Current data indicate the manatee population has increased slowly since the 1970s, though there are no statistically-based current estimates of abundance for the entire Florida manatee population (Haubold et al. 2006). The Florida Fish and Wildlife Conservation Commission utilizes data from rangewide, synoptic surveys which are used to calculate minimum documented abundance. Surveys have been conducted annually since 1991 and are timed to coincide with periods of extreme cold weather when manatees aggregate at warm-water refuges and can thus be easily surveyed using aircraft and observers in boats. Based on highest minimum counts, manatees are approximately equal in abundance on Florida's East and West coasts (USFWS 2001). The most recent biological status review of the manatee in Florida (Haubold et al. 2006) reported a minimum documented population size of 3,300, which reflects the Winter survey of 2001, and is the highest number of manatees ever recorded. The most recent survey, conducted in February, 2006, reported a total of 3,116 manatees in Florida. Of this, 1,642 were reported on the East coast, with 1,474 reported on the West coast (FWRI 2006 press release). Population viability analysis for the Florida manatee using derived age-specific data on reproduction and survival shows a slightly negative growth rate of -0.003, with a 44% probability that Trichechus manatus latirostris will persist as a species over 1000 years. The main factors affecting population projections are adult survival and fecundity. Marmontel et al. (1997) reported that as little as a 10% increase in adult mortality, or a 10% decrease in reproduction, would likely drive the population to extinction over 1000 years. However, this model also showed that if manatee mortality were to be reduced by 10%, slow population growth would likely occur. Four regional subpopulations of Trichechus manatus latirostris have been identified in Florida. These subpopulations consist of individuals that tend to return to the same warm-water refuges each winter, and tend to disperse similarly in the warmer months. Based on telemetry data, these subpopulations tend to have only limited exchanges (Deutsch et al 2003; USFWS 2001; Bengtson 1981) with one another, and each has somewhat distinctive population characteristics as outlined below.Figure 1. Four subpopulations of the Florida manatee with percentage estimates based on winter survey counts in each region between 1996 - 2000. Atlantic subpopulation: The Atlantic subpopulation includes all manatees occurring along Florida's east coast. During the January 2001 survey, 1447 manatees were counted. Data from Craig and Reynolds (2004) was in agreement with this estimate, and reported the population size of manatees using power plants on the Atlantic Coast during winter 2001 was 1607 (within a 95% Bayesian credible interval of 1353 - 1972) (Haubold et al. 2006). Over the most recent 10-year period, Runge et al. (2004) estimated the Atlantic subpopulation has grown 1.0% per year, but this figure is not statistically different from zero, meaning the subpopulation may have increased slowly or it may have declined slightly. What is concerning is that over the last 5 years, lower adult survival rates have been observed, suggesting that this subpopulation may be declining by approximately 3.0% each year (Haubold et al. 2006). Langtimm et al. (1998) estimated adult survival in this subpopulation to be 90.7%. While this figure may appear to represent high survival, the low reproductive rates and slow rate of population increase in manatees makes adult survival vital to population stability. The USFWS (2001) reports that adult survival below 90% in the subpopulation would be a cause for concern. St. Johns River subpopulation: The St. Johns River subpopulation utilizes the upper St. Johns River Basin and its tributaries, but occurs in a significantly smaller area, primarily south of Palatka, when winter temperatures turn cold. During the 2001 survey, 112 manatees were counted. However, observations attempting to identify nearly every individual using Blue Spring, the main overwintering site in the region, showed that at least 141 different manatees visited the spring during winter of 2001. A total of 154 manatees were counted at Blue Spring during the most recent synoptic survey in January 2005 (Haubold et al. 2006).Runge et al. (2004) reported that the St. Johns River subpopulation has shown strong growth over the last 10 years, increasing by 6.2% (95% CI: 3.7 - 8.1%) per year. Coupled with the strong growth rate are high adult survival and reproductive rates. Thus, the smallest of the four subpopulations, accounting for only 5% of the total population, is growing at the fastest rate (Haubold et al. 2006).Northwest subpopulation: The Northwestern subpopulation occurs from the Pasco-Hernando County line northward through the Florida Panhandle and includes the Gulf coast to Louisiana. During the January, 2001 survey, 377 manatees were counted (Haubold et al. 2006).Runge et al. (2004) reported that this subpopulation has grown by 3.7% (95% CI: 1.6 - 5.6%) per year from 1991 - 2001. This subpopulation is the second smallest in number and accounts for approximately 11% of the total manatee count. Its population characteristics are similar to those of the St. Johns Basin subpopulation in terms of having high adult survival rates, but reproduction seems to be somewhat lower (Haubold et al. 2006). Southwest subpopulation: The Southwestern subpopulation occurs from approximately the Pasco-Hernando County line southward to Whitewater Bay in the Everglades. During the Winter 2001 survey, 1364 individuals were counted in this region (Haubold et al. 2006).Runge et al. (2004) reported that from 1994 - 2002, the southwestern subpopulation has declined at an estimated rate of 1.1% per year (95% CI: -5.4 to +2.4%). There is somewhat greater uncertainty about survival and reproductive rates in this region, as is reflected by the relatively wide confidence interval. This population has lower estimates of adult survival than those of all other subpopulations, likely due to the combined effects of watercraft mortality and episodic mortality events caused by red tide, but possibly also due to the geographic extent of current sampling efforts (Haubold et al. 2006). Interestingly, the 2 smallest subpopulations (St. Johns River Basin and Northwest) are growing in number at rates of 6.2% and 3.7% respectively (Runge et al. 2004). Eberhardt and O'Shea (1995) estimated these growth rates are currently adequate to sustain these populations. However, the 2 largest subpopulations (Atlantic and Southwestern), which together account for 74% of the total population, are either just stable, or declining by 1-3% annually (Haubold 2006; Runge et al. 2004). The most likely cause for this lack of growth is depressed adult survival rates, reported at 90.7% for adults in the Atlantic population (Langtimm et al 1998), and as yet undetermined for the Southwest population (USFWS 2001). Model predictions presented in Haubold et al. (2006) projected future decline in both the Atlantic and Southwestern populations.Locomotion: Stabilization in the water arises from the elongate body shape of the manatee, dorsal position of the lungs, and the heavy bones of the manatee, which contribute to the animal's neutral buoyancy by acting as ballast. Most swimming activity in manatees is accomplished solely by dorsoventral undulations of the wide, rounded tail, which also serves as a rudder. Manatees are able to steer, bank, and roll simply by adjusting tail position. Newborns tend to use the flippers exclusively while swimming (Hartman 1979). Flippers are used primarily for turning, but also for precise movements, course corrections, stabilizing position, and for orientation while feeding, idling or socializing. When idling on the bottom while resting or feeding, the flippers provide the sole source of movement, with many manatees using the tips of their flippers to balance upon while resting. Manatees have often been observed "walking" along the bottom using alternating flipper movements (Hartman 1979). Swimming speeds in adults are approximately 18 - 21 strokes per minute while idling; 24 - 36 strokes per minute while cruising; and 45 - 50 strokes per minute while escaping. Average idling speeds of adults were clocked at approximately 2 - 5 km/hr. while idling; 3 - 7 km/hr. while cruising; and 18 - 25 km/hr. in flight. Fleeing sprints are generally short in duration, usually not more than 100 meters. Calves, being smaller, must stroke at a higher rate to keep pace with adults; however, observations show that cows tend to swim more slowly when accompanied by calves. (Hartman 1979).
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