FORENSIC METHODS FOR CHARACTERIZING WATERCRAFT FROM WATERCRAFT-INDUCED WOUNDS ON THE FLORIDA MANATEE (TRICHECHUS MANATUS LATIROSTRIS)

Watercraft-related mortality represents 1,253 (24.9%) of 5,033 Florida manatee ( Trichechus manatus latirostris ) deaths recorded between 1 January 1979 and 31 December 2004. Wound patterns generated by collisions with watercraft are diagnostic. Sets of cuts and scrapes that are roughly equidistant and perpendicular to the direction of vessel travel are consistent with lacerations made by propeller blades. From these lesions, estimates of propeller diameter, pitch, rotation, and direction of travel may be obtained. Considerable overlap of propeller sizes and pitches on different size vessels, common use of counter rotation propellers, and numerous other complicating factors may confound efforts to accurately predict vessel size and type from propeller wounds. Of the more than one million watercraft registered in Florida, 98% are ≤12.2 m (40 ft), yet watercraft 5.3–36.6 m (17.5–120 ft) are known to have killed manatees. Analysis of a 5-yr subset of mortality data suggests that a disproportionate number of propeller-caused watercraft-related mortalities could be attributed to propeller diameters ≥43.2 cm (17 in.), inferring that these were caused by watercraft ≥12.2 m (40 ft).     

DISTRIBUTION PATTERNS OF INDIVIDUALLY IDENTIFIABLE WEST INDIAN MANATEES (TRICHECHUS MANATUS) IN FLORIDA

Photographs of distinctively scarred manatees ( Trichechus manatus ) were taken at aggregation sites throughout Florida and assembled into a catalog for identifying individuals. Resightings of known manatees in different years or at different locations enabled us to document site fidelity and long-distance movements. Of the 891 individuals included in the catalog as of August 1986, 470 (53%) were resighted at least once. We documented 219 cases of seasonal returns to specific aggregation sites and 98 instances of movements between areas. Movements in excess of 820 km were documented for individuals on the east coast of Florida. Rapid movements south during early winter, as well as northerly spring movements, verify a seasonal migration pattern for many individuals along Florida's east coast. The wide-ranging migratory habits of manatees in Florida must be considered if meaningful management strategies are to be developed and implemented.     

TRENDS IN MANATEE (TRICHECHUS MANATUS LATIROSTRIS) COUNTS AND HABITAT USE IN TAMPA BAY, 1987–1994: IMPLICATIONS FOR CONSERVATION

Aerial surveys ( n = 88) were used to document locations and count sightings of manatees ( Trichechus manatus latirostris ) in the inshore waters of Tampa Bay, Florida, between November 1987 and May 1994. We made 5,358 sightings of manatees in 1,958 groups. Calves represented 8% of the manatees sighted. Counts were significantly higher in winter ( = 79, n = 29 flights) than in non-winter ( = 46, n = 47) months. Counts of manatees in winter increased significantly during the study, but warm-season counts did not. Regression models demonstrated a relationship between counts and environmental factors. Year-round counts were related to air temperatures and seasons, with highest counts in winter. However, in the winter season, counts were significantly correlated only with wind speed, not air temperature. Yearround counts were predicted to be curvilinear with highest counts at 15°C average air temperature. Areas used differed with season: in cold weather, 76% of all sightings occurred in zones with warm-water sources. High-use areas were identified for summer months. Spatial filter analysis was used to compare manatee density in high-use areas between two two-year time periods. The data indicate that (1) manatee use of Tampa Bay was high and increasing in winter, (2) there are particular zones of the bay where conservation of manatees and habitat should be a priority, and (3) sufficient information has been collected for management agencies to develop and implement manatee protection plans.     

