Program, Inland Navigation District, FFWCC]