Species Description: Trichechus manatus latirostris, like all Sirenians, are non-ruminant herbivores that live in loosely associated social groups (Hartman 1979). The body is massive and fusiform, resembling that of a seal. Body color is generally light to dark gray or brown. Calves are somewhat darker at birth, but lighten gradually within the first month. Adults may reach an average of 3 m (9.8 ft.) in length and 1000 kg (2,200 lbs). The largest adults grow to 4.6 m (15 ft.) and 1,620 kg (3,570 lbs) (Rathbun et al. 1990). Females tend to be larger than males of the same age.
Hair is sparse, but distributed all over the entire body surface, with stiff whiskers around the face and muzzle. The upper lip is flexible and lined with bristles on both the upper and lower lip pads. The tail is large, rounded and horizontally flattened. No hind limbs are present but forelimbs are rounded and paddle-like. Females have 2 mammary glands at the axilla of each forelimb. The head is bulbous, with the small eyes set widely apart. Eyes close by contraction of a sphincter muscle around the eye. Orbits are lined with oil glands that bathe the corneas and a nictitating membrane is present. Nostrils are located on the dorsal surface of the snout. Ear openings are small, located immediately behind the eyes, and lack pinnae (USFWS 2001).
Bones are massive and heavy, with the ribs and long bones of the limbs lacking marrow cavities (O'Dell 1982). A layer of blubber underlies the skin, and fat deposits are found around the intestines and muscles. Molars designed for crushing are grown continuously in the posterior portion of the jaw and move forward as old teeth wear down (Domning and Hayek 1986). Analysis of ear structure suggests that manatee hearing is not acute, and they may have difficulty in localizing sound that occurs outside of a narrow, low frequency range (Ketten et al. 1992). The brain is small in comparison to other similarly sized mammals.
Regional Occurrence: Based on highest minimum counts of the manatee population in the United States, the Florida manatee (Trichechus manatus latirostris), found only in the southeastern U.S., constitutes the largest known group of West Indian manatees anywhere within the species' range (USFWS 2001). In contrast, the Antillean manatee (T. manatus manatus), which occurs in the Greater Antilles, eastern Mexico, Central America, north and northeastern South America and Trinidad (Lefebvre et al 2001) has a wider distribution area, but significantly smaller population sizes. It is suggested that Antillean manatees do not achieve larger population sizes because they are subject to poaching, incidental take in gillnets, and severe habitat loss in many areas within the range (USFWS 2001).
Florida manatees are found only in the southeastern U.S., though a few have been documented in the Bahamas (Lefebvre et al 2001). When waters are warm, from approximately March - late November, some manatees travel along the east coast into Georgia, North and South Carolina, Virginia, and occasionally further north into New York and even Rhode Island (Deutsch et al. 2003). On Florida's west coast, some manatees move into Alabama, Mississippi, Louisiana and Texas during the spring and summer months (Powell and Rathbun 1984).
During winter months (December - February), manatees are temperature-restricted to peninsular Florida, and their geographic range constricts to approximately the 20°C isotherm, including many warm-water refuge areas around artesian springs, power plants, and other industrial sites where thermal effluents occur. Natural refuges include Blue Spring, Crystal River, Homasassa Spring, and Warm Mineral Spring. Ten major thermal refuges around power plants have also been identified, 3 on the west coast, and 7 on the east coast (Reynolds and Wilcox 1986, 1994).
The northern extent of the Florida manatee was believed to be south Florida; however, loss of habitat in south Florida, coupled with expansion of human population with consequent construction of power plants and other industrial sites that discharge warm effluents, have caused manatees to expand their wintering grounds into previously unsuitable areas.
The historical ranges of T. m. latirostris and T. m. manatus may overlap along the Gulf coast of Texas where strays from Mexico and Florida sometimes co-occur (USFWS 2001).
IRL Distribution: Manatees are distributed throughout the IRL. Historical accounts of manatee occurrence and movement suggest that manatees are likely to be as geographically widespread as they are at present, but are less abundant in many regions (USFWS 2001; Lefebvre et al. 2001), including the IRL.
