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Manatees, Bioacoustics and Boats

Hearing tests, environmental measurements and acoustic phenomena may together explain why boats and animals collide

Edmund Gerstein

It's 2 o'clock in the morning, and, wouldn't you know it, Stormy is "in love" with that big Navy transducer again. Now I have to get in the cold water and pry him off so we can set up for Dundee's session. Oh, the joys of working with manatees under the Tampa moonlight! Even though I'll be tired, cold and wet, before sunrise we will have measured another critical aspect of the Florida manatee's hearing abilities. Over the next seven years of extended late-night auditory testing—more than 30,000 threshold trials in all—my wife Laura and I will measure two teenage manatees' ability to hear, locate and discriminate different underwater signals under various controlled acoustical conditions. In the end, we will have laid the groundwork for a sensory explanation for why manatees are hit repeatedly by boats.

Figure 1. Boats and manatees . . .Click to Enlarge Image

The endangered Florida manatee, a subspecies of the West Indian manatee (Trichechus manatus), is a gentle, bewhiskered herbivore that can reach 4 meters in length, weigh up to 1,300 kilograms and live over 60 years. Designated as Florida's official marine mammal, the manatee has been the focus of more controversy and polarization over conservation and protection than perhaps any other mammal. "Sea cows," as they are sometimes affectionately called, inhabit shallow coastal, estuarine and riverine habitats throughout peninsular Florida, where they graze on sea grasses and are routinely injured and sometimes killed by collisions with recreational boats, barges and commercial ships. These collisions are so prevalent that the majority of wild manatees are identified by their characteristic boat scars.

After more than two decades of manatee-protection policies that have focused on slowing boats passing through manatee habitats, the number of injuries and deaths associated with collisions has increased and reached record highs in the past two years. To help track the population, Florida and federal wildlife agencies maintain a growing scar catalogue of recognized living individuals who have survived collisions. Some of these manatees have propeller wounds from as many as 16 different boat strikes. Why does this happen?

When startled or frightened, manatees explode with a burst of power and can reach swimming speeds of up to 6.4 meters per second in an instant. My colleagues and I wondered: Given that manatees have the cognitive ability to recognize danger, a fear-flight reaction and the physical prowess to evade boats, why, after an individual has been hit once, twice or three times, doesn't it learn to avoid boats? Is it possible that manatees are unaware of the danger? Can they hear boats approaching, and if so, from how far away, from which direction and under what acoustic conditions?

Figure 2. Behavioral audiogram . . .Click to Enlarge Image

These basic questions suggested a number of interdisciplinary behavioral and acoustic investigations that I conducted over the past decade with Joseph E. Blue, retired director of the Naval Undersea Warfare Center and the Naval Research Laboratory's Underwater Sound Reference Detachment and now president of Leviathan Legacy, Inc.; Steven E. Forsythe of the Naval Undersea Warfare Center; and Laura. No one had previously conducted rigorous, controlled underwater psychoacoustic (audiometric) studies, which are necessary to understand what sounds manatees can hear in their environment. In conjunction with audiometric studies, we conducted a comprehensive series of underwater acoustic surveys of various wild manatee habitats, along with critical boat-noise propagation measurements, to further understand why animals are so vulnerable to collisions. Defining and applying the physics of near-surface acoustic propagation are also necessary if collisions between boats and animals are to be reduced, not only in Florida's aquatic byways but also on the open seas, where great whales are regularly injured and often killed by large ships.

Our test results contradict several long-held beliefs that form the basis of current protection strategies. Manatees have good hearing abilities at high frequencies, however, they have relatively poor sensitivity in the low frequency ranges associated with boat noise. Ironically, manatees may be least able to hear the propellers of boats that have slowed down in compliance with boat speed regulations intended to reduce collisions. Such noise often fails to rise above the noisy background in manatee habitats until the boat is literally on top of the manatee. In addition, near-surface boundary effects can cancel or severely attenuate the dominant low-frequency sound produced by propellers. In many situations, ship noise is not projected in directional paths where hearing these sounds could help the animals avoid collisions. Our basic and applied research results suggest that there may be a technological solution to address the underlying root causes of the collision problem and resolve the clash between human and animal interests.

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