Boat Device Alerts Manatees
of Approaching Craft


A Florida Atlantic University researcher, Edmund Gerstein, studying manantees observed they have difficulty in hearing low frequency sounds of the nature of those generated by boats. He, his wife and some associates have developed a small, hull mounted, directional sonar alarm broadcasting high frequency tones ranging from 3 kHhz to 20 kHz to alert manatees of oncoming boats. In addition, they are studying pulsing the signal (beep - beep) 2 to 25 times per second in relation to boat speed, hoping manatees will learn to associate pulse rate with boat speed. Further development is currently stalled due to the U.S. Fish and Wildlife Service Office of Management Authority denying their permit for open water testing.

The Research

Edmund Gerstein set out to gain a better understanding of the underwater audiogram) hearing capabilities of the West Indian manatee (the Florida manatee is a subspecies). His test subjects were two adult males (Stormy and Dundee) at the Lowry Park Zoo in Tampa Florida.

He and his wife, Laura, of Florida Atlantic University, Steven Forsythe of the Navy Undersea Warfare Center Newport Division and Dr. Joseph Blue a retired longtime Navy researcher specializing in underwater sounds, now with the Leviathan Legacy, Inc. (a company developing the manatee alert technology) published the paper, "The Underwater audigram of the West Indian Manatee" in the June 1999 issue of the Journal of the Acoustic Society of America. This article brought their work to international attention. The description of research below comes from it.

Stormy and Dundee were trained individually in the evening and early morning hours in a large test tank to avoid daytime noises. The tank was equipped with 5 underwater viewing panels allowing excellent visual observations and recordings to be made. Tests were conducted approximately 3 meters of water.

The test apparatus had 5 components:

Note: this is the simplified layman's description. See the actual reference for a more detailed description.
  1. A water filled PVC stationing hoop in which the manatee placed his head toward the transducer.
  2. A hydrophone to measure the actual sounds and background noise inside the hoop.
  3. A sound generating transducer
  4. A structural platform above the waters surface used to suspend the sound transducer and paddles into the water.
  5. Two PVC response paddles for the manatees to push to indicate the presence or absence of sound.

The Training Procedure

Positive reinforcement was used to train the manatees to discriminate between the tone and no-tone paddles (heard the sound or did not hear the sound). They used light and dark colors on the paddles and surfaces with different "feels" to help the manatees more rapidly distinguish the paddles. It took six months for the manatees to learn to discriminate the paddles and perform the threshold tests.

The chain of behaviors required the manatees to
  1. Swim to the experimenter at the surface of the water.
  2. Wait for a hand signal to start the trial
  3. Turn dive and swim to the stationing hoop 6 meters away and 1.5 meters below the surface.
  4. Place their head in the stationing hoop facing the transducer
  5. Listen and wait for the strobe light
  6. Back out of the stationing hoop and select one of the two paddles (tone or no-tone)
  7. Swim back to the researcher for reward and/or start of next trial
Each trial required about 2 minutes, not including the time spent rewarding the subjects. Sessions consisted of 30-80 trials over 1 to 2 hours. The animals also had to be physically conditioned to endure the long working sessions.

Basically, the researchers provided an underwater sound at a specific frequency, measured the sound pressure level at the hoop, then the manatee was to indicate if he heard it or not. Once the manatee reported and was rewarded, they tested another data point.

A system of warmup and cool downs trials were conducted at levels about 15 db above their anticipated threshold for each frequency (easy for them to hear). If they did not perform 80% or better on the warmup and/or cool down, the data from that session was dropped.

Using the warmup and cool down screening of the data above, they still collected 7962 good data points. Both subjects averaged over 90 percent in the warmup and cool down tests.


Manatees once thought to be a large slow moving mammal were observed making very rapid burst speed movements. They appear to be very capable of avoiding oncoming boats. The research indicated the manatees hear best in the range from 3 kHz to 20 kHz, slightly above the low frequencies associated with oncoming boats. Combining these two observations led to speculation manatees do not use their burst speed maneuvering capability to avoid oncoming boats because they do not hear them.