Age, Size, Lifespan: Manatees may live as long as 60 years based on analysis of growth layers in the ear bones (Marmontel et al. 1996). Perinatal survival are rates low, with half of all carcasses recovered in Florida between 1976 - 1991 aged from 0 - 2 years of age (Marmontel et al. 1997). Survival rates increase steadily to Age 4, then remain constant (89.9%) from approximately Age 4-25 years of age (Marmontel et al. 1997).
Newborns average 1.2 - 1.4 m in length (4 - 4.5 ft.) and 30 kg (66 lbs.) (O'Dell 1981). Maximum adult size is approximately 4.6 m (15 ft.) and 1,620 kg (3,570 lbs.), with females tending to grow larger than males of the same age (Rathbun et al. 1990).
Abundance: 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.
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).
Southwestern 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 two 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 two 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 Southwestern 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).
Reproduction: Manatees have an approximately 1:1 sex ratio and low reproductive rates. Males mature at 2 - 3 years of age, while females first become pregnant at 3 - 5 years of age. Most produce a calf by Age 7 (USFWS 2001; Garcia-Rodriguez et al. 1998; Marmontel 1995). Females produce 1 calf every 2 - 3 years, with twins occurring approximately 2% of the time. Cows that lose calves tend to come into estrous faster than those feeding calves. Females continue to produce calves into at least their thirties (Haubold et al 2006; Marmontel 1995), with observations of some captive cows giving birth at older ages.
Cohesive social interactions among manatees occur only in mating herds, which typically consist of a female in heat being pursued by courting bulls. Herds may persist up to 4 weeks with different males cycling in and out of the herd daily (Haubold et al. 2006; Rathbun 1999; Rathbun et al. 1995). Hartman (1979) observed one estrous herd on Florida's West coast consisting of a single female and as many as 17 mature bulls that constantly pursued her. Juvenile males in the population joined and exited the pursuit at various times as well. Bulls generally are tireless while courting a female, constantly embracing or mouthing her as she swims. The constant harassment often leads females to swim into very shallow water, where they are known to beach themselves to avoid the attention of males. Rathbun et al. (1995) suggested that older, larger males dominate access to females in mating herds and are responsible for the majority of pregnancies.
Breeding is reported in all seasons, and successive copulation frequently occurs. However, peak sperm production (as analyzed in recovered carcasses) occurs primarily from March through November. Only 20% of adult males showed evidence of sperm production from December - February (Hernandez et al. 1995).
Marmontel et al. (1997) calculated the gross annual recruitment rate of manatees in Florida as 8% using the following formula: (# females in population) X (% mature females) X (Annual pregnancy rate). Life tables reported in this study indicated approximately zero population growth with a net reproductive rate of 1.09, and a finite rate of increase of 0.005, which is not statistically different from zero (Marmontel et al. 1997). Further, the authors suggested that manatee survival in Florida did not follow the pattern observed in other large mammals species, but rather resembled survivorship curves calculated in exploited populations.
Embryology: The gestation period in manatees lasts 11 - 14 months (USFWS 2001; Rathbun et al. 1995; Hartman 1979). Most calves are born from May through September, but young calves are observed throughout the year (Hartman 1979). Newborns average 1.2 - 1.4 m (4 - 4.5 ft.) in length and 30 kg (66 lbs.) (O'Dell 1981).
Survival rates are low among the youngest calves, with half of all recovered carcasses (n = 1,212) from 1976 - 1991 belonging to Age Classes 0, 1 and 2 (Marmontel et al. 1997).
Metabolism: In comparison to other large mammals, manatees have fairly low basal metabolic rates that measure only 36 - 50% those of similarly sized terrestrial mammals. This is perhaps the lowest weight-specific metabolic rate for any mammal known (Irvine 1983; Best 1981; Scholander and Irving 1941). Irvine speculated that low basal metabolism in the manatee may be an ecological adaptation of a large, tropical animal to a relatively low-quality food source.
Temperature: Manatees have high thermal conductance, so are not well-adapted to the cold water temperatures that occur during the winter months (Irvine 1983). At temperatures below 20°C (68°F) manatees become susceptible to cold stress and cold-induced mortality; thus, many consider 20°C the lower tolerance limit of the manatee (Irvine 1983). Historical winter range limits support this notion, with manatee winter ranges roughly corresponding to the 20°C isotherm.
