Manatees, Bioacoustics and Boats
Hearing tests, environmental measurements and acoustic phenomena may together explain why boats and animals collide
Sound Near the Water Surface
Manatees are not the only animals that collide with boats. Other passive-listening marine mammals, including great whales, are vulnerable to collisions when near the surface or in shallow water. Here, the physics of near-surface sound propagation significantly affects their ability to detect low-frequency sounds.
A phenomenon known as the Lloyd mirror effect can attenuate or cancel the propagation of lower-frequency sounds generated near the surface. The Lloyd mirror effect does its damage at the surface, where the risk of collisions with ships and boats is greatest. At the surface, sound reflections can be 180 degrees out of phase with incident waves and can cancel the low-frequency sounds of boats and ships. The sound pressure approximates 0, as the water's surface is a pressure-release boundary that is free to move in response to pressure in the water. The increase of pressure away from the water's surface is proportional to frequency, with pressure at shallow depths being inversely proportional to wavelength and thus proportional to frequency (the lower the frequency, the lower the acoustic pressure near the surface).
The details of these fluctuations at short distances depend on many factors, the most important of which are water depth, bottom shape and density, and surface roughness. Even if manatees or whales could ordinarily hear such sounds, the Lloyd mirror effect can attenuate them to levels that are indistinguishable from the ambient noise. Although some whales, unlike manatees, may have acute low-frequency hearing, it is no advantage at the surface. Animals cannot react to sounds that never reach them, regardless of their auditory abilities.
In concert with the Lloyd mirror effect, another acoustic phenomenon may be the cause of many ship and barge strikes on marine mammals. Acoustical shadowing is caused when the sound rays from the propellers of a ship are blocked by the ship's hull from projecting forward.
Acoustical shadowing is particularly a problem when propellers are located above the keel depth of ships. Most large ships that strike whales, as well as tugboats that kill manatees, have this propeller configuration. The propellers of a traditional tug are recessed to reduce surface venting from propeller cavitation, to drive the tug in line with its center of mass and to protect the propeller from damage in case the keel strikes the bottom. With the propeller in this position, a sound ray reflected from a shallow bottom will again be reflected by the tug's structure before the sound can propagate forward. This causes both an acoustical shadow ahead of the tug or tug-and-barge combination and severe attenuation from multiple reflections. The attenuation loss alone is 60 to 100 decibels; thus propeller noise ahead of the tug-and-barge combination is completely masked by the ambient noise at the surface.
An acoustical shadow is cast all around a ship whenever the width of the ship is greater than the wavelength of the sound. A hull 10 meters wide will cast a shadow at frequencies higher than 150 hertz. The shadow's extent depends on the number of wavelengths across the ship. Little diffraction around this barrier will occur. Of course, extremely low-frequency (long-wavelength) sound can diffract around most hulls, but at these frequencies the Lloyd mirror effect loss is severe. The two effects together have significant ecological consequences.
We used a vertical array of hydrophones to document these combined effects from tugs and ships. Data obtained for tugs with barges show a more pronounced "quiet zone" subtending a large angle from the tug's propellers. The same shadowing effect is present with large ships that hit whales. A recent study by David Laist and his colleagues indicates that ships hitting whales tend to be 80 meters or more in length. In the shallow coastal sea lanes where whales are most frequently killed, the depth of the water is shallower than the length of these vessels—resulting in even greater shadowing effects. The same relative conditions are found in shallow manatee habitats, where the vessels are not as large but the water is much shallower.
The sounds generated by an 18.3-meter tug pushing a 76.2-meter barge are significantly shadowed. A sample of these measurements helps to illustrate how the sounds of the tug remained undetectable until 45.7 meters of the barge had passed the hydrophone array (Figure 10, graph). A manatee or whale in the direct path of the barge would not have been able to acoustically detect the barge before the animal had been run over by it or become entrapped by the hydrodynamic force .
Large ships and barges differ in the way they reflect sound. A barge in shallow water may reflect sound between the sea bottom and the flat bottom of the boat several times. A ship, with its V- or U-shaped hull, will reflect sound off to the side rather than straight ahead, creating a more pronounced shadow zone with relatively loud noise radiating off to the sides. Such noise may confuse animals and even cause them to swim into the quiet zone to seek refuge—placing them directly in the path of the approaching vessel.