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Florida manatees, Trichechus manatus latirostris, respond to approaching vessels
BIOLOGICAL CONSERVATION
Biological Conservation 119 (2004) 517–523 www.elsevier.com/locate/biocon
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
c
Mote Marine Laboratory, 1600 Ken Thompson Parkway, Sarasota, FL 34236, USA b Chicago Zoological Society, USA 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
Abstract 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
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: [email protected] (D.P. Nowacek).
0006-3207/$ - see front matter. Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2003.11.020
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
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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.
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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-
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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
Table 1 Description of behavioral parameters used to evaluate responses to boat approaches Parameter
Description
Heading Mobility Swimming speed Inter-animal distance Channel Swimming depth
Compass orientation if stationary, or swimming direction Whether the animal is traveling or stationary Paddle stroke frequency or creation of white water Distance between the focal manatee and the nearest manatee Turning toward, or moving toward or into deeper water (>2 m) Depth of water in which focal animal is swimming
Behaviors were scored during video review and then compared between approach and control segments.
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Table 2 Description of approach boat characteristics and habitat types encountered during the study Parameter
Description
Approaching boat speed Approaching boat type Approach distance Manatee habitat Approaching boat habitat
Categorized as idle, plow, or plane Categorized by engine type: outboard, inboard, jet or non-motor Distance between approaching boat and focal manatee at closest approach (m) Shallow ( 6 2 m), edge (within 1 m of channel edge), or deep/channel (>2 m) Shallow ( 6 2 m), edge (within 1 m of channel edge), or deep/channel (>2 m)
StatisticaÒ . 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 1999 2000 Total
9 9 8 22b
Opportunistic Experimental Opportunistic
9 16 9 30c
33 96 41 170
123a 64 187
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.
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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
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(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
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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
S.M. Nowacek et al. / Biological Conservation 119 (2004) 517–523 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.
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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. McGrawHill, 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.
Biological Conservation 119 (2004) 517–523 www.elsevier.com/locate/biocon
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
c
Mote Marine Laboratory, 1600 Ken Thompson Parkway, Sarasota, FL 34236, USA b Chicago Zoological Society, USA 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
Abstract 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
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: [email protected] (D.P. Nowacek).
0006-3207/$ - see front matter. Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2003.11.020
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
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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.
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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-
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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
Table 1 Description of behavioral parameters used to evaluate responses to boat approaches Parameter
Description
Heading Mobility Swimming speed Inter-animal distance Channel Swimming depth
Compass orientation if stationary, or swimming direction Whether the animal is traveling or stationary Paddle stroke frequency or creation of white water Distance between the focal manatee and the nearest manatee Turning toward, or moving toward or into deeper water (>2 m) Depth of water in which focal animal is swimming
Behaviors were scored during video review and then compared between approach and control segments.
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Table 2 Description of approach boat characteristics and habitat types encountered during the study Parameter
Description
Approaching boat speed Approaching boat type Approach distance Manatee habitat Approaching boat habitat
Categorized as idle, plow, or plane Categorized by engine type: outboard, inboard, jet or non-motor Distance between approaching boat and focal manatee at closest approach (m) Shallow ( 6 2 m), edge (within 1 m of channel edge), or deep/channel (>2 m) Shallow ( 6 2 m), edge (within 1 m of channel edge), or deep/channel (>2 m)
StatisticaÒ . 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 1999 2000 Total
9 9 8 22b
Opportunistic Experimental Opportunistic
9 16 9 30c
33 96 41 170
123a 64 187
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.
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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
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(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
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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
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