In recent years, manatees have expanded their winter ranges to include warm-water refuge areas such as artesian springs; canals and turning basins where water temperatures remain above ambient water temperatures; power plants; and other industrial sites releasing warm-water effluents. Manatees, which generally do not form large groups except during breeding periods, begin aggregating in large numbers in thermal refuges when water temperatures drop in late Autumn. Many individuals return to the same refuge each year, with most remaining in wintering areas until approximately March, when ambient water temperatures begin to rise above 20°C (Hartman 1979).
Manatee deaths in Florida not linked to human causes are often from cold-induced stress, with sub-adults disproportionately affected (Marmontel et al. 1997). Much of the cold-induced mortality observed in Florida manatees is linked directly to areas lacking warm-water refuges (Hartman 1979).
Salinity: Trichechus manatus latirostris is euryhaline, tolerating widely ranging salinities. Some evidence suggests that manatees may prefer salinities of less than 25 ppt. They regularly seek out fresh water sources for osmoregulation and perhaps to rid themselves of accumulated parasites (Lefebvre et al. 2001; Hartman 1979; Haubold et al. 2006); and have often been observed drinking fresh water from hoses and pipes when these are available to them.
Manatees live in loosely associated social groups (Hartman 1979), with most cohesive social interaction among manatees occurring only in mating herds. They are known to vocalize, with calls thought to assist in establishing and maintaining contact between individual animals, rather than as an aid to navigation. Bengtson and Fitzgerald (1985) reported that the manatee call rate increases as levels of excitement and activity increase.
Trophic Mode: Manatees are non-ruminant herbivores that specialize in hind-gut fermentation. They man consume approximately 8% of body weight daily and feed for 6 - 8 hours per day (Best 1981; Hartman 1979). Manatees and are apparently unparticular in their choice of feeding sites when food is abundant, and will often continue to exploit specific areas until food resources are depleted (Best 1981; Hartman 1979).
The stomach is small relative to body size. A fingerlike projection, called the cardiac gland, protrudes from the cardiac portion of the stomach and secretes most enzymes used in digestion (Best 1981). Digestive efficiencies in manatees are similar to those of terrestrial herbivores such as the horse (Best 1981). Intestines may measure up to 40 m (130 ft) in length (Reynolds 1979).
Manatees feed exclusively on submerged, emergent and floating vegetation in freshwater, brackish and marine waters, with seagrasses an important staple in the diet (USFWS 2001). They are indiscriminate feeders that ingest whatever vegetation is available, and move freely between habitats in search of food (Hartman 1979). Feeding generally occurs in shallow water 1- 4 m deep. Plants are consumed in situ or taken below the surface to be manipulated into the mouth using the foreflippers. The upper lips of a manatee are bilobed, prehensile, and covered with stiff bristles. To feed, the lobes are everted so they project forward to the food source. Upon contact with food, the lips then close laterally, and the bristles grasp the food to tuck it into the mouth (Hartman 1979).
In fresh water, manatees tend to favor submerged vascular plants such as Hydrilla verticillata, Myriophyllum spicatum, Ceratophyllum demersum, Vallisneria sp. and Callisneria neotropicalis (Snipes 1984; Best 1981). In marine and estuarine habitats, algae such as Anabena, Cladophora, Enteromorpha, Gracilaria, Oscillatoria and Spirogyra are often consumed, as well as a variety of seagrasses, especially Thalassia testudinum (turtle grass) (Best 1981, Hartman 1979). Provancha and Hall (1991) found spring aggregations of manatees feeding in seagrasses dominated by Syringodium filiforme (manatee grass) and reported an apparent preference for both Syringodium and Halodule wrightii (shoalgrass) over other seagrasses and macroalgae such as Caulerpa spp. Manatees continuously replace worn teeth. As anterior-most teeth wear and are lost, replacement teeth grow at the posterior end of the tooth row (Best 1981).
Competitors: Manatees do not strongly compete with other herbivores, likely due to their flexibility in making food choices based on availability (Best 1981).
