Overview

Comprehensive Description

The Florida manatee is a long-lived marine mammal, dark gray in color, and averaging about 10 feet (3 m) in length and between 800 and 1,200 pounds (363 to 544 kg) in weight. It has a round, flattened, paddle-shaped tail and two front flippers that are used for steering while swimming (Federal Register, 75 FR 1574-1579).

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Distribution

Winter range generally confined to warm water effluents and Southern Florida; summer range includes confirmed sightings from Massachusetts to Texas (coastal) and up the Mississippi as far as Memphis, Tennessee. Also, confirmed travel by known individuals to Cuba and the Bahamas.
  • North-West Atlantic Ocean species (NWARMS)
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Florida manatees constitute the largest known group of West Indian manatees anywhere in the species’ range. Outside the United States, manatees occur in the Greater Antilles, on the east coast of Mexico and Central America, along the north and northeastern coast of South America, and in Trinidad (Lefebvre et al. 2001, cited in USFWS 2001).

In the southeastern United States, manatees occur primarily in Florida and southeastern Georgia, but individuals can range as far north as Chesapeake Bay or even Rhode Island on the Atlantic coast, and probably as far west as Texas on the Gulf coast (USFWS 2001). In summer, is now not uncommon to find manatees in coastal waters of Georgia, North and South Carolina, Alabama, and Louisiana (Federal Register, 75 FR 1574-1579).

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Physical Description

Morphology

The following is derived from the 2001 Florida Manatee Recovery Plan (USFWS 2001, which should be consulted for original references): Florida manatees are massive spindle-shaped animals (wide in the middle and tapering at both ends) with skin that is uniformly dark gray, wrinkled, sparsely haired, and rubber-like. Manatees have paddle-like forelimbs, no hind limbs, and a spatulate, horizontally flattened tail. Females have two axillary mammae, one at the posterior base of each forelimb. Their bones are massive and heavy with no marrow cavities in the ribs or long bones of the forearms. Adults average about 3.0 m (9.8 ft) in length and 1,000 kg (2,200 lbs) in weight, but may reach lengths of up to 4.6 m (15 ft) and weigh as much as 1,620 kg (3,570 lbs). Newborns average 1.2 to 1.4 m (4 to 4.5 ft) in length and about 30 kg (66 lbs). The nostrils, located on the upper snout, open and close by means of muscular valves as the animals surface and dive. A muscular flexible upper lip is used with the forelimbs to manipulate food into the mouth . Bristles are located on the upper and lower lip pads. Molars designed to crush vegetation form continuously at the back of the jaw and move forward as older ones wear down. The eyes are very small, close with sphincter action, and are equipped with inner membranes that can be drawn across the eyeball for protection. Externally, the ears are minute with no external ear flaps.

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Ecology

Habitat

Habitat and Ecology

Systems
  • Freshwater
  • Marine
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The Florida manatee lives in freshwater, brackish, and marine habitats, including coastal tidal rivers and streams, mangrove swamps, salt marshes, and freshwater springs. Submerged, emergent, and floating vegetation are their preferred foods. During the winter, cold temperatures keep the population concentrated in peninsular Florida and many manatees rely on the warm water from natural springs and power plant outfalls. During the summer they expand their range and on rare occasions are seen as far north as Rhode Island on the Atlantic coast and possibly as far west as Texas on the Gulf coast (USFWS 2001, 2007).

The following summary of habitat use by the Florida manatee is based on the review in the West Indian Manatee 5-Year Review (USFWS 2007 and references therein): Shallow grass beds, with ready access to deep channels, are generally preferred feeding areas in coastal and riverine habitats. In coastal Georgia and northeastern Florida, manatees feed in salt marshes on smooth cordgrass (Spartina alterniflora) by timing feeding periods with high tide. Manatees use springs and freshwater runoff sites for drinking water; secluded canals, creeks, embayments, and lagoons for resting, cavorting, mating, calving and nurturing their young; and open waterways and channels as travel corridors. Manatees occupy different habitats during various times of the year, with a focus on warm-water sites during winter. Manatees have adapted to changing ecosystems in Florida. Industrial warm-water discharges and deep-dredged areas are used as wintering sites, stormwater/freshwater discharges provide manatees with drinking water, and the imported exotic plant, Hydrilla sp. (which has replaced native aquatic species in some areas), has become an important food source at wintering sites.


