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
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.
II. HABITAT AND DISTRIBUTION
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).
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.
III. LIFE HISTORY AND
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).
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).
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.
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).
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).
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.
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
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.
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).
IV. PHYSICAL TOLERANCES
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.
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
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
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).
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.
V. COMMUNITY ECOLOGY
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
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
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, Lips then close laterally, the bristles
grasping the food and tucking it into the mouth (Hartman 1979).
In fresh water, manatees tend to favor submerged,
vascular plants such as Hydrilla verticilata, 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).
Manatees do not strongly compete with other herbivores, likely due
to their flexibility in making food choices based on availability
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 (Hartman1979;
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 and they have been protected
since that time (USFWS 2001).
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).
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 feet) 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 feet), 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 inches.) deep. Estrous females
often escape the attentions of following males by moving into waters
as shallow as 60 cm (23.6 inches), 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.
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.
Observations of manatee behavior show that manatees largely ignore
the presence of other animals, though they are sometimes startled
by them (Hartman 1979).
VI. 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.
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
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.
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.
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.
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.
Table 1. Causes of mortality in Florida
manatees recovered under the Marine Mammal Rescue and Recovery Program
showing total numbers of deaths in each category from 1985 - 2005.
Florida Fish and Wildlife Research Institute (2006).
Figure 2. Major causes of documented manatee mortality in
Florida from 1985 - 2005.
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
Figure 3. Total manatee mortality in
Florida by year from 1985 - 2005. Data from Florida Fish and
Wildlife Research Institute (2006).
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.
Figure 4. Watercraft collisions from
1985 - 2005. Data from Florida Fish and Wildlife Research
Institute (2006). Linear regression line is shown in red.
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
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 1000 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).
In the 5-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.
Figure 3. Manatee mortality
in the 5-county area of the IRL.
Causes of manatee mortality in IRL Counties from 1985 - 2005
with statewide totals given in each category.
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.
1985 - 2005
Florida Counties having the highest
watercraft-related manatee mortality from
1985 - 2005. Data from Florida Fish and
Wildlife Research Institute (2006).
Table 4 below shows annual total
mortality statistics by year for IRL Counties from 1985 - 2005.
Table 4. Annual manatee
mortality in IRL Counties from 1985 - 2005.
Note that Brevard County accounts for 46 - 78% of IRL mortality
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
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,
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
Another significant problem for
managers is the insuring 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).
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 man-made factors are reduced;
minimum spring flows
in warm-water refuges are identified; critical
foraging habitats are
protected; and human-caused deaths are
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
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:
Table 5. Listing criteria
for imperiled species under procedures adopted in April 2005 by
the Florida Fish and Wildlife Conservation Commission.
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 7500 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
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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