Predators: Manatees have few known predators, but all members of the Sirenia have been hunted for food (Rathbun 1984; Best 1981; Domning 1978), with some populations, especially in the southern hemisphere, still noted to be at risk from poaching and subsistence hunting (IUCN 2006, Jiménez 2002; Domning 1982). A related species, Steller's sea cow (Hydrodamalis gigas), was a 25-foot kelp-feeder that inhabited the Bering Sea. It was hunted to extinction by fur seal hunters within 25 years of its discovery in 1741 (Hartman 1979; Marmontel et al. 1997; Domning 1978).
In Florida, evidence shows manatees were hunted by pre-Columbian societies. After Spanish occupation of Florida, the increase in human population increased hunting pressures on the manatee, heavily impacting population levels. Commercial and subsistence hunting during the 1800s also significantly reduced the population in Florida. In 1893, the state of Florida passed legislation banning the killing of manatees; they have been protected since that time (USFWS 2001).
Parasites: Manatee skin supports a variety of parasitic or commensal organisms. Ectoparasites and commensals include the copepod Harpacticus pulex; Lyngbya, a blue-green algae; diatoms, balanid barnacles, protozoans, nematodes, isopods, small gastropods, and leeches (Hartman 1979). Endoparasites include digenetic trematodes and a variety of nematodes (Hartman 1979).
Habitats: Manatees are habitat generalists that utilize canal systems, mangrove creeks, saltmarshes, estuaries, bays and nearshore coastal waters. They move freely between habitat types and regularly seek out fresh water sources for osmoregulation (Haubold et al. 2006; Lefebvre et al. 2001; Hartman 1979).
Manatees typically inhabit waters less than 3.7 m (12 ft) in depth (Haubold et al. 2006). Hartman (1979) reported manatees in Western Florida generally inhabited depths of 1.5 - 2 meters (4.9 - 6.6 ft), and observed that manatees tended to avoid waters less than 1.5 m deep unless these areas had access to deeper waters nearby. Feeding is sometimes observed on grassflats less than 50 cm (19.7 in) deep. Estrous females often escape the attentions of following males by moving into waters as shallow as 60 cm (23.6 in), with some even stranding themselves for brief periods.
Hartman (1979) reported that fast moving currents, generally over 5 km/hr. (3 miles/hr), discourage manatees from occupying certain areas. In Sebastian Inlet, currents can exceed 11 km/hr. (6.8 miles/hr) at certain points during the day. These strong currents likely deter manatees from entering or exiting the inlet during periods of peak velocity. Rather, manatees may swim adjacent to the shoreline, or slip in or out of the inlet during slack tides.
Activity Time: Manatees are generally arrhythmic (Hartman 1979), with feeding, resting, traveling, socializing, and other activities showing no consistent differences between day and evening hours. Feeding occurs over 1 - 2 hour periods and totals 6 - 8 hours daily. Socialization among manatees is highly variable, but is apparently higher in winter when animals aggregate in warm-water refuges. Though manatees spend much of their days seeking out food, sleeping, or traveling (Best 1981), they also spend approximately 6 - 10 hours at rest with no apparent pattern their activities.
Associated Species: Observations of manatee behavior show that manatees largely ignore the presence of other animals, though they are sometimes startled by them (Hartman 1979).
Special Status: As of 1996, the International Union for the Conservation of Nature and Natural Resources (IUCN), which maintains the IUCN Red List of species threatened with extinction, listed the Florida manatee as Vulnerable to extinction based on habitat loss or degradation; harvesting for subsistence, materials or medicinal purposes; accidental mortality; and pollution effects. However, a more recent assessment, completed in 2005, proposes to list the manatee as Endangered. This assessment has not, to date, been accepted.
The U.S. Fish and Wildlife Service first listed the manatee as an Endangered species in 1967, predating implementation of the Endangered Species Act of 1973. This initial listing and subsequent designation of critical manatee habitat was historic in that it was one of the first designations of critical habitat for an endangered species, and the first for any marine mammal. Manatees are also federally protected under provisions of the Marine Mammal Protection Act of 1972.
Developed in 1980, the initial Federal recovery plan for manatees covered both subspecies of Trichechus manatus (the Florida manatee and the Antillean manatee, which occurs in Puerto Rico and the U.S. Virgin Islands). This recovery plan was revised in 1989 to focus solely on Florida manatees. Subsequent revisions of this manatee recovery plan occurred in 1996 and 2001.