Historically, manatees relied on the warm, temperate waters of south Florida and on natural warm-water springs scattered throughout Florida as buffers against the lethal effects of cold winter temperatures. In part, as a result of human disturbance at natural sites, they have expanded their winter range to include industrial sites and associated warm-water discharges as refuges from the cold. Although manatees overwinter at major springs throughout peninsular Florida, nearly two-thirds of the population winters at industrial warmwater sites, which are now made up almost entirely of power plants.

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Depth range based on 6735 specimens in 1 taxon.
Water temperature and chemistry ranges based on 82 samples.

Environmental ranges
  Depth range (m): 0 - 0
  Temperature range (°C): 25.997 - 26.823
  Nitrate (umol/L): 0.384 - 0.740
  Salinity (PPS): 35.999 - 36.175
  Oxygen (ml/l): 4.582 - 4.769
  Phosphate (umol/l): 0.076 - 0.101
  Silicate (umol/l): 1.289 - 2.334

Graphical representation

Temperature range (°C): 25.997 - 26.823

Nitrate (umol/L): 0.384 - 0.740

Salinity (PPS): 35.999 - 36.175

Oxygen (ml/l): 4.582 - 4.769

Phosphate (umol/l): 0.076 - 0.101

Silicate (umol/l): 1.289 - 2.334
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

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Trophic Strategy

Seagrasses appear to be a staple of the manatee diet in coastal areas. When feeding in coastal seagrass beds, manatees may either consume virtually the entire plant or consume only the exposed grass blades without disturbing the roots or sediment. Manatees may return to specific seagrass beds to graze on new growth. Favored foods include manatee grass (Syringodium filiforme) and shoalgrass (Halodule wrightii), which may be preferred over the macroalga Caulerpa spp. Along the Florida-Georgia border, manatees feed in salt marshes on smooth cordgrass (Spartina alterniflora) by timing feeding periods with high tide (USFWS 2001 and references therein).

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Population Biology

Obtaining accurate estimates of the total number of Florida manatees has been technically challenging. Best estimates from the 1980s suggested a total population of at least 1200 individuals. Beginning in 1991, the Florida Department of Natural Resources (FDNR) [now the Florida Fish and Wildlife Conservation Commission (FWC)] initiated a statewide aerial survey program to count manatees in potential winter habitat during periods of severe cold weather. These surveys, known as the synoptic surveys, involve a coordinated series of statewide aerial surveys and ground counts of manatees at warm-water refuges and adjacent areas. They are carried out one to three times each winter and are much more comprehensive than those used to estimate a minimum population during the 1980s. The highest two-day minimum count of manatees from these surveys tallied approximately 3,300 manatees in January 2001, but it remains uncertain what proportions of the total manatee population was actually counted (USFWS 2001). Although surveys have been conducted more recently than 2001, the weather conditions for that particular survey were ideal. As a result, the count from that year may provide the best minimum population estimate (USFWS 2007).

On a far more limited basis, it has been possible to monitor the number of manatees using the Blue Spring and Crystal River warm-water refuges. At these locations, numbers have clearly increased since the early 1970s (USFWS 2001 and references therein).

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Life History and Behavior

Life Expectancy

Maximum determined age for a West Indian manatee is 59 years (O'Shea et al. 1995, cited in USFWS 2001).

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Reproduction

The reproductive biology of the Florida manatee was summarized in the 2001 Recovery Plan (USFWS 2001 and references therein). Breeding takes place when one or more males are attracted to an estrous female to form an ephemeral mating herd (ranging in size from about 5 to 22 individuals). The highest frequency of mating herds is from February to July and mating herds can persist up to 4 weeks, with different males joining and leaving the herd daily. Permanent bonds between males and females do not form. During peak activity, the males in mating herds compete intensely for access to the female. Successive copulations involving different males have been reported. Some observations suggest that larger, presumably older, males dominate access to females early in the formation of mating herds and are responsible for most pregnancies, but males as young as three years old may produce sperm. Although breeding has been reported in all seasons, studies of reproductive organs from carcasses of males found evidence of sperm production in 94% of adult males recovered from March through November, but in only 20% of adult males recovered from December through February. Females appear to reach sexual maturity by about age five but have given birth as early as four, and males may reach sexual maturity at 3 to 4 years of age. Manatees may live in excess of 50 years and evidence for reproductive senescence (reduced reproduction rate with age) is unclear. The length of the gestation period is uncertain but is thought to be between 11 and 14 months. The normal litter size is one (with an equal sex ratio at birth); twins are rare (less than 2% of pregnancies) and the largest number of births occur between May and September. Calf dependency usually lasts one to two years after birth. Calving intervals vary greatly among individuals. They are probably often less than 2 to 2.5 years, but may be considerably longer depending on age and possibly other factors. Females that abort or lose a calf at birth may become pregnant again within a few months or even weeks.

