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.     

USING TREND INDICES FOR ENDANGERED SPECIES

Initial appraisals of the status of endangered large-mammal populations may have to depend on indices of population trend. Such indices may possibly be improved by using auxiliary variables. Various models were studied for populations of the Florida manatee (Trichechus manatus latirostris), Yellowstone grizzly bear (Ursus arctos horribilis), and Hawaiian monk seal (Monachus schauinslandi). Several criteria for checking validity of the fitted models were considered, and the simple R² criterion appears to provide useful comparisons. Multiple regression models overestimated the rate of change of the East Coast manatee population as determined from three other sources (a covariance model, a non-linear model, and the rate estimated from reproductive and survival data). A multiple regression model for grizzly bears using three auxiliary variables exhibited a fairly high R² (0.84) and appeared to provide a better fit than did a non-linear model. A beach count index for Hawaiian monk seals seemed to be unreliable for year-to-year comparisons in contrast to total population counts and estimates from a capture-recapture method. The use of auxiliary variables for checking and improving trend index data appears feasible and well worthwhile.     

INTERSPECIFIC AND INTRASPECIFIC MORPHOLOGICAL VARIATION IN MANATEES (SIRENIA: TRICHECHUS)

The three living species of Trichechus are clearly defined and well exemplify the degree of variability and taxonomic value of morphological characters in a well-understood mammalian genus. Statistical analysis of the largest sample of manatee skulls yet studied has allowed us to identify small suites of characters that effectively distinguish these species. The two subspecies of T. manatus proposed by Hatt (1934) can likewise be distinguished, and their use as taxonomic categories seems justified. This suggests that the cool winters of the northern Gulf Coast, on the one hand, and the deep water and strong currents of the Straits of Florida, on the other, are effective barriers to gene flow between Florida and Antillean manatees. Alleged taxonomic distinctions within T. senegalensis , however, have no demonstrated basis. No significant sexual dimorphism was detected in skulls of any of the species. Many interspecific differences can be correlated with feeding ecology, but others remain unexplained. Features of tooth crown morphology are among the most constant characters examined, but some osteological characters are equally good. T. manatas and T. senegalensis (which are phenetically the most similar) also seem to share a more recent common ancestor than either does with T. inunguis. However, the three species probably separated from each other at nearly the same time. T. inunguis has since become the most derived species, while T. senegalensis has changed the least.     

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.     

Manatees as Sentinels of Marine Ecosystem Health: Are They the 2000-pound Canaries?

The order Sirenia is represented by three species of manatees and one species of dugong distributed in tropical and subtropical regions of the world and considered vulnerable to extinction. The sentinel species concept is useful to identify indicators of the environment and may reflect the quality of health in marine ecosystems. The single species approach to evaluate ecological health may provide a series of snap shots of environmental changes to determine if animal, human, or ecosystem health may be affected. Under this concept, marine vertebrates may be good integrators of changes over space and time, and excellent sentinels of ecosystem health. Based on their life history, manatees may or may not be ideal sentinels, as they are robust, long-lived species and appear remarkably resilient to natural disease and the effects of human-related injury and trauma. These characteristics might be the result of an efficient and responsive immune system compared to other marine mammals. Although relatively immune to infectious agents, manatees face other potentially serious threats, including epizootic diseases and pollution while in large aggregations. Manatees can serve as excellent sentinels of harmful algal blooms due to their high sensitivity, specifically to brevetoxicosis, which has caused at least two major die-offs in recent times. Threats to manatees worldwide, such as illegal hunting and boat collisions, are increasing. Habitat is being lost at an alarming rate and the full effects of uncontrolled human population growth on the species are unknown. The manatee may serve as a sentinel species, prognosticating the deleterious effects of unhealthy marine and aquatic ecosystems on humans. We have identified a number of critical research needs and opportunities for transdisciplinary collaboration that could help advance the use of the sentinel species concept in marine ecosystem health and monitoring of disease emergence using our knowledge on these magnificent sirenians.     

