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

Abstract
Florida manatees, Trichechus manatus latirostris, respond
to approaching vessels
Stephanie M. Nowacek a,b , Randall S. Wells a,b , Edward C.G. Owen a ,
Todd R. Speakman a,b , Richard O. Flamm c , Douglas P. Nowacek a,*
a Mote Marine Laboratory, 1600 Ken Thompson Parkway, Sarasota, FL 34236, USA
b Chicago Zoological Society, USA
c Florida Marine Research Institute, Florida Fish and Wildlife Conservation Commission, USA
Received 18 April 2003; received in revised form 28 August 2003; accepted 5 November 2003
Florida manatees inhabit shallow coastal and estuarine waters of the southeast US, a range that brings them into frequent
contact with vessels. More than 30% of documented annual mortalities are attributed to vessel collisions, and most living animals
bear the scars of multiple, non-lethal encounters. To document the behavior of manatees in the presence of vessels, we recorded their
movements with an overhead video system. We scored six aspects of behavior during 170 vessel approaches, and compared their
behavior with 187 control segments when no boats were present. Manatees in shallow waters and at the edge of the channel responded
to approaches by orienting towards the nearest deep water, a boat channel, and increasing their swimming speed. Close
boat approaches and shallow water depths exacerbated these responses. Our results indicate that manatees detect and respond to
approaching vessels with an apparent flight response, a response which includes movement towards deeper water. If given sufficient
time, i.e., approached or passed slowly, the manatees may then be able to reach deeper water and safe depths.
Ó 2004 Elsevier Ltd. All rights reserved.
Keywords: Manatee; Behavior; Vessel; Collision; Conservation
Biological Conservation 119 (2004) 517–523
The West Indian manatee, Trichechus manatus, inhabits
the warm, coastal, marine and fresh water systems
fringing the Caribbean Sea, Gulf of Mexico, and
western Atlantic Ocean (Lefebvre et al., 2001). Within
this range, these slow-moving, shallow-water animals
face a variety of serious threats from humans, including
hunting, habitat loss, entanglement in fishing gear, entrapment
in water control structures, and collisions with
boats. These threats vary in intensity over time and
between locations (OÕShea, 1988). Even where manatees
are subject to seemingly strong protective measures,
mortality and morbidity from human activities can be
exceedingly common (Reynolds, 1999).
The Florida subspecies of the West Indian manatee,
Trichechus manatus latirostris, ranges throughout the
* Corresponding author. Present address: Department of Oceanography,
Florida State University, 509 OSB, West Call St., Tallahassee,
FL 32306-4320, USA. Tel.: +1-850-645-1547; fax: +1-850-644-2581.
E-mail address: nowacek@ocean.fsu.edu (D.P. Nowacek).
0006-3207/$ - see front matter. Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biocon.2003.11.020
BIOLOGICAL
CONSERVATION
www.elsevier.com/locate/biocon
waters of Florida, sometimes venturing into other states
along the Gulf and Atlantic coasts during warmer
months. The Florida manatee, more than any other
subspecies, is faced with frequent encounters with boats,
sometimes leading to death or serious injury from collision
impact or cuts from propellers (Ackerman et al.,
1995; Wright et al., 1995). On average, about 30% of
documented manatee deaths each year are due to collisions
with vessels (Ackerman et al., 1995; Commission,
2001; Wright et al., 1995). Deaths from boat strikes involve
all age classes and occur year-round over much of
the subspeciesÕ range (Ackerman et al., 1995; OÕShea,
1988; OÕShea, 1995).
The number of documented mortalities is only a
minimum estimate of the direct impact of boats on
manatees; it does not include animals struck by vessels
but not killed, or those killed but whose carcasses were
not recovered or found. Many Florida manatees bear
scars of multiple past collisions with boats; some of
these injuries appear debilitating (OÕShea, 1995). Of the

518 S.M. Nowacek et al. / Biological Conservation 119 (2004) 517–523
1184 living individuals in the identification scar catalog
that bear boat strike scars, 97% have scar patterns indicative
of more than one strike (OÕShea et al., 2001).
The long-term effects, both at the individual and population
levels, of repeated serious injuries on manatee
survivorship, reproduction, and quality of life remain to
be evaluated.
