Abstract
To survive cold winter periods most, if not all, Florida manatees rely on warm-water refuges in the southern two-thirds of the Florida peninsula. Most refuges are either warm-water discharges from power plant and natural springs, or passive thermal basins that temporarily trap relatively warm water for a week or more. Strong fidelity to one or more refuges has created four relatively discrete Florida manatee subpopulations. Using statewide winter counts of manatees from 1999 to 2011, we provide the first attempt to quantify the proportion of animals using the three principal refuge types (power plants, springs, and passive thermal basins) statewide and for each subpopulation. Statewide across all years, 48.5% of all manatees were counted at power plant outfalls, 17.5% at natural springs, and 34.9 % at passive thermal basins or sites with no known warm-water features. Atlantic Coast and Southwest Florida subpopulations comprised 82.2% of all manatees counted (45.6% and 36.6%, respectively) with each subpopulation relying principally on power plants (66.6% and 47.4%, respectively). The upper St. Johns River and Northwest Florida subpopulations comprised 17.8% of all manatees counted with almost all animals relying entirely on springs (99.2% and 88.6% of those subpopulations, respectively). A record high count of 5,076 manatees in January 2010 revealed minimum sizes for the four subpopulations of: 230 manatees in the upper St. Johns River; 2,548 on the Atlantic Coast; 645 in Northwest Florida; and 1,774 in Southwest Florida. Based on a comparison of carcass recovery locations for 713 manatees killed by cold stress between 1999 and 2011 and the distribution of known refuges, it appears that springs offer manatees the best protection against cold stress. Long-term survival of Florida manatees will require improved efforts to enhance and protect manatee access to and use of warm-water springs as power plant outfalls are shut down.
Highlights
The Florida manatee, Trichechus manatus latirostris, is a subspecies of West Indian manatee that occurs almost exclusively in the southeastern United States at the northern limit of the species’ range [1]
Some areas of Florida are less prone to cold temperatures, even in southernmost Florida water temperatures can fall several degrees below 18uC for a week or more at a time and to 10uC for shorter periods in cold winters [3]. To survive such periods almost all Florida manatees remain near pockets of warm water called ‘‘warm-water refuges.’’ Two functional categories of warm-water refuges have been identified [3,4]: (1) discharges formed by the constant outflow of warm water mainly from natural springs or power plants, and (2) passive thermal basins (PTBs) heated by solar radiation, ground water seeps, or microbial degradation of benthic organic material
This distribution is virtually identical to regional proportions reported by the Fish and Wildlife Service from synoptic surveys between 1996 and 2000 – 12%, in Northwest Florida, 37% in Southwest Florida, 47% along the Atlantic Coast, and 4% in the upper St
Summary
The Florida manatee, Trichechus manatus latirostris, is a subspecies of West Indian manatee that occurs almost exclusively in the southeastern United States at the northern limit of the species’ range [1]. Calves learn to use individual refuges or sets of refuges by following their mothers during the first year of life and typically continue to use those sites as they age [6] Because of their fidelity to refuges, Florida manatees occur in four relatively discrete regional subpopulations [7], called management units (Figure 1): (1) the upper St. Johns River, (2) the Atlantic Coast, (3) Northwest Florida, and (4) Southwest Florida. As water temperatures rise in spring, animals disperse from refuges into overlapping ranges along the Atlantic and Gulf of Mexico coasts, but rarely move between coasts [6]
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