It is all about SOUND science: Manatees, masking and boats

Abstract

A series of controlled underwater psychoacoustic tests with captive manatees, along with comprehensive acoustic surveys of manatee habitats and boat noise propagation measurements, were conducted to investigate manatee hearing and acoustical factors that render Florida manatees vulnerable to repetitive boat collisions. Masked thresholds, critical ratios, and directional hearing of various sounds, including species-specific calls and recorded boat noise were measured against white noise and natural ambient backgrounds. Significantly high acoustic levels were required for manatees to detect approaching vessels. Slow moving boats generate relatively low power and frequency spectra compared to the louder higher frequency cavitation noise produced by faster boats. Counterintuitive to convention, the sounds of slower moving boats are more difficult for manatees to detect and they are often masked by the noise of snapping shrimp and distant faster moving boats. Furthermore, physical boundary effects inherent in shallow water attenuate and limit the propagation of dominant low-frequency sounds produced by many recreational and commercial vessels. While not adapted for hearing or localizing these lower frequencies, manatees can hear and localize an array of narrow band higher FM sounds that could be incorporated into a low-intensity directional alarm to alert them of approaching boats.