In spite of the large numbers of injuries and deaths
resulting from watercraft, few observations of collisions
or of manatee behavior in response to approaching
boats have been recorded (Wright et al., 1995). In the
absence of systematic observations of manatee-boat interactions,
much speculation exists about whether or
how manatees respond to approaching boats. Detailed
information on the behavioral responses of manatees to
boat approaches would be valuable for devising boattraffic
management plans to reduce manatee mortality.
Such information has been difficult to obtain, because
they submerge for extended periods of time, very little of
the animal is visible when it surfaces, and water clarity is
poor in much of the speciesÕ range where boats are operating
near them. Observations may be further complicated
by the presence of the observer.
Recent efforts to devise techniques for making
continuous observations of marine mammals have resulted
in the development of a remotely-operated
overhead video recording system suspended from a
helium-filled aerostat tethered to a 6 m support vessel
(Nowacek D.P. et al., 2001). The video camera can
provide excellent visual penetration into the water
column, allowing observation of behavioral details
that cannot be obtained in any other manner. Because
of concerns raised by reports of boat collisions with
bottlenose dolphins (Wells and Scott, 1997), this
technique was applied to experimental studies of behavioral
responses of dolphins to approaches by vessels
(Nowacek S.M. et al., 2001). The overhead video
system has also been used in videogrammetry studies
of manatees (Flamm et al., 2000). Given the adaptability
of the system, we extended the overhead video
approach to studies of the behavioral responses of
manatees to vessel approaches.
1. Methods
We observed manatees in the waters near City Island,
Sarasota, FL, adjacent to Mote Marine Laboratory
(Fig. 1). The channel to the north side of the City Island
seagrass meadows is a designated ‘‘Water Sports Area’’,
near a boat ramp, and is used heavily by water skiers on
weekends and holidays. The Water Sports Area ranged
in depth from 0.7 to 4.8 m, though most of the channel
was less than 3 m deep and the fringing seagrass
meadows were typically less than 2 m deep. Manatees
were found in seagrass meadows and channels, and
along the edges where the shallows slope into the
channels.
We conducted focal animal behavioral observations
on manatees under circumstances where continuous
observations were possible (Altmann, 1974). Focal
manatees were selected on the basis of their occurrence
in relatively clear water, and the existence of distinctive
features that would facilitate their recognition
Fig. 1. The study area was a small area of the inshore waters near Sarasota, FL composed primarily of seagrass beds with a deep channel nearby.
Observations most often started when manatees were in the shallows or at the edge of the channel. We followed animals according to the behavioral
protocol explained in the text, including attempts to maintain contact even when they moved into the channel.

throughout the course of our observations. We refrained
from approaching manatee mothers with young calves.
During two field seasons, two types of vessel approaches
were used: opportunistic (1999 and 2000) and
experimental (1999 only). Opportunistic approaches
were recordings of vessels operated by individuals independent
of our study that happened to pass within the
field of view of our aerial video camera while a focal
manatee was visible in the frame. During experimental
approaches, a 5.5 m long, 150 hp outboard-powered
center console vessel under our control was directed via
VHF radio toward manatees under observation via the
aerostat. This kind of boat represents one of the more
common classes of vessels in the study area. As per our
US Fish and Wildlife Service Research Permit, the vessel
was directed along a straight-line course so that it would
pass greater than three manatee body lengths from the
focal manatee. The experimental boat approach began
about 100 m away from the manatee(s), and ended the
same distance past the animal(s). The approach vessel
attempted to maintain a course and speed in a channel
as it reached its point of closest approach to the focal
manatee. To maximize the safety of the manatees, the
experimental approach vessel carried a dedicated observer
to watch for manatees moving into the path of the
boat and a short-range television system that carried the
transmitted view from the overhead video camera.
The support vessel was anchored during boat approaches,
when possible. This provided a stable view of
the waters surrounding the focal manatee, and eliminated
the support vesselÕs engine as a possible confounding
source of disturbance. Observers onboard the
support vessel collected real-time data on the timing and
circumstances of the experimental and opportunistic
approaches, videotape counter information, weather
and sea surface conditions, and the presence of other
manatees and vessels in the area. Upon completion of a
set of experimental trials or a series of opportunistic
approaches on a focal manatee, the approach vessel
crew collected data on water depth and turbidity (via
Secchi disk) at the manateeÕs location.