PREHENSILE USE OF PERIORAL BRISTLES DURING FEEDING AND ASSOCIATED BEHAVIORS OF THE FLORIDA MANATEE (TRICHECHUS MANATUS LATIROSTRIS)

The use of perioral bristles (modified vibrissae) by 17 captive Florida manatees and approximately 20 wild manatees was analyzed. Captive manatees were fed six species of aquatic vegetation normally eaten in the wild (four freshwater species and two seagrasses). Inanimate objects were placed in the holding tanks with manatees at Lowry Park Zoological Gardens (Tampa, FL) to determine the degree to which perioral bristles were used in exploration and to define the range of manipulative behavior. In addition, behavioral observations were made on the use of perioral bristles during social interactions with conspecifics. Observations were recorded using a Hi8-format video camera. Florida manatees possess an unusually large degree of fine motor control of the snout and perioral bristles. The large and robust perioral bristle fields of the upper lip were used in a prehensile manner during feeding. Bristle use by manatees feeding on submerged vegetation differed from that seen during feeding on floating vegetation. Other behavioral use of the perioral bristles shows variation depending upon the situation encountered. The degree of plasticity of perioral bristle use supports our hypothesis that the vibrissal-muscular complex of the Florida manatee has evolved to increase the efficiency of grazing and browsing on aquatic vegetation and to fully maximize the potential of the manatee as a generalist feeder. The manipulative and sensitive nature of the manatee snout is likely a manifestation of a complex sensory and motor system which has evolved for marine mammal aquatic herbivores living in shallow turbid habitats.     

INFLUENCE OF POWER PLANTS AND OTHER WARM-WATER REFUGES ON FLORIDA MANATEES

Because of limited tolerance to cold, most Florida manatees survive cold winter periods by aggregating at warm‐water discharges from power plants and natural springs in central and northern Florida. Many power plants used by manatees may soon be retired. When this occurs, some people assume manatees will move to warmer areas in southern Florida; others fear they will stay near retired plants and sustain high levels of cold‐related deaths causing a decline in abundance. To assess these possibilities, we examine warm‐water habitats, population structure and movement, cold‐related deaths, and information on possible historical manatee distribution. Winter water temperatures even in southernmost Florida periodically fall below manatee tolerance levels. To survive such periods, manatees use two types of warm‐water refuges: warm‐water discharges, and passive thermal basins that cool slowly, thereby temporarily retaining warm temperatures. During the coldest periods, perhaps 60% of all manatees use 10 power plants and 15% use four natural springs; most others use thermal basins in southern Florida. Site fidelity to these refuges appears to be the principal factor segregating manatees into at least four subpopulations. Since 1986, rates of cold‐related deaths in southernmost Florida (10.0%) have exceeded those in areas with natural springs in central and northern Florida (8.8%). Our findings suggest that warm‐water springs in northern Florida offer better winter habitat than thermal basins in southern Florida and are better able to support large numbers of manatees. Although evidence is scant, we suggest that manatees historically overwintered principally at northern springs, but that Pre‐Columbian and European hunting restricted their winter range to southernmost Florida by the early 1900s. We also suggest that southernmost Florida may not be able to sustain a large influx of displaced of manatees in the absence of power plants, and that warm‐water springs in northern Florida should be considered the most important source of natural warm‐water habitat.     

DISTRIBUTION AND INNERVATION OF FACIAL BRISTLES AND HAIRS IN THE FLORIDA MANATEE (TRICHECHUS MANATUS LATIROSTRIS)

The distribution and anatomy of sirenian perioral bristles (modified vibrissae) and facial hairs are of interest because of their use during feeding and tactile exploration. In the present study we have identified six fields of perioral bristles on the face of the Florida manatee (T. manatus latirostris), four (U1-U4) on each side of the upper lips and oral cavity, and two (L1-L2) on each side of the lower lip pad, inside the oral cavity and rostral to the horny mandibular pad. Each field has a characteristic location, number of bristles, and range of bristle length and diameter. There is a mean of 110 (± 19) bristles per side, with no left-right differences. Branches of the infraorbital nerve innervate the bases of the largest bristles (U2 group) on the upper bristle pad, and the inferior alveolar nerve supplies the bristles of the lower bristle pad. The dorsal and ventral buccal branches of the facial nerve innervate the superficial facial musculature, which is likely to be involved in bristle eversion and other movements which constitute feeding behavior. Hair is denser in the facial region than on the remainder of the body. Within the face, hair is denser on the oral disk than on the supradisk. The oral disk contains bristle-like hair, whereas the supradisk region possesses hair that is similar in length and diameter to that on the postcranial body. The mean total of bristles and hairs per face was 1,942. Means for the subregions were 220 (± 39) bristles on the perioral bristle pads, 601 (± 115) bristlelike hairs in the oral disk region, 710 (± 229) typical hairs in the supradisk region, and 411 (± 108) typical hairs on the chin. There were no significant differences between left and right side counts. Facial hair density was inversely correlated with facial area and body size. These data provide new information on the anatomical basis of the exceptional orofacial activities characteristic of manatees during feeding and tactile exploration.     

