Florida manatees, Trichechus manatus latirostris, respond to ...

522 S.M. Nowacek et al. / Biological Conservation 119 (2004) 517–523
problem or environmental factor. With regard to the
first category, many manatees live in areas of extremely
dense vessel traffic and so may habituate to the sounds
of approaching/passing boats, and/or they may not be
able to discriminate between multiple approaching
boats. In these situations they may simply miscalculate
which approaching boat actually poses a threat. Another
possibility is that they are unable to localize the
position of the boat accurately enough to avoid collision
and therefore move in the wrong direction or fail to
move soon enough. Given the anatomy of the manatee
auditory system (Ketten et al., 1992), they are unlikely
to possess the ability to localize the acoustic frequencies
produced by boats as effectively as other marine mammals
that inhabit similar environments, e.g., bottlenose
dolphins (Au, 1993).
With regard to the second category, we have demonstrated
that manatees are capable of detecting and
responding to approaching boats at relatively long
ranges. Factors affecting an individualÕs physical ability
or motivation to respond were discussed earlier, but
environmental characteristics may also affect their ability
to correctly estimate critical pieces of information
about the noise source. Noise from other sources, including
other boats, could mask or partially cover the
sounds of a boat that poses an actual threat. In addition,
transmission of sound in shallow water is affected by
water depth, physiography, substrate type, and vegetative
cover (Kinsler et al., 2000; Urick, 1983), and as
these factors vary so might a manateeÕs ability to correctly
assess the speed, position, and range of an approaching
boat, all of which could contribute to a
collision.
The most prevalent current management strategy to
reduce manatee morbidity and mortality from boat
strikes is to slow vessels in areas of high manatee density.
Our results support this strategy in two ways.
Vessel speed was not a significant factor in eliciting a
response, i.e., fast boats elicited no more or fewer responses
than slow boats, and collisions at high speed
would obviously cause more trauma. Secondly, we have
demonstrated that manatees are capable of appropriately
responding to approaching boats. So, despite
longer exposure time, slower vessel speeds likely afford
the manatees and the vessel operators extra time that
both may need to assess the threat and, if necessary, to
take appropriate action to avoid a collision.
Acknowledgements
Primary support for this project was provided by
the Save-the-Manatee Trust Fund administered by the
Florida Marine Research Institute (FMRI) of the
Florida Fish and Wildlife Conservation Commission.
We especially thank Dr. James Powell of FMRI, now of
Wildlife Trust, for his support and encouragement. We
would also like to thank Dr. John Reynolds for his insight
and review of early versions of this manuscript.
Bob BondeÕs comments also helped to improve the final
manuscript. Staff time for the project and access to the
aerostat, support vessel, and approach vessel were provided
by the Chicago Zoological Society, Woods Hole
Oceanographic Institution, and Dolphin Biology Research
Institute. Many of our statistical analyses are
based on the advice of Andy Solow from the Woods
Hole Oceanographic Institution and Colin Simpfendorfer
from Mote Marine Laboratory. This research
was conducted under Federal Fish and Wildlife Research
Permit No. MA843809-0.
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