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Florida manatee

The Florida manatee (Trichechus manatuslatirotris latirostris) lives in freshwater, brackish and marine environments and inhabits shallow coastal waters, estuaries, bays, rivers, and lakes [1]. Florida manatees occur in waters of the southeastern United States, primarily in Florida and southeastern Georgia, but individuals can range as far north as Rhode Island and probably as far west as Texas [2]. During winter, cold water temperatures keep the population concentrated at warm water sites in peninsular Florida [2].
Collisions with boats represent the greatest current threat to Florida manatees but water control structures also account for significant mortality. Habitat loss caused by residential and commercial development also remains a problem and the loss of warm water refuges could pose a significant threat [2].
Historical data for Florida manatee numbers are poor [2]. Early aerial survey studies conducted in the late 1980’s concluded that there were at least 1,200 animals [4].  Beginning in 1991, the state initiated synoptic aerial surveys to count manatees in potential winter habitat during periods of extreme cold weather. Aerial survey results are highly variable however, and results depend on weather conditions [2]. The number of manatees counted increased from 1,478 in 1991 to 2,812 in 2007 [2]. Warm water refuge counts, which are more accurate, also show an increase since the 1970s [2]. An analysis of trends at winter aggregation sites suggest a mean annual increase of 7 -12% occurred at sites on the east coast from 1978-1992 [3]. More detailed demographic data is needed, however, and more widespread accurate surveys must be conducted, before population trends can be determined with certainty.
Over the past 25 years, there has been an increase in Florida manatee deaths. It is unclear whether this represents a proportional increase relative to overall population size [2]. This needs further investigation because Florida manatee recovery likely depends on controlling human caused mortality of adult manatees [2]. "Red tides" caused by the dinoflagellate (Gymnodinium breve) have also resulted in three incidents (1982,1991, and 1996) of high mortality [2]. Observed declines in the percentage and number of calves seen at power plant aggregation sites could be cause for concern [4].
[1] Lefebvre, L. W., et al. 1989. Distribution, status, and biogeography of the West Indian manatee. Pages 567-610 in C. A. Woods (editor). Biogeography of the West Indies. Sandhill Crane Press, Gainesville, Florida
[2] U.S. Fish and Wildlife Service. 2001. Florida Manatee Recovery Plan, (Trichechus manatus latirostris),Third Revision. U.S. Fish and Wildlife Service. Atlanta, Georgia. 144 pp. + appendices.
[3] U.S. Fish and Wildlife Service. West Indian Manatee (Trichechus manatus latirotris) Florida Stock Assessment. U.S. Fish and Wildlife Service Jacksonville Florida. Revised 1995
[4] 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): 143-177
[5] Associated Press. 2007. Report: U.S. may take manatees off endangered list. April 9, 2007.
[6] Florida Fish & Wildlife Conservation Commission. Synoptic aerial surveys (1991-2006). Available at www.savethemanatee.org/population4a.htm.
Banner photo © Phillip Colla