THE USE OF STABLE CARBON ISOTOPE ANALYSIS FOR DETERMINING THE DIETARY HABITS OF THE FLORIDA MANATEE, TRICHECHUS MANATUS LATIROSTRIS

The sloughed skin from three captive manatees at Lowry Park Zoological Garden (Tampa, Florida) was examined over a period of one year to determine its stable carbon isotopic composition (δ¹³C). The food consumed by these manatees in a controlled diet was also sampled and its δ¹³C values determined. The sloughed skin δ¹³C values from the captive manatees were enriched by an average of +4.1%0 relative to lettuce (generally >98% of the diet) the animals consumed. δ¹³C values of the skin were shown to be related to changes in δ¹³C values of the lettuce.
The stable carbon isotopic composition of internal tissues (liver, kidney, and blubber) and skin from dead, stranded manatees was also determined. These values were compared to values of vegetation that manatees are known to eat in the wild. The δ¹³C values of the internal tissues and skin of wild manatees were consistent with the range of δ¹³C values of their expected diet.     

REPRODUCTION IN FEMALE MANATEES OBSERVED IN SARASOTA BAY, FLORIDA

Resightings of recognizable individual Florida manatees, Trichechus manatus latirostris , in Sarasota Bay provided information on life history and reproductive parameters. These parameters have been documented at winter aggregation sites, but few data existed for Florida's west coast at non-winter sites. Life history data are needed to estimate annual reproduction and survival rates and construct population models for manatees along west central Florida. Observations of 113 recognizable manatees of known sex sighted between 1993 and 1997 indicated a 1: 1 sex ratio. Fifty-three females produced at least 55 calves during five years of observation. Two large, presumably sexually mature females were never observed with calves in over five years of monitoring. Not surprisingly, several reproductive parameters matched those measured in previous studies at winter sites with a mean of 1.8 seasons of calf dependence, 2.2 seasons between births, and age of sexual maturity at 3–4 yr. Birth rates, fecundity, and proportions of females with calves varied only slightly among investigations.     

HABITAT SELECTION BY MANATEES IN SARASOTA BAY, FLORIDA

Habitat selection by Florida manatees ( Trichechus manatus latirostris ) is influenced by, among other things, availability of food, thermal, and freshwater resources. However, habitat selection by females with dependent calves may differ from that of other demographic groups with regard to the relative importance of these factors. Additional factors that may influence habitat selection for females with dependent calves include ambient noise, strong currents, or increased foraging requirements. We examined distributional data for manatees from aerial surveys of the coastal waters near Sarasota, Florida, between 1994 and 2004 to determine whether habitat selection by groups of manatees that included calves differed from that of other groups. We characterized groups according to their location within seven habitat types. Enclosed bays not traversed by the Intracoastal Waterway had the highest proportions of groups with calves. Groups with calves used a No Entry refuge (from which almost all human use is barred) to a greater extent than did other groups. Overall, groups with calves exhibited significantly different habitat selection from groups without calves ( P < 0.001, χ²= 43.0, df = 6), but this was not consistent across seasons. During the winter and spring, thermal requirements influenced manatees to such an extent that all demographic groups selected habitat similarly.     