The state of Florida's Fish and Wildlife Conservation Commission downlisted the manatee from Endangered to Threatened status on June 7, 2006 under newly developed criteria for listing imperiled species.
Why are manatees endangered?
Trichechus manatus latirostris is endangered due to the many near- and long-term effects of human activities on population growth and adult survival. Habitat loss as the result of unconstrained coastal development is a major consideration, as are human-related factors such as death due to watercraft collisions, entrapment in flood gates and canal locks, entanglement in gear or lines, etc. Beyond human-related causes, natural factors such as cold stress, depressed rates of survival in young manatees, red tides, and stochastic events all contribute to annual losses to the population that may be unsustainable over the long term.
Because manatees are long-lived, their natural history and population biology operate over extended time periods (Marmontel et al. 1997). The manatee is characterized as a species with a low maximum rate of potential increase (USFWS 2001). It is slow to mature, generally produces only one offspring per pregnancy, has long periods of offspring dependency, and long interbirth periods. Thus, like other large mammals, for populations to remain stable over time, high rates of adult survival are vital.
Survival rates, however, are depressed in both young manatees and in adults. Marmontel et al. (1997) analyzed population viability in the manatee and noted survival rates between 1976 - 1991 were low among the youngest calves, with half of the manatee carcasses recovered recovered under the Florida Fish and Wildlife Research Institute's Marine Mammal Rescue and Recovery Program belonging to Age Classes 0, 1 and 2. Under the provisions of this program, manatee carcasses are recovered and transported to the State's Marine Mammal Pathobiology Laboratory in St. Petersburg for necropsy and determination of cause of death. Using data from 1974 - 2005, Florida Marine Research Institute (FWRI 2006) reported 5,725 manatee carcasses recovered statewide. Of this, 1,199 (21%) deaths occurred in manatees measuring less than 5 feet in length.
From 1985 - 2005, there were a total of 4,850 manatee carcasses recovered in Florida. Table 1 and Figure 2 below show the mortality categories and numbers of documented manatee deaths in each category. Note that when a cause of death could be assigned, human activities account for the greatest proportion (29.8%) of total documented manatee mortality, with watercraft collisions (24.7%) having the most detrimental effects.
Figure 3 below shows documented annual manatee mortality in Florida for the years 1985 - 2005. As shown, there has been a clear increase in the number of manatee deaths. USFWS (2001) reported a 6.0% annual increase in manatee deaths between 1976 - 2000. Note that 1996 was the deadliest year for manatees, followed by 2005 and 2003. In 1996 and 2005, red tides may have increased mortality to greater levels than was observed in other years (FWRI 2006 press release). A large red tide outbreak on Florida's west coast killed 151 manatees in 1996, while red tides were believed to be responsible for the deaths of 81 manatees in 2005.
However, most of the increase in manatee mortality is attributable to increased watercraft-related death and perinatal death (USFWS 2001; Marine Mammal Commission 1993).
The Importance of Watercraft-Related Mortality:
Because the manatee is slow to mature and has a low reproductive capacity, population stability and growth are linked closely to adult survival rates. Marmontel et al. (1997) reported that manatee survivorship curves resembled those of exploited populations and explained this observation as being due primarily to human-related causes that account for 30% or more of total documented mortality annually. Watercraft collisions alone account for 25% of annual mortality (FWRI 2006; Haubold et al 2006; FFWCC 2003; USFWS 2001; Marmontel et al 1997) and approximately 3.5 - 5.3% of the annual observed total population (Haubold et al. 2006). Many researchers (Haubold et al. 2006; USFWS 2001; Marmontel et al. 1997; Wright et al. 1995) have emphasized that besides habitat protection, population stability and growth can be achieved by reducing watercraft-related deaths. However, all indicators show that on an annual basis, there is likely to be even greater watercraft pressure on manatee populations as Florida's human population grows and the number of watercraft increases.