Video recordings of opportunistic and experimental
approaches were reviewed on a high quality, flat screen
24 in. video monitor (Sony Trinitron) and categorized
based on quality. The ability to continuously track be-
S.M. Nowacek et al. / Biological Conservation 119 (2004) 517–523 519
Table 1
Description of behavioral parameters used to evaluate responses to boat approaches
Parameter Description
Heading Compass orientation if stationary, or swimming direction
Mobility Whether the animal is traveling or stationary
Swimming speed Paddle stroke frequency or creation of white water
Inter-animal distance Distance between the focal manatee and the nearest manatee
Channel Turning toward, or moving toward or into deeper water (>2 m)
Swimming depth Depth of water in which focal animal is swimming
Behaviors were scored during video review and then compared between approach and control segments.
haviors of the focal animal was imperative, so segments
in which the focal animal was not continuously viewable
were considered ‘‘unusable’’ and were not included in
the analyses (for example, turbidity during early experiments
often precluded continuous observations). Two
sets of video segments were analyzed. The first set included
the approach segments, which were typically
about 30 s long. The second set included randomly selected
control segments, also lasting about 30 s. Each
control segment occurred at least 5 min before or after a
vessel approach. Control segments were spaced at least 2
min apart to ensure independence between segments.
Changes in behavior, along with distance between the
focal manatee and the approach vessel, were measured
and recorded. Two independent observers watched each
approach segment twice to ensure scoring consistency.
Distance estimates were made by direct measure on the
flat screen, using known-length reference objects (approach
vessel and components) in appropriate orientations
to minimize problems associated with parallax
(Flamm et al., 2000).
We measured six behavioral parameters (Table 1).
Accurate measures of amount of change (e.g., number
of degrees of heading change) were not possible from the
video image as there was no way to calibrate measurements
given differences in zoom and viewing angle. Instead,
each approach segment was scored using either
‘‘change’’ or ‘‘no change’’ for each of the parameters.
When changes in the behavioral parameters were detected,
we recorded the direction of the change, e.g., an
increase in swim speed or a decrease in inter-animal
distance.
The analyses were hierarchical. First, we assessed
whether the frequency of change in any of the behavioral
categories varied between the vessel approach
segment (i.e., experimental treatments) and the control
segments by applying a test for an additive effect (due to
treatment) at the logistic scale, but allowing for heterogeneity
between individual animals (McCullagh and
Nelder, 1989). Second, if differences were found we then
compared vessel approach segments to assess the specific
circumstances where changes occurred by applying a
generalized linear model using a binary distribution and
a logit link function. Data on habitat and boat features
were categorical (Table 2) and tests were conducted with

520 S.M. Nowacek et al. / Biological Conservation 119 (2004) 517–523
Table 2
Description of approach boat characteristics and habitat types encountered during the study
Parameter Description
Approaching boat speed Categorized as idle, plow, or plane
Approaching boat type Categorized by engine type: outboard, inboard, jet or non-motor
Approach distance Distance between approaching boat and focal manatee at closest approach (m)
Manatee habitat Shallow ( 6 2 m), edge (within 1 m of channel edge), or deep/channel (>2 m)
Approaching boat habitat Shallow ( 6 2 m), edge (within 1 m of channel edge), or deep/channel (>2 m)
Statistica Ò . These methods have been successfully applied
in testing the responses of other marine mammals
to vessel approaches (Nowacek S.M. et al., 2001). Significance
was assigned at the 6 0.05 level.
It should be noted that the terms ‘‘approaches’’ and
‘‘passes’’ are considered interchangeable, referring to
vessels observed moving within the same field of video
view as focal manatees. Few ‘‘approaches’’ actually
brought vessels on an interception vector with the focal
manatees, rather the vessels passed by the manatees at
distances ranging up to 145 m (mean ¼ 19.9 m,
SD ¼ 20.5 m, n ¼ 170).