Additional heard problems occur at the near the surface. Background noise is intensified and a phenomenon called the Lloyd Mirror Effect especially attenuates low frequency sounds near the surface.

Manatees once thought to be dumb, were found to be very teachable. This finding lends some credence to them being able to associate the pulse frequency of an oncoming boat with its speed.

The manatee alert device actually generates two very high pitched tones and the difference between the two becomes the proper range of tones in the water. One of the tones is varied in a manner making the resulting frequency move across the range best heard by manatees. They refer to it as a nonlinear parametric sonar project system or sometimes as a parametric transducer. It is approximately 4 inches in diameter.

Project History

Paraphrased from 17 June e-mail from Mr. Joe Blue
Edmund Gerstein started the manatee hearing project about 1991 with a colleague. Joe Blue soon began assisting with some of the technical equipment issues and Steve Forsythe began helping with signal processing and computer data logging. The work was done at the Lowry Park Zoo manatee exhibit pool in Tampa Florida. When Edmund's original colleague left the project, his wife Laura joined the team.

Measurements consisted of manatee audiograms, directional hearing capability, critical bandwidth and masked threshold measurements using several kinds of signals to the manatee.

As the experiment progressed, they learned manatees can be trained and are capable of learning complex associations. They also can move quite rapidly for a large marine mammal when startled.

They also learned manatees hear at a much higher frequency than earlier thought, their most sensitive hearing range being in the 12 kHz to 20 kHzs.

About 1993, Ed and Joe devised a thorough test program that included acoustic measurements of manatee habitats and motor boat noise propagation to go along with manatee audiometric measurements. They also began to generate a patent based on their findings which was filed in 1994. Another patent was later filed in conjunction with a well known bioacoustician, Dr. Whitlow Au.

Acoustic measurements were made on a prototype device indicating nonlinear or parametric devices could achieve the directionality and sound pressure levels needed to alert manatees to approaching boats. Those not in the path would not hear the sound.

When they applied for a permit to document what manatees hear when boats approach with and without the device and to test the effectiveness of the manatee alerting device, it was denied by the US Fish and Wildlife Service Office of Management Authority.

Objections raised to the permit centered on possible manatee strikes during the testing and concern for them being scared of the alert signal.

RBBI Permit Comments - it does seem a bit odd they denied the permit to field test the device based on possible manatee strikes during testing when in 1998 they approved Permit #PRT-843809 to "conduct controlled approaches to manatees using a variety of vessels and operating conditions as part of a study of responses of manatees to vessels." from the Mote Marine Laboratory in Sarasota FL.

The 1999 manatee alert application read, "to study the reactions of West Indian manatees to and possible impacts from controlled boat approaches, underwater acoustic projections of approaching boats, and narrow beam manatee alerting sounds."

Research Sponsors

This research was made possible by the following entities providing personnel, money and/or facilities. As well as by the assistance of many professional colleagues and volunteers.

Reference Materials

Many of these references can be found in major libraries or online from the references themselves (journal and newspaper web sites) or from web based information providers such as Dow Jones, Uncover and Dialog.

Note: Leviathan Legacy, Inc is the commercial arm of the manatee alert project.


  • U. S. Patent No. 5,559,759 issued 24 September 1996, Method of Alerting Marine Mammals and Other Mammals Underwater of the Danger of Approaching Motor Vessels, Inventors: Joseph E. Blue, Edmund R. Gerstein and Laura Gerstein.

  • U. S. Patent No. 5,850,372 issued 15 December 1998, Method of Alerting Sea Cows of the Danger of Approaching Motor Vessels, Inventor: Joseph E. Blue, Assigned to: Leviathan Legacy, Inc., Boca Raton, FL.

