Manatees, Bioacoustics and Boats
Hearing tests, environmental measurements and acoustic phenomena may together explain why boats and animals collide
In Search of Solutions
Just as speed limits for small boats in inland waters can reduce propeller noise and sound frequency, so reducing ship speeds could conceivably increase the risk of collision by increasing exposure time (and thus opportunities for collisions) while diminishing the ships' audibility. Current surveillance and avoidance programs are ineffective at night or in poor weather, just when the animals also must rely on sound detection to avoid ships. Today wildlife managers are focused on other protection methods that still do not address the underlying sensory and acoustic causes of collisions. These methods include active sonar to detect animals ahead of ships and passive-listening sensors that light up to indicate that manatees are in the area.
Interested in tracking manatees in turbid water conditions, we investigated the use of active sonar to detect manatees and conducted the first and only sonar target strength measurements on manatees (Gerstein and Blue 1997). Using direct and echo-reduction methods, we evaluated echo-ranging and shadowing sonar technologies. Unfortunately, the problems relating to bottom and volume scattering and reflection from the surface in the manatees' shallow-water environments make reliable detection at safe enough distances from boats impractical. Similar surface-reflection, false-alarm and absorption problems are confounding other investigators trying to detect whales (much bigger targets) near the surface in front of ships.
The use of sound-activated light sticks to alert boaters to manatees in their vicinity is also being explored. The proposed system would employ passive listening for manatee vocalizations to trigger warning lights on pilings along waterways frequented by manatees. Unfortunately, manatees are relatively quiet animals. Not only does this characteristic of manatee behavior reduce the effectiveness of a sound-activated device, it could also give boaters a false impression that no manatees are present, placing undetected animals at additional risk. We have an extensive catalogue of calibrated manatee vocalizations, and our analysis of these signals shows that although mothers and calves communicate more regularly, manatee calls are low-intensity signals averaging only 12 decibels over the ambient noise. We have documented that manatees can detect and locate these low-intensity calls below the ambient noise, but underwater acoustic instruments cannot filter these calls below the ambient noise as manatees can. Consequently, animals would need to be very close to a listening station in order to be detected above typical ambient levels. Another aspect that may be unrecognized is that many components of their calls are directional. Only if the hydrophone array is in line with a vocalizing manatee would it receive these impulses.
We decided that the best way to protect animals would be to address the underlying sensory and acoustical causes of collisions. Manatees and whales may be well adapted to hear and detect significant biological sounds in their environments; however, boats, ships and barges were never part of their evolutionary histories. Thus these animals are faced with modern ecological challenges for which they are at a sensory disadvantage. In light of the psychoacoustic measurements described above, the known acoustical characteristics of shallow-water habitats, the spectra of boat noise and the dangerous, deceptive problem of acoustical shadowing, it is apparent that manatees, and perhaps other passive-listening marine mammals, could benefit from an acoustic warning device designed to fit on the front of boats, ships and barges.
With this in mind, we developed an acoustic alerting device specifically designed to exploit the manatees' optimum hearing abilities. Using wavelet-derived acoustic signals that manatees can readily detect and locate at or near ambient levels, we were able to develop an environmentally friendly device that projects very low-intensity and highly directional acoustic signals in front of boats. The signals are designed to defeat the challenges posed by acoustical shadowing and the Lloyd mirror effect. Such highly directional, low-intensity sounds would pose no threat of cumulative noise effects even with thousands of devices operating simultaneously. Compared with 200-watt fish finders and depth sounders, our 10-watt device imposes no noise-pollution concerns. It provides a set of consistent, highly directional acoustic cues which marine mammals, most notably manatees, could quickly learn to associate with boats, ships and barges.
The bow-mounted manatee-alerting device we have been testing in manatee habitats incorporates a through-the-hull-mounted parametric transducer that creates a stable, directional beam of sound just under the surface of the water for distances of up to 200 meters (Gerstein and Blue 1996, 1997). The device incorporates a parametric design to deliver this narrow beam with a small transducer. It projects two ultrasonic source frequencies that are beyond the measured hearing limits of marine animals. The resulting difference or parametric frequency from the two oscillating source frequencies of 230,000 and 250,000 hertz results in a 20,000-hertz, centered parametric wave that is audible to manatees and dolphins but falls below the detection limits of fish and outside the hearing range of turtles and aquatic birds. As it is not designed to scare or harm manatees, the device could provide a consistent set of highly directional cues that manatees might learn to associate with boats. Being highly directional, manatees would only hear the signals when they are in the direct line of an approaching boat and in imminent danger of injury. Ignoring the signals would have negative consequences; thus the manatees would not become habituated to them. Critics have suggested that such an approach is untenable because manatees may need to get injured before they can associate the alerting signals with danger. However, manatees are getting hit repeatedly every day, not because they don't know boats are dangerous, but because they can't locate them at safe distances in time and space. If these devices were placed on slow-moving boats and barges, manatees could soon learn to associate the sounds with approaching vessels without having to suffer injuries repeatedly. Animals cannot learn to avoid boats that they cannot detect or locate.
For all our custodial efforts and regulations to protect manatees, even the most conscientious and best-intentioned boaters can still strike manatees they cannot see. When an animal cannot hear or locate a boat, it is at risk whether the boat is going fast or slow. In the end, the most reliable, motivated and responsive individual that can save any manatee at any place and time is the manatee itself—provided it has the sensory awareness to do so. An acoustic alerting device could give animals the opportunity to save themselves.
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