Florida's human population has increased 130% since 1970, and will exceed 18 million by 2010 (USFWS 2001; Bureau of Economics and Business Research 1993). Net immigration of new permanent residents to Florida is approximately 760 persons per day, with 80% of immigrants choosing to settle within 16 km (7 miles) of a coast (Bureau of Economics and Business Research 1993). Haubold et al. (2006) noted that by 2004, the number of registered boats in Florida had doubled since 1980 to over 982,000, excluding those that are brought to Florida by seasonal visitors and tourists. As of December 2005, the number of registered vessels in all size classes in Florida was 1,010,370 (FDHSMV 2005), approximately 1 vessel for every 17 persons. Wright et al. (1995) projected that this number is expected to increase by approximately 2.9% each year, and will continue to increase in concert with increases in Florida's human population.
Ackerman et al. (1995) reported that boating-related mortality in manatees increased by 10.3% annually since 1976. Additionally, the sub-lethal effects of watercraft collisions are also cause for alarm. Based on a photo identification database, more than 1,000 Florida manatees are documented to have at least one healed scar caused by a boat strike. Moreover, 97% of manatees in this sample bear scars from multiple collisions (O'shea et al. 2001; Wright et al 1995; Beck and Reid 1995).
IRL Manatee Mortality Statistics:
In the five-county area of the Indian River Lagoon, manatee mortality from 1985-2005 totaled 1,414 animals. A disproportionately high amount of total mortality occurred in Brevard County, with 896 manatee deaths (63%). Volusia County followed distantly with 222 deaths (16%). Martin County accounted for 134 deaths (9%), while deaths in Indian River and St. Lucie counties numbered 104 (7%) and 58 (4%) respectively. Figure 3 and Table 2 below illustrate total manatee mortality in IRL counties from 1985 - 2005.
As in other areas, watercraft collisions are the leading factor in manatee deaths in IRL counties, with Brevard County accounting for 57% of watercraft-related deaths in the IRL, and leading all Florida counties in the number of watercraft-related manatee deaths, as is shown in Table 3 below.
|County||Watercraft Mortality |
1985 - 2005
Management and Recovery Plans:
Marmontel et al. (1997) stressed that manatee management and recovery efforts must focus on retaining and improving the conditions under which manatee demography operates, with research and management priorities centered on habitat protection and reduction of adult mortality.
Trichechus manatus latirostris is Federally listed as an Endangered species under the Endangered Species Act of 1973, and was downlisted in Florida to Threatened status as of June 7, 2006.
The most significant and controllable threat to manatee recovery remains human-related mortality, specifically boat strikes that cause death or debilitating injury (USFWS 2001). As a result, the challenge for managers has increasingly become how to modify human, not manatee, behavior (Reynolds 1999). The USFWS and FFWCC will both continue to evaluate needs for additional protection areas that may be necessary to achieve manatee recovery, with the goal of considering manatee needs at the ecosystem-level, thus assuring that protection regulations as well as quality and quantity of habitat are sufficient to ensure recovery of the species.
Recently, both the USFWS and FFWCC have used targeted enforcement strategies in an attempt to increase boater compliance with manatee protection areas and slow-speed zones, which ultimately will reduce manatee injuries and deaths. Current priority actions in manatee conservation and protection include improved boater education, increased enforcement of existing regulations, improved maintenance of signs and buoys, compliance assessment, and periodic re-evaluation of the effectiveness of the rules. However, increased protection and enforcement efforts have generally met with vehement opposition by recreational boaters, boat manufacturers, legislators, and development interests throughout Florida.
Another significant problem for managers is ensuring the stability and longevity of warm-water winter refuges. Historically, manatees relied on warm temperate waters of south Florida and on natural warm-water springs throughout their range as winter refuges. However, with the increased construction of power plants and other industrial plants discharging thermal effluents, manatees expanded their winter ranges to include these sites as well. Currently, approximately 66% of the total manatee population relies on industrial sites for winter refuge from cold waters. Should the stability of these sites come into question, as could occur as the power industry continues to deregulate, then manatees in these areas could become vulnerable to escalating mortality rates.
Intensive coastal development throughout Florida poses an additional long-term threat to the Florida manatee (USFWS 2001). There are three major approaches to address this problem.
