Manatees, Bioacoustics and Boats
Hearing tests, environmental measurements and acoustic phenomena may together explain why boats and animals collide
The Sounds Boats Make
Boat noise is different in character from biological noise. Underwater it has two domains, or operating conditions: noncavitating and cavitating noise, the latter arising from turbulence caused by the propeller rotations. The frequency and power of boat noise is directly related to the speed of the vessel. The faster the propeller rotation, the more cavitation is created. As tiny bubbles form and collapse, they produce a broad range of frequencies above prevailing ambient conditions at frequencies up to 20,000 hertz. Conversely, when the rotation of the propeller is reduced and a boat is traveling slowly, the turbulence is minimal, and both the frequency and power spectrum of the noise are significantly reduced. The dominant noise spectra are below 1,000 hertz at sound-pressure levels that barely reach the manatees' audiogram thresholds.
Under typical ambient conditions, the sounds of an approaching small boat can be indistinguishable from the background until they are loud enough to cross the masked thresholds. Since the intensity for a given sound source decreases with increased distance, a slow-moving boat with propellers turning at 400 rotations per minute needs to be virtually on top of a manatee before the sound can be detected. Unfortunately, propellers turning at 400 rpm can slice up a manatee just as readily as can those of a fast-moving boat going 3,500 rpm.
We recorded the sounds of approaching boats at the Outboard Motor Test Facility in Stuart, Florida. At this location we found physical and ambient conditions typical of manatee habitats: water 5 meters deep, isothermal conditions and ambient biological noise coming primarily from snapping shrimp. We suspended recording hydrophones 1.5 meters below the surface and sampled sounds from representative boats operating at various speeds
When we played the recordings of these sounds to manatees under controlled masking conditions, the higher-frequency broadband cavitation noise made by fast boats was detectable at 9 decibels above the ambient level—a relatively low critical ratio. Manatees could not detect the noise from idling boats under the ambient-noise conditions recorded in the field. The low-frequency spectra did not breach the audiogram threshold limits, and the remaining higher-frequency sounds were so low that they were masked by moderate ambient conditions of only 70 and 80 decibels. This noise had to be amplified 29 decibels above the ambient noise before it was detectable.
What do these results suggest? For one thing, a boat with a slowly rotating propeller generates low-frequency sounds impossible to locate and indistinguishable from the ambient noise until it is dangerously close to a manatee. A key management strategy used in Florida for protecting manatees over the past 20 years has been to slow boats in waters frequented by manatees by creating idling and slow-speed zones. This strategy can actually exacerbate the problem when it is implemented in turbid water conditions (which, along with tannin staining, are prevalent in Florida). Under such conditions, manatees and boaters cannot actively avoid each other using visual cues, and acoustic signals are the only means of detection available to the animals.
Consider the results from our boat-measurement studies simulating an encounter between an 8.2-meter boat and a manatee (Figure 8). When the boat approaches at high speed, the noise level crosses the manatees' critical ratio approximately 16 seconds before the propellers reach the hydrophone—about 198 meters away from impact. The noise of the same boat approaching slowly remains undetectable and does not cross critical ratios until the propellers are only 0 to 2 seconds away, less than 3.7 meters from impact. Under moderately noisy ambient conditions, the sounds associated with slow-moving boats can become acoustically transparent.
Although slow-moving boats may arguably cause fewer fatalities than do fast-moving boats, they are also more likely to cause repeated injuries to manatees that cannot detect their approach. The manatee scar catalogue of living survivors is consistent with, and bears some testimony to, this observation.