2. Results
We observed boat approaches on 22 days during 13–
26 May 1999 and 29 April–10 June 2000 (Table 3). La
Ni~na-related conditions of higher than normal winds
and turbid waters hampered our efforts during 2000. Of
a total of 317 passes recorded during the two years of
the study, 170 involving 30 manatees met our criteria for
being considered usable. Additionally, 187 control segments
were scored.
A detectable change in at least one behavioral category
relative to boat approaches was noted in 84 (49%) of the
170 usable passes. However, significant changes were
only noted in two behavioral parameters. Significantly
more changes in behavior were found during approach
segments than during control segments for the variables
‘‘channel’’ (test statistic value ¼ 3.353, p < 0:05), and
‘‘swimming speed’’ (test statistic value ¼ 5.097, p < 0:05).
In 42 of 112 cases (37.5%), manatees turned toward or
moved toward or into a channel as a boat approached.
Similarly, in 30 of 152 cases (19.7%) manatees changed
their swimming speeds as boats approached, 90% of these
cases involved increases in swimming speed. No clear
patterns were found for the remaining behavioral variables
of heading, swimming depth, inter-animal distance,
and mobility as indicators of response to boat
approaches.
We examined these findings with respect to approach
boat and habitat parameters, including the distance of
the boat from the manatee at the point of closest approach
or passage. Channel behavior, an apparent
generalized response involving turning toward or moving
toward or into deep water, occurred without specific
regard to boat type, boat speed, distance from the
manatee (within the upper limit of our visual field, about
145 m), the kind of habitat in which the boat was operating,
or the kind of habitat occupied by the manatee.
A total of 152 experimental or opportunistic segments
was suitable for evaluation of changes in swimming
speed and associated factors. Changes were
observed in 30 of these segments and 27 of them involved
increases in swimming speed. Significant changes,
however, were detected only during close
approaches; no significant changes in swimming speed
were seen when the boat passed farther than 25 m from
the focal animal. Swimming speed was significantly affected
by the following factors during approaches that
occurred less than 25 m from the focal animal: focal
animal (F ¼ 3:18, df ¼ 24, p < 0:001), manatee habitat
(F ¼ 5:08, df ¼ 2, p < 0:01) and approaching boat
habitat (F ¼ 5:86, df ¼ 2, p < 0:01). Additionally, approaching
boat distance was a significant covariate
Table 3
Summary of effort from each field season including number of approaches suitable for analysis
Year No. field days Approach type No. focal manatees No. vessel passes No. control segments
1999 9 Opportunistic 9 33 123a 1999 9 Experimental 16 96
2000 8 Opportunistic 9 41 64
Total 22b 30c 170 187
Approach types are explained in the text; briefly, ÔopportunisticÕ approaches consisted of observations of transiting vessels not under our control,
and ÔexperimentalÕ were those conducted with boats under our control.
a
123 control segments for all of 1999 (opportunistic and experimental combined).
b
In 1999, both experimental and opportunistic observations were conducted on the same days.
c
In 1999, some of the focal animals were subject to both experimental and opportunistic approaches.

Fig. 2. Percent change in swimming speed in observed manatee habitats.
Distance categories, 0–9 and 10–19 m, refer to the distance observed
between an approaching vessel and a manatee at the point of
closest approach. See Table 2 and text for complete descriptions of the
habitats and for statistical comparisons.
(F ¼ 14:79, df ¼ 1, p < 0:001) such that the frequency
of change was correlated with approach distance.
To further investigate the relationship between approach
distance and frequency of change in swimming
speed, we looked at responses in limited distance categories:
0–9 and 10–19 m from the boat. We found that
when a boat approached or passed less than 10 m from
the focal animal, changes in swimming speed were significantly
affected by manatee habitat (F ¼ 8:37, df ¼ 1,
p < 0:05; Fig. 2) and approach distance (covariate,
F ¼ 10:77, df ¼ 1, p < 0:01). When a boat approached
or passed 10–19 m from the focal animal both manatee
habitat and approaching boat habitat were significant
factors affecting the frequency of change in swimming
speed (manatee habitat: F ¼ 4:22, df ¼ 2, p < 0:05,
Fig. 2; approaching boat habitat: F ¼ 5:10, df ¼ 2,
p < 0:05). Within each manatee habitat type, manatees
were more likely to change their swimming speed when
boats moved to within 9 m than when they were at 10–
19 m, with the highest occurrence of swimming speed
change (87.5%) when the manatees were approached
closely in the shallowest waters (Fig. 2). Of 54 cases in
which approaching boat habitat was recorded, analyses
show that manatees were more likely to change their
swimming speed if the approaching boat was in the
shallow habitat (25.0%) than in the edge habitat (12.5%)
or the deep habitat (11.9%).
