Florida Manatees
by
Lynn W. Lefebvre
National Biological Service
Thomas J. O'Shea
National Biological Service
| Major efforts have concentrated on better quantification of Florida manatee populations, emphasizing reproduction, population size, and mortality. Most of the information on manatee reproduction (Table) comes from long-term studies based on recognizable individuals at winter aggregation sites (e.g., Rathbun et al. in press). Florida manatees are at the northern limit of the species' range and must seek warmer waters when water temperatures drop below about 20 ºC. Natural springs, such as those found in Crystal River on the west coast and Blue Spring on the St. Johns River, and discharges from industrial plants provide warmwater refuges for hundreds of manatees during cold periods. |
Table. Estimated population traits of the Florida manatee based on long-term life-history research (data are from the National Biological Service and the Florida Department of Environmental Protection). | |
| Individual manatees are recognized at these sites largely through their unique scar patterns, caused by boat strikes (Figs. 1a and 1b). National Biological Service personnel have cataloged almost 1,000 recognizable manatees and maintained their sighting histories in a computer-based system (Beck and Reid in press). |
Fig. 1a. Female manatee and calf. Individuals can be identified by their unique scar patterns; scars are usually the result of collisions with boats. Courtesy J.P. Reid, NBS |
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| Estimates of manatee reproductive traits are similar across study sites (Table), despite large habitat differences among study areas. There is also agreement in reproductive estimates obtained from salvaged carcasses (Marmontel in press), indicating that Florida manatees have probably achieved a maximum level of reproduction (O'Shea and Hartley in press). | ||
Aerial Surveys |
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| Fig. 1b. A manatee often bears scars from multiple boat collisions. Courtesy R.K. Bonde, NBS | |
The population of Florida manatees cannot be directly estimated because they are often difficult to see. They occupy waters that may be turbid or obscured by overhanging branches; they can move long distances between counting areas over a short time; and many environmental factors, particularly temperature, influence their distribution and behavior (Lefebvre et al. in press). |
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| Three statewide aerial surveys, coinciding with maximum manatee use of winter aggregation sites, resulted in counts of 1,268 (January 1991), 1,465 (February 1991), and 1,856 (January 1992; Ackerman in press). The differences in these counts are thought to reflect the influence of different environmental conditions, not changes in population size. Manatee presence at winter aggregation sites varies within and between winters, depending upon the pattern and severity of winter cold fronts. | ||
| Garrott et al. (1994) developed a population index by using a temperature covariate to model a simple linear trend in annual aerial survey data from the winters of 1977-78 through 1991-92. Their analyses showed an increasing trend in the temperature-adjusted counts of 7%-12% annually on the Atlantic coast, but the degree to which these increases are related to true population growth is unknown. No pronounced temporal trend was detected at the largest aggregation site on the southwest coast. | ||
| While this result seems promising because it shows no evidence for major declines, it is tempered by other factors. The number of human-related manatee deaths on the Atlantic coast is more than twice as high as on the gulf coast (Ackerman et al. in press). This fact is reflected in the lower survival rate of adult manatees on the Atlantic coast than at Crystal River and Blue Spring (O'Shea and Langtimm in press). Reynolds and Wilcox (1994) found that the number of calves sighted at winter aggregation sites has decreased since 1982, and that in three recent winters, the percentage of manatees sighted that are calves has also decreased. They note that mortality of calves at or near time of birth is the fastest-growing type of manatee mortality, thus the downward trend in aerial survey calf counts is a cause for concern and further investigation. | ||
Recovery Criteria |
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| Species recovery criteria for the Florida manatee are three-fold: the population trend must be stable or increasing; mortality must be stable or declining; and threats to manatee habitat must be under control (USFWS 1989). Better population and life-history data suggest a greater potential for increase and higher numbers than previously recognized, and strong steps taken by local, state, and federal governments are increasing the number and area of sanctuaries and slow boat-speed zones. These steps may reduce mortality if they are continued and expanded, allowing the population to recover more quickly. | ||
| Management has focused on ways to reduce human-related mortality. Of greatest concern has been an increase over the years in the number of human-caused deaths, particularly those caused by collisions with boats (Fig. 2). Boat strikes account for 78% of human-related manatee mortality and 25% of all documented deaths (Wright et al. in press). A moderate reduction in the number of boat-related deaths in the last 2 years caused optimism; however, watercraft collisions accounted for 49 manatee deaths in 1994, almost matching the record number of 51 in 1991 (Fig. 2). |
Fig. 2. Number of manatee deaths from watercraft collisions and number of Florida registered vessels from 1976-93 (data from National Biological Service and Florida Department of Environmental Protection). |
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Habitat Threats |