    Newspaper Articles

  • Device developed to protect manatees. Boca Raton News. 9 Aug. 1999.
  • Beep, Beep: Danger. Naples Daily News editorial. Date unknown.****************
  • A device to help save the manatee. Tampa Tribune. 6 May 2000. (discusses the permit problem).
  • Giving manatees a fair warning. Editorial. The Tampa Tribune. 10 Sep. 1999.
  • Manatee extremists oppose warning device. Chris Christian. *******************************
  • Give a beep for manatee. Palm Beach Post. Editorial. 11 Aug. 1999.
  • Safe passage for the manatee. The Atlanta Journal - Constitution. 29 Aug. 1999.

    Consumer Publications

  • Deaf, not dumb. Phil Cohen. New Scientists. 31 July 1999. A version of this article is online.
  • Deaf to danger: Manatees can't hear boats. National Geographic. Feb. 2000. Earth Almanac section.
  • Manatee harm alarm. Wildlife Conservation. Apr. 2000. Page 18.
  • What a manatee can't hear. Sports Afield. Winter 1999/2000. Page 67.
  • Device might alert manatees to nearby boats. Florida Today. 5 Aug. 1999.

    Thesis & Dissertations

  • (1999) Gerstein, E.R. "Psychoacoustic Evaluations of the West Indian Manatee (Trichechus manatus latirostis)". Edmund Gerstein. Ph.D. Dissertation, College of Science, FAU, Boca Raton, Florida.

  • (1995) Gerstein, E.R."Underwater Audiogram of a West Indian Manatee (Trichechus manatus latirostris)". Edmund Gerstein. M.S. Thesis, Biological Sciences, FAU, Boca Raton, Florida.

    Invited Presentations

  • (1995) Gerstein E.R. The Behavior and Ecology of West Indian Manatees. National Science For Conservation Series, hosted at Indianapolis Zoo, Indiana.

  • (1994) Gerstein E.R. "Underwater Audiogram of a West Indian Manatee (Trichechus manatus) and Ecological Correlates." Marine Mammal Sensory Symposium, Zeedierenpark, Harderwijk, Holland.

  • (1994) Gerstein E.R. "The Psychophysics of Manatee Hearing and Environmental Consequences" Acoustical Society of America, Southeastern Regional Conference, Gainesville, Florida.

    Formal Presentations

  • (1997) Gerstein E.R., Gerstein L., Forsythe S., Blue J.B., and C.L. Salisbury. "Underwater Hearing Abilities of the West Indian Manatee: A Key to Protection and Conservation Provided Through Captive Research" American Zoo and Aquarium Association Conference, Albuquerque, New Mexico.

  • (1995) Gerstein E., Gerstein L., Forsythe S., and J. Blue. Hearing abilities of the West Indian manatee (Trichechus manatus). 11th Conference on the Biology of Marine Mammals (BMM). Orlando, Florida.

  • (1995) Gerstein, E., Gerstein L., Forsythe S., and J. Blue. "Underwater Hearing in the West Indian manatee." 23rd International Marine Animal Trainers Association (IMATA) Conference, Las Vegas, Nevada.

  • (1994) Gerstein E., Gerstein L., Forsythe S., and J. Blue . "Underwater Audiogram of a West Indian Manatee (Trichechus manatus) and Ecological Correlates of Measured Boat Frequencies". First International Manatee and Dugong Research Conference, Gainesville, Florida.

  • (1993) Gerstein E., Gerstein L., Forsythe S., and J. Blue . "Underwater Audiogram of a West Indian Manatee (Trichechus manatus)." 10th Conference BMM, Galveston, Texas.

  • (1992) Gerstein, E. "The Manatee Mind: Discrimination Training for Sensory Perception Testing of West Indian manatees (Trichechus manatus)." 20th IMATA Conference, Freeport, Grand Bahamas.

  • (1987) Gerstein E. R., Patton, G. W., Tavolga, W. N., "Preliminary Underwater Acoustical Thresholds of a Captive West Indian Manatee, A Quantitative Behavioral Approach". 7th Conference BMM, Miami, Florida.

    Publications, Technical Reports, Books and Abstracts

  • Gerstein E., Gerstein L., Forsythe S., and J. Blue (1999). Underwater Audiogram of a West Indian Manatee (Trichechus manatus) Journal of the Acoustical Society of America 105, 3575-83.