3. Discussion
Our results show that manatees do in fact respond to
approaching or passing boats, and it appears that they
often begin to respond when the vessels are at distances
of 25–50 m, though we can not say at what distance the
manatees initially detected the approaching vessels. It is
not surprising that manatees can detect boats given the
frequencies of sound boats produce (Richardson et al.,
1995) and the range of frequencies manatees can hear
S.M. Nowacek et al. / Biological Conservation 119 (2004) 517–523 521
(Bullock et al., 1980; Bullock et al., 1982; Gerstein et al.,
1999; Ketten et al., 1992). That they actually respond,
primarily by moving toward or into deeper water
(channels) and increasing speed, has not, to our
knowledge, been reported in the scientific literature.
Unfortunately, this response may take the manatee into
the path of the approaching vessel before it can reach
water of sufficient depth to be able to move safely beneath
the boat.
While we have demonstrated that manatees do respond
to boat approaches, these responses varied with
the individual manatee. General patterns of response do
not emerge until changes in behavior are considered
within very limited approach distance categories. Differential
responses among individuals might be attributed
to any of a variety of factors, possibly including,
but not limited to age, prior exposure to boats, reproductive
state, hearing ability, or activity. However, these
potential factors diminish in importance for approaches
at close range since at these distances we began to see a
uniform response: increased swimming speed and
movement toward or into deeper water.
Our results also show that distance is not the only
significant factor affecting changes in behavior. Manatee
habitat and approaching boat habitat also significantly
affected frequencies of behavioral change. Sound propagation
is typically poor in shallow water and manatees
may have no way of clearly localizing the direction from
which a vessel is approaching. Manatees in deep water
have more options for responding to approaching boats.
In channels they can increase their swimming speed,
change their heading, and/or change their swimming
depth. Often manatees in channels were observed to dive
to depths greater than the visibility limit indicated by
our Secchi disk, placing them safely beneath the keels
and propellers of almost all of the vessels operating in
the area.
Our data clearly demonstrate that manatees make
changes in their behavior in response to boat approaches
at distances of at least 25 m. Responses were
noted to occur as close as 1 m and as far as 68 m from
the approaching vessel. Frequencies of behavioral
change vary depending especially on the distance from
the approaching boat and the manateeÕs habitat. Approaching
closely and/or approaching in shallow water
increase risk of collision as it becomes more difficult for
manatees to move safely out of or below the path of the
approaching vessel.
If the animals are capable of avoiding approaching
boats, which they did successfully with every approach
during our study, it seems fair to ask why they continue
to be hit. Several possibilities exist, and they fall into
two major categories: (i) manatees detect approaching
vessels but do not respond appropriately or in sufficient
time to avoid being hit, or (ii) manatees are unable
to detect approaching boats due to some individual

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.
References
Ackerman, B.B., Wright, S.D., Bonde, R.K., Odell, D.K., Banowetz,
D.J., 1995. Trends and patterns in mortality of manatees in
Florida, 1974–1992, In: T.J. OÕShea, B.B. Ackerman, H.F. Percival
(Eds.), Population Biology of the Florida Manatee, National
Biological Service Information Report, Ft. Collins, CO, pp. 223–
258.
Altmann, J., 1974. Observational study of behavior: sampling methods.
Behaviour 49, 227–267.
Au, W.W.L., 1993. The Sonar of Dolphins. Springer-Verlag, New
York.
Bullock, T.H., Domning, D.P., Best, R.C., 1980. Evoked brain
potentials demonstrate hearing in a manatee (Trichechus inunguis).
Journal of Mammalogy 61 (1), 130–133.