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| Habitat threats are far from under control, however. Florida has one of the fastest-growing human populations in the nation, with an estimated net gain of close to 1,000 people per day (Fernald et al. 1992). Much growth has occurred along the coast, with inevitable consequences for coastal habitats. For example, about a third of the 600,000 ha (1.5 million acres) of seagrass meadows present in coastal Florida in the 1940's no longer exist (Lewis 1987). One of the most important regions for manatees on the Atlantic coast is the Indian River Lagoon. Over the past 20 years, losses of submerged aquatic vegetation in some areas of the lagoon have exceeded 95% (Busby and Virnstein 1993). Submerged freshwater plants have also been affected adversely by increases in turbidity and nutrients. | ||
| Debris, particularly monofilament line, plastics, and unattended fishing nets and ropes, directly threatens manatees, who may ingest or become entangled in these materials (Beck and Barros 1991). Manatees are also vulnerable to natural and human-caused catastrophes, such as disease and oil spills, particularly when the animals are concentrated at winter aggregation sites. | ||
Future |
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| Population and life-history information suggests that the potential long-term viability of the Florida manatee population is good, provided that strong efforts are continued to curtail mortality, habitat quality is maintained or improved, and steps are taken to offset potential catastrophes. | ||
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National Biological Service Sirenia Project 412 NE 16th Ave., Rm 250 Gainesville, FL 32601 |
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| References | |
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Ackerman, B.B. Ongoing manatee aerial survey programs--a progress report. In T.J. O'Shea, B.B. Ackerman, and H.F. Percival, eds. Population biology of the Florida manatee (Trichechus manatus latirostris). National Biological Service, Biological Rep. Series. In press. Ackerman, B.B., S.D. Wright, R.K. Bonde, D.K. Odell, and D.J. Banowetz. Trends and patterns in manatee mortality in Florida, 1974-1992. In T.J. O'Shea, B.B. Ackerman, and H.F. Percival, eds. Population biology of the Florida manatee (Trichechus manatus latirostris). National Biological Service, Biological Rep. Series. In press. Beck, C.A., and N.B. Barros. 1991. The impact of debris on the Florida manatee. Marine Pollution Bull. 22(10):508-510. Beck, C.A., and J.P. Reid. An automated photo-identification catalog for manatee life history studies. In T.J. O'Shea, B.B. Ackerman, and H.F. Percival, eds. Population biology of the Florida manatee (Trichechus manatus latirostris). National Biological Service, Biological Rep. Series. In press. Busby, D.S., and R.W. Virnstein. 1993. Executive summary. Pages iii-viii in L.J. Morris and D.A. Tomasko, eds. Submerged aquatic vegetation and photosynthetically active radiation. Special Publ. SJ93-SP13, St. Johns River Water Management District, Palatka, FL. Fernald, E.A., E.D. Purdum, J.R. Anderson, Jr., and P.A. Krafft. 1992. Atlas of Florida. University Press of Florida, Gainesville, FL. 280 pp. Garrott, R.A., B.B. Ackerman, J.R. Cary, D.M. Heisey, J.E. Reynolds III, P.M. Rose, and J.R. Wilcox. 1994. Trends in counts of Florida manatees at winter aggregation sites. Journal of Wildlife Management 58(4):642-654. Lefebvre, L.W., B.B. Ackerman, K.M. Portier, and K.H. Pollock. Aerial surveys for estimating manatee population size and trend--problems and prospects. In T.J. O'Shea, B.B. Ackerman, and H.F. Percival, eds. Population biology of the Florida manatee (Trichechus manatus latirostris). National Biological Service, Biological Rep. Series. In press. |
Lewis, R.R., III. 1987. The restoration and creation of seagrass meadows in the Southeast United States. Pages 153-173 in M.J. Durako, R.C. Phillips, and R.R. Lewis III, eds. Proceedings of the Symposium on Subtropical-tropical Seagrasses of the Southeastern United States, 12 August 1985. Florida Marine Research Publications Number 42, St. Petersburg, FL. Marmontel, M. Age and reproductive parameter estimates in female Florida manatees. In T.J. O'Shea, B.B. Ackerman, and H.F. Percival, eds. Population biology of the Florida manatee (Trichechus manatus latirostris). National Biological Service, Biological Rep. Series. In press. O'Shea, T.J., and W.C. Hartley. Longitudinal studies of manatee reproduction and early age survival at Blue Spring, upper St. Johns River, Florida. In T.J. O'Shea, B.B. Ackerman, and H.F. Percival, eds. Population biology of the Florida manatee (Trichechus manatus latirostris). National Biological Service, Biological Rep. Series. In press. O'Shea, T.J., and C.A. Langtimm. Adult survival estimates for Florida manatees at Crystal River, Blue Spring, and the Atlantic coast. In T.J. O'Shea, B.B. Ackerman, and H.F. Percival, eds. Population biology of the Florida manatee (Trichechus manatus latirostris). National Biological Service, Biological Rep. Series. In press. Rathbun, G.B., J.P. Reid, R.K. Bonde, and J.A. Powell. Reproduction in free-ranging Florida manatees (Trichechus manatus latirostris). In T.J. O'Shea, B.B. Ackerman, and H.F. Percival, eds. Population biology of the Florida manatee (Trichechus manatus latirostris). National Biological Service, Biological Rep. Series. In press. Reynolds, J.E., III, and J.R. Wilcox. 1994. Observations of Florida manatees (Trichechus manatus latirostris) around selected power plants in winter. Marine Mammal Science 10(2):163-177. USFWS. 1989. Florida Manatee (Trichechus manatus latirostris) Recovery Plan. Prepared by the Florida Manatee Recovery Team for the U.S. Fish and Wildlife Service, Atlanta, GA. 98 pp. Wright, S.D., B.B. Ackerman, R.K. Bonde, C.A. Beck, and D.J. Banowetz. Analysis of watercraft-related mortality of manatees in Florida, 1979-1991. In T.J. O'Shea, B.B. Ackerman, and H.F. Percival, eds. Population biology of the Florida manatee (Trichechus manatus latirostris). National Biological Service, Biological Rep. Series. In press. |