LONG-TERM TRENDS IN ABUNDANCE AND DISTRIBUTION OF MANATEES (TRICHECHUS MANATUS) IN THE NORTHERN BANANA RIVER, BREVARD COUNTY, FLORIDA

Four aerial survey projects were conducted between 1977 and 1986 to determine the abundance, density and distribution of West Indian manatees (Trichechus manatus), in the northern Banana River, Kennedy Space Center, Florida. Manatee density and distribution within selected portions of the 78.5 km² study area were determined. Peak numbers of manatees occurred in spring of each year. The maximum counts increased from 56 in 1978 to 297 in 1986. Manatee abundance was lowest in the winter of each year. Mean density per flight increased from 0.52 manatees/km2 in 1977–78 to 2.73/km² in 1984–86. This increase may reflect increases in the east coast population or shifts in the population distribution. Distributional changes were observed in the study area through time, with a lower percentage of manatees occurring in industrial areas and a correspondingly higher percentage of manatees in nonindustrial areas by 1985.     

AN EPIZOOTIC OF FLORIDA MANATEES ASSOCIATED WITH A DINOFLAGELLATE BLOOM

Over a 10-wk period in early 1982, 39 Florida manatees ( Trichechus manatus latirostris ) were found dead in the lower Caloosahatchee River and nearby waters of southwestern Florida. Two were killed by boats. The remainder showed no evidence of trauma. Lesions indicative of infectious agents were not identified, and bacteriological and contaminant residue findings were unremarkable. Nonspecific lesions of congestion and hemorrhage were identified in brain tissue. Numerous reports were also received of manatee morbidity. Some distressed manatees showed no biochemical lesions in clinical analyses of blood samples and recovered quickly. Timing of manatee illnesses coincided with fish and double-crested cormorant ( Phalacrocorax auritus ) mortality and morbidity. A widespread bloom of the dinoflagellate red tide organism ( Gymnodinium breve ) also coincided with these incidents. G. breve produces potent neurotoxins (brevetoxins). Circumstantial evidence links these events, and possible routes of exposure may include ingestion of filter-feeding ascidians. Ecological conditions that magnified the extent of the epizootic included an early dispersal of manatees into the area from a nearby winter aggregation site and unusually high salinities that facilitated the inshore spread of the red tide bloom. Management responses to future episodes of red tide in manatee areas are suggested.     

SURVIVAL ESTIMATES FOR FLORIDA MANATEES FROM THE PHOTO-IDENTIFICATION OF INDIVIDUALS

We estimated adult survival probabilities for the endangered Florida manatee ( Trichechus manatus latirostris ) in four regional populations using photoidentification data and open-population capture-recapture statistical models. The mean annual adult survival probability over the most recent 10-yr period of available estimates was as follows: Northwest - 0.956 (SE 0.007), Upper St. Johns River - 0.960 (0.011), Atlantic Coast - 0.937 (0.008), and Southwest - 0.908 (0.019). Estimates of temporal variance independent of sampling error, calculated from the survival estimates, indicated constant survival in the Upper St. Johns River, true temporal variability in the Northwest and Atlantic Coast, and large sampling variability obscuring estimates for the Southwest. Calf and subadult survival probabilities were estimated for the Upper St. Johns River from the only available data for known-aged individuals: 0.810 (95% CI 0.727–0.873) for 1st year calves, 0.915 (0.827–0.960) for 2nd year calves, and 0.969 (0.946–0.982) for manatee 3 yr or older. These estimates of survival probabilities and temporal variance, in conjunction with estimates of reproduction probabilities from photoidentification data can be used to model manatee population dynamics, estimate population growth rates, and provide an integrated measure of regional status.     

THE MUSCULAR HYDROSTAT OF THE FLORIDA MANATEE (TRICHECHUS MANATUS LATIROSTRIS): A FUNCTIONAL MORPHOLOGICAL MODEL OF PERIORAL BRISTLE USE

Facial musculature was examined in the Florida manatee, Tricbecbus manatus latirostris , in order to develop a functional model of perioral bristle use. Muscles identified include the M. levator nasolabialis, M. buccinatorius, M. maxillonasolabialis, M. centralis nasi, M. lateralis nasi, M. spbincter colli profundus pars oris, M. orbicularis oris, M. mandibularis, and M. mentalis. A new muscle, M. centralis nasi , has been named and is an integral part of perioral bristle movement. The snout of the Florida manatee is capable of performing complex movements. The prehensile ability of Florida manatees can be explained in the context of a muscular hydrostat as defined by Kier and Smith (1985). Eversion of certain bristles in the upper lip occurs by shortening longitudinal, transverse, and semicircular muscles in combination with volume displacement due to compensatory changes in the shape of the snout. Midline sweeping of these bristles is accomplished by the contraction of M. centralis nasi. Eversion of bristles on the lower jaw is a result of shortening of M. mentalis. Contraction of M. orbicularis oris pushes vegetation into the oral cavity. All observed movement patterns and uses of perioral bristles can be explained by variation of these sequences within the context of muscular hydrostat function.     