HYDROSTASIS IN THE SIRENIA: QUANTITATIVE DATA and FUNCTIONAL INTERPRETATIONS

It has long been assumed that the heavy bones and horizontal lungs of sirenians are adaptations for maintaining neutral buoyancy and horizontal trim. These assumptions are examined in the light of detailed measurements of skeletal weight distribution in the Florida manatee ( Trichechus manatus latirostris ), measurements of the positions of body and skeletal centers of gravity in the Amazonian manatee (T. inunguis ), and available data on lung morphology in sirenians. The detailed arrangement of bones, muscle masses, and viscera in a sirenian ( T. inunguis ) is illustrated in cross sections for the first time. The hypotheses that skeletal weight is appropriately distributed to serve as hydrostatic ballast, and that the lungs are properly designed to help maintain horizontal trim, are confirmed. We suggest that selection for maintenance of trim and maximization of turning moments of the flippers may help account, respectively, for hindlimb loss and shortening of the neck in both sirenians and cetaceans. We recommend that the term "pachyostosis" be used to refer only to thickening of bones regardless of their density; that "osteosclerosis" be used for the replacement of cancellous with compact bone; and that "pachyosteosclerosis" be used for the joint occurrence of these conditions. Pachyosteosclerosis in sirenians, which may be brought about during ontogeny by mechanisms involving thyroid and parathyroid hormones, is fully normal and adaptive in this order and in no sense "pathological."     

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.     

Phylogeography, phylogeny and hybridization in trichechid sirenians: implications for manatee conservation

Abstract The three living species of manatees, West Indian (Trichechus manatus), Amazonian (Trichechus inunguis) and West African (Trichechus senegalensis), are distributed across the shallow tropical and subtropical waters of America and the western coast of Africa. We have sequenced the mitochondrial DNA control region in 330 Trichechus to compare their phylogeographic patterns. In T. manatus we observed a marked population structure with the identification of three haplotype clusters showing a distinct spatial distribution. A geographic barrier represented by the continuity of the Lesser Antilles to Trinidad Island, near the mouth of the Orinoco River in Venezuela, appears to have restricted the gene flow historically in T. manatus. However, for T. inunguis we observed a single expanding population cluster, with a high diversity of very closely related haplotypes. A marked geographic population structure is likely present in T. senegalensis with at least two distinct clusters. Phylogenetic analyses with the mtDNA cytochrome b gene suggest a clade of the marine Trichechus species, with T. inunguis as the most basal trichechid. This is in agreement with previous morphological analyses. Mitochondrial DNA, autosomal microsatellites and cytogenetic analyses revealed the presence of hybrids between the T. manatus and T. inunguis species at the mouth of the Amazon River in Brazil, extending to the Guyanas and probably as far as the mouth of the Orinoco River. Future conservation strategies should consider the distinct population structure of manatee species, as well as the historical barriers to gene flow and the likely occurrence of interspecific hybridization.     

CAPTURE-RECAPTURE ANALYSIS FOR ESTIMATING MANATEE REPRODUCTIVE RATES

Modeling the life history of the endangered Florida manatee ( Trichechus manatus latirostris ) is an important step toward understanding its population dynamics and predicting its response to management actions. We developed a multi-state markresighting model for data collected under Pollock's robust design. This model estimates breeding probability conditional on a female's breeding state in the previous year; assumes sighting probability depends on breeding state; and corrects for misclassification of a cow with first-year calf, by estimating conditional sighting probability for the calf. The model is also appropriate for estimating survival and unconditional breeding probabilities when the study area is closed to temporary emigration across years. We applied this model to photo-identification data for the Northwest and Atlantic Coast populations of manatees, for years 1982–2000. With rare exceptions, manatees do not reproduce in two consecutive years. For those without a first-year calf in the previous year, the best-fitting model included constant probabilities of producing a calf for the Northwest (0.43, SE = 0.057) and Atlantic (0.38, SE = 0.045) populations. The approach we present to adjust for misclassification of breeding state could be applicable to a large number of marine mammal populations.