First, the USFWS, and the state agencies of Florida, Georgia, and the Gulf states, as well as other regional recovery partners, continue to review and comment on permit applications for construction projects that could occur in manatee habitat areas so that detrimental impacts can be minimized.
Second, coastal counties in Florida have been required to develop their own plans for manatee protection. Third, habitat protection is being accomplished through land acquisition. Both the USFWS and the state of Florida have taken steps to acquire new areas of critical manatee habitat for protected areas. The state of Florida has acquired important areas through several programs, most notably the Florida Forever Program (USFWS 2001).
The USFWS Manatee Recovery Plan (2001) sets the long-term goals of minimizing the causes of manatee disturbance, harassment, injury and death; determining and monitoring the status of the manatee population; identification, protection, evaluation and monitoring of critical manatee habitats; facilitation of manatee recovery efforts through improved public awareness and education. There are also threats from natural events such as red tides and cold events that may require additional efforts. It is anticipated that full recovery may not be possible for another 14 years or more.
Federal downlisting of the manatee will occur when the following conditions are met:
1. Threats to manatee habitat or range, as well as threats from
natural or human-made factors are reduced; minimum spring flows
in warm-water refuges are identified; critical foraging habitats are
protected; and human-caused deaths are reduced.
2. In each of the 4 regional subpopulations over 10 years, the
following benchmarks are achieved: the average adult survival rate
is 90% or greater; there is statistical confidence that the average
number of adult females accompanied by first- or second-year
calves in winter is at least 40%; and there is statistical confidence
that the annual rate of population growth equals or is greater than
State of Florida:
In Florida, new criteria for evaluating imperiled species were implemented in 2005. Under this new process, imperiled species must qualify under at least one of the criteria listed in Table 5 below:
In August 2001, the Coastal Conservation Association petitioned the Florida Fish and Wildlife Conservation Commission to assess the Endangered status of Trichechus manatus latirostris with a view to delisting it as an endangered species. Manatee status was reviewed, but in December 2003, FFWCC postponed most listing decisions to re-evaluate the listing process. This new process was adopted in April 2005, and the manatee was reviewed under the criteria for this new process.
It was found that though manatee numbers appear to have risen over the last several decades, many human-related (habitat loss, coastal development, watercraft-related mortality) and natural threats (uncertainty of future warm-water refuges, red tide events, hurricanes and other stochastic events) to the population remained.
Under Criterion A, which addresses population reduction, it was found that the manatee had a 12.1% probability of a 50% population reduction within 3 generations; and a 46.5% probability of a 30% population reduction in 3 generations. It thus qualified as a Threatened species under Criterion A.
Criterion B concerns the extent of occurrence and area of occupancy. It was found that the extent of manatee occurrence is approximately 7,500 square miles, with an area of occupancy of 100 - 300 square miles. Manatees met the condition of a decline in area of occupancy, but the Biological Review Committee did not feel that manatees occur only in a limited number of locations, nor was their habitat severely fragmented. Hence, manatees did not qualify under Criterion B.
Criterion C addresses population size and population trends. The total minimum population size for the manatee was calculated to be 2,310 adults, with no evidence of extreme fluctuation in population numbers, and no subpopulation contained greater than 90% of the total population. However, the probability of a 20% reduction in population size within 2 generations was calculated to be 55.5%; and there was a 77.1% probability of a 10% population reduction. Manatees thus qualified as Threatened under Criterion C.
Criterion D concerns the number of mature individuals in the population. With a minimum adult population size of 2,310, and an area of occupancy of 100 - 300 square miles, the manatee did not qualify under Criterion D.
Criterion E addresses the probability of imminent extinction. It was calculated that manatees had an overall probability of extinction in the next 100 years of only 1%. Further, the total population is not likely to approach any of the qualifying thresholds outlined in the listing criteria; though one of the 4 subpopulations would qualify for listing if the subpopulations were considered separately. Thus, manatees did not meet the listing qualifications under Criterion E.
On June 7, 2006, the state of Florida downlisted the manatee to Threatened status under Criteria A and C above.
Economic Importance: As a protected species, the manatee has no direct commercial importance. Indirectly, manatees have an influence of Florida's ecotourism industry and adjunct enterprises, which are valued at approximately $1.8 billion annually (IFAS 2006).
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