DETERMINATION OF MANATEE POPULATION TRENDS ALONG THE ATLANTIC COAST OF FLORIDA USING A BAYESIAN APPROACH WITH TEMPERATURE-ADJUSTED AERIAL SURVEY DATA

In many animal population survey studies, the construction of a stochastic model provides an effective way to capture underlying biological characteristics that contribute to the overall variation in the data. In this paper we develop a stochastic model to assess the population trend and abundance of the Florida manatee, Trichechus manatus latirostris , along the Atlantic coast of the state, using aerial survey data collected at winter aggregation sites between 1982 and 2001. This model accounts for the method by which the manatees were counted, their movements between surveys, and the behavior of the total population over time. The data suggest an overall increase in the population from 1982 to 1989 of around 5%–7%, a reduction in growth or a leveling off (0%–4% annual growth) from 1990 to 1993, and then an increase again of around 3%–6% since 1994. In winter 2001–2002 (the most recent survey season for which analyses were done), we estimated the adult manatee population along the east coast of Florida to be 1,607 individuals (range = 1,353–1,972; 95% credible interval). Our estimate of manatee abundance corresponds well with maximum counts (approximately 1,600 manatees) produced during synoptic aerial surveys under optimal conditions. Our calculations of trends correspond well with mark and recapture analyses of trends in survival of adult manatees along the east coast through the early 1990s. Our population trend estimates since that time are more optimistic than those generated by mark-recapture models.     

AGE DETERMINATION IN MANATEES USING GROWTH-LAYER-GROUP COUNTS IN BONE

Growth layers were observed in histological preparations of bones of known-age, known minimum-age, and tetracycline-marked free-ranging and captive Florida manatees ( Trichechus manatus latirostris ), substantiating earlier preliminary findings of other studies. Detailed analysis of 17 new case histories showed that growth-layer group (GLG) counts in the periotic bone were consistent with known age, or time since tetracycline administration, but were less reliable in other bones. GLG counts were also made in periotic bones of 1,196 Florida manatees of unknown age found dead from 1974 through 1991. These counts were conducted in order to assess variability and to determine relationships among estimated age, size, sex, and degree of bone resorption. Resorption can interfere with accuracy of GLG counts. This effect does not occur until ages greater than about 15 yr and body lengths greater than 300 cm are attained. GLGs were also observed in periotic bones of Antillean manatees ( Trichechus manatus manatus ) but were not validated against known-age specimens. Use of GLG counts in the periotic bone is suitable for application to studies of population dynamics and other age-related aspects of manatee biology.     

Tactile discrimination of textures by Florida manatees (Trichechus manatus latirostris)

Two male Florida manatees (Trichechus manatus latirostris) demonstrated sensitive tactile discrimination in a two‐alternative forced choice task, using a modified staircase method. Stimuli were acrylic plates with vertical gratings of ridges and grooves. The standard stimulus, present on every trial, had 2 mm gratings and the comparison stimuli had wider gratings. The blindfolded subjects were trained to demonstrate discrimination by pressing the target with wider gratings. Discrimination thresholds (75% correct) for the subjects were 2.05 mm and 2.15 mm, corresponding to Weber fractions of 0.025 and 0.075, respectively. These results indicate thresholds on similar stimuli comparable to humans (index finger tasks) and better than harbor seals, Phoca vitulina, and the closely related Antillean manatee, Trichechus manatus manatus. Memory for the tactile task was quite stable for both subjects, over 2 yr in the case of one of the subjects. Video analysis of responses indicated that bristle‐like hairs, perioral bristles, and skin on the oral disk were involved in the discrimination response.