Bullock, T.H., OÕShea, T.J., McClune, M.C., 1982. Auditory evoked
potentials in the West Indian manatee (Sirenia: Trichechus manatus).
Journal Comparative Physiology 148, 547–554.
Commission, Marine Mammal, Annual Report to Congress, 2001.
Flamm, R.O., Owen, E.C.G., Weiss, C.F., Wells, R.S., Nowacek, D.P.,
2000. Aerial videogrammetry from a tethered airship to assess
manatee life-stage structure. Marine Mammal Science 16 (3), 617–
630.
Gerstein, E.R., Gerstein, L., Forsythe, S.E., Blue, J.E., 1999. The
underwater audiogram of the West Indian manatee (Trichechus
manatus). Journal of the Acoustical Society of America 105 (6),
3575–3583.
Ketten, D.R., Odell, D.K., Domning, D.P., 1992. Structure, function,
and adaptation of the manatee ear. In: Thomas, J. (Ed.), Marine
Mammal Sensory Systems. Plenum Press, New York, pp. 77–95.
Kinsler, L.E., Frey, A.R., Coppens, A.B., Sanders, J.V., 2000.
Fundamentals of Acoustics, fourth ed. Wiley, New York.
Lefebvre, L.W., Marmontel, M., Reid, J.P., Rathbun, G.B., Domning,
D.P., 2001. Status and biogeography of the West Indian manatee.
In: Woods, C.A., Sergile, F.E. (Eds.), Biogeography of the West
Indies: Patterns and Perspectives. CRC Press, Boca Raton, pp.
425–474.
McCullagh, P., Nelder, J., 1989. Generalized Linear Models. Chapman
and Hall, London.
Nowacek, D.P., Wells, R.S., Tyack, P.L., 2001. A platform for
continuous behavioral and acoustic observations of free-ranging
marine mammals: overhead video combined with underwater
audio. Marine Mammal Science 17 (1), 191–199.
Nowacek, S.M., Wells, R.S., Solow, A., 2001. Short-term effects of
boat traffic on bottlenose dolphins, Tursiops truncatus, in Sarasota
Bay, Florida. Marine Mammal Science 17 (4), 673–688.
OÕShea, T.J., 1988. The past, present, and future of manatees in the
southeastern United States: realities, misunderstandings, and

enigmas presented at the Southeastern Non-game and Endangered
Wildlife Symposium.
OÕShea, T.J., 1995. Waterborne recreation and the Florida manatee.
In: Knight, R.L., Gutzwiller, K.J. (Eds.), Wildlife and Recreationists.
Island Press, Washington, DC, pp. 297–311.
OÕShea, T.J., Lefebvre, L.W., Beck, C.A., 2001. Florida manatees:
perspectives on popluations, pain and protection. In: Dierauf, L.A.,
Gulland, F.M.D. (Eds.), CRC Handbook of Marine Mammal
Medicine. CRC Press, Boca Raton, pp. 31–43.
Reynolds, J.E., 1999. Efforts to conserve the manatees. In: Twiss, J.R.,
Reeves, R.R. (Eds.), Conservation and Management of Marine
Mammals. Smithsonian Institution Press, Washington, DC, pp.
267–295.
S.M. Nowacek et al. / Biological Conservation 119 (2004) 517–523 523
Richardson, W. John, Greene, Charles R., Malme, Charles I.,
Thomson, Denis H., 1995. Marine Mammals and Noise. Academic
Press, San Diego.
Urick, R.J., 1983. Principles of Underwater Sound, third ed. McGraw-
Hill, New York.
Wells, R.S., Scott, M.D., 1997. Seasonal incidence of boat strikes on
bottlenose dolphins near Sarasota, Flordia. Marine Mammal
Science 13, 475–480.
Wright, S.D., Ackerman, B.B., Bonde, R.K., Beck, C.A., Banowetz,
D.J., 1995. Analysis of watercraft-related mortality of manatees in
Florida, 1979–1991 In: T.J. OÕShea, B.B. Ackerman, H.F. Percival
(Eds.), Population Biology of the Florida Manatee, National
Biological Service Information Report, Ft. Collins, CO.