  • Gerstein E.R. and J.E. Blue (1997). "Near surface acoustic properties of manatee habitat in Broward county and vicinity". Tech Report 116 Florida Department of Environmental Protection.

  • Gerstein E.R. and J.E. Blue (1997). "Some acoustic consideration for development of a manatee alert device" Tech Report #4 DACW39-92R-0112, Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS.

  • Gerstein E. R. and L. A Gerstein (1997). "Psychoacoustic investigations of the West Indian manatee: underwater hearing of tones and environmental sounds". Tech Report 1. 64. Florida Department of Environmental Protection.

  • Gerstein E. R. and L. A Gerstein (1996). "Underwater masked thresholds of pulsed and continuous tones in West Indian manatees" Tech Report #2 DACW39-92R-0112, Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS.
  • Gerstein E.R. and J.E. Blue (1996). "Near surface acoustic properties of manatee habitats at King's Bay submarine base". Tech Report #3 DACW39-92R-0112, Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS.

  • Gerstein E. R. (1994). "Auditory Assessment of the West Indian Manatees (Trichechus manatus) Potential Impacts of Low Frequency Activities on Manatee Acoustic Behavior and Communication. Tech Report # 1 DACW39-92R-0112, Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS.

  • G. W. Patton and E.R. Gerstein (1992). "Toward understanding mammalian hearing tractability: Preliminary acoustical perception thresholds in the West Indian manatee (Trichechus manatus). In : D. B. Webster, R.R Fay, and A.N Popper (eds.), The Evolutionary Biology of Hearing . New York and Berlin Springer - Verlag pp.783.

  • Gerstein, E.R. (1994). The manatee mind: discrimination training for sensory perception testing of West Indian manatees (Trichechus manatus). Marine Mammals: Public Display and Research. Vol. 1. pp. 10-21. Cambridge, MA.

    Fish and Wildlife permit applications

  • Leviathan Legacy, Inc request PRT-018197 announced. Federal Register 17 Dec. 1999. Page 70723.
  • Leviathan Legacy, Inc request PRT-018197 rejected. Federal Register 28 Apr. 2000. Page 24977.
  • Mote Marine Laboratory request PRT-843809 granted. Federal Register 18 June 1998. Pages 33383-33384.

    Press Coverage

  • World Wildlife Conservation Yearbook
  • CNN, ABC, NBC, CBS, PBC affiliates
  • The Weekly Reader and Scholastic News

    General Manatee Information

    Making Sense of Manatees in the Apr./May 2000 issue of National Wildlife does a nice job covering the gratefulness of these large animal and briefly mentions the manatee alert research.

    RBBI Closing Comments

    We recently became aware of this research and are very impressed by the amount of groundwork laid in this area by these individuals. I called the technology to the attention of a major magazine and have been asked to write a brief news item for them.

    Currently we are still coming up to speed, gathering additional information and generating a list of ideas that might assist them in receiving a permit from the U.S. Fish and Wildlife Service Office of Management Authority.

    The permit was rejected due to concern for possible manatee strikes during testing, even though a fairly similar study to "conduct controlled approaches to manatees using a variety of vessels" was approved. Some suggest underlying factors may have been behind these concerns.

    1. If the device is successful and widely implemented, speed limits in manatee areas might be raised
      1. Increasing danger the animals
      2. Increasing boat traffic and noise in those areas, some of which border residential homes.
    2. The alert may be harmful to the manatee or other marine habitants.
    3. The alert may actually drive manatees from the area.

    The researchers counter saying manatee safe areas will always be needed. Manatees will not want to be ducking oncoming boats all the time. They are not promoting higher boating speeds or removal of Manatee Safe areas.

    The device is an alert, the manatees can hear it, but it is in no way destructive to their hearing. The researchers call the device an ALERT not an ALARM and state some dolphins and fish finders generate underwater sounds thousands of times louder.

    The device projected to cost about $100, is about 2 inches by 5 inches, mounts to the bow of the and projects a high frequency sound manatees can hear for about 200 meters in front of the boat.

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