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Evolution and Systematics

Evolution

Systematics and Taxonomy

The West Indian manatee, Trichechus manatus Linnaeus, 1758, is one of four living species of the mammalian Order Sirenia. The other three sirenians are the West African manatee (T. senegalensis), the Amazonian manatee (T. inunguis), and the dugong (Dugong dugon). All four species are aquatic herbivores listed as endangered or threatened throughout their ranges by the U.S. Fish and Wildlife Service (USFWS 2001). A fifth species, Steller’s sea cow (Hydrodamalis gigas), once lived in sub-Arctic waters of the Bering Sea. Steller’s sea cow was a toothless sirenian that fed on kelp and reached lengths of up to 8 m (26 ft). It was hunted to extinction within 27 years of its discovery in 1741 (Reynolds and Odell 1991, cited in USFWS 2001).

Two subspecies of West Indian manatee are now recognized (Domning and Hayek 1986, cited in USFWS 2001; USFWS 2001, 2007): the Florida manatee, T. manatus latirostris, which occurs in the southeastern United States, and the Antillean manatee, T. manatus manatus, found throughout the remainder of the species’ range. The Florida manatee was originally described as a distinct species, Manatus latirostris, but for many decades now has been recognized as a subspecies of T. manatus. The historical ranges of the two subspecies may overlap on the coast of Texas, where the origin of occasional strays (from Florida or Mexico) is uncertain.

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Conservation

Conservation Status

IUCN Red List Assessment


Red List Category
EN
Endangered

Red List Criteria
C1

Version
3.1

Year Assessed
2008

Assessor/s
Deutsch, C.J.

Reviewer/s
Reynolds III, J.E. & Powell, J.A. (Sirenia Red List Authority)

Contributor/s

Justification
The Florida manatee subspecies is listed as Endangered on the basis of a population size of less than 2,500 mature individuals and the population is estimated to decline by at least 20% over the next two generations (estimated at ~40 years) due to anticipated future changes in warm-water habitat and threats from increasing watercraft traffic over the next several decades.

History
  • 2007
    Endangered
  • 1996
    Vulnerable
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Population

Population Trend
Stable
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According to the USFWS (2007), the best available data suggest that manatees are increasing or stable throughout most (but not all) of their range in Florida.

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Threats

The most significant problem presently faced by Florida manatees is death or injury from boat strikes (USFWS 2001; 2007 and references therein), both direct impacts and propeller cuts. Collisions with watercraft accounted for an average of 24% of known manatee deaths in Florida annually (1976-2000), with 30% in 1999 and 29% in 2000. The second major concern is the long-term availability of warm-water refuges for manatees, which is uncertain if minimum flows and levels are not established for the natural springs on which many manatees depend, and as deregulation of the power industry in Florida occurs (manatees take advantage of warm power plant outfalls) (USFWS 2001; 2007 and references therein). Deaths attributed to water control structures and navigational locks (e.g., entrapment and crushing in gates or locks) represents 4% of known deaths (USFWS 2001) and other threats, such as entanglement in fishing gear, appear to be relatively minor.

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Legislation

The Florida manatee is protected by federal law under both the U.S. Endangered Species Act of 1973 (it had been previously listed as endangered in 1967 under the Endangered Species Preservation Act of 1966) and the Marine Mammal Protection Act of 1972. In 1893, the State of Florida passed legislation prohibiting the killing of manatees (USFWS 2001). In 1979, the Florida Game and Freshwater Fish Commission listed the Florida manatee as an endangered species. In 2007, after much public debate, the Florida Fish and Wildlife Conservation Commisission voted to defer a proposal to reclassify the Florida manatee from "endangered" to merely "threatened" on the state imperiled species list.

Critical habitat was designated for the Florida manatee on September 24, 1976 (Federal Register, 41 FR 41914). (The term "critical habitat" under the U.S. Endangered Species Act has specific technical legal meaning, but in essence it is habitat that is deemed essential to the conservation of a species, whether or not it is currentlly utilized by that species.) This designation delineated specific waterways in Florida that were known to be important concentration areas for manatees at that time. In 2008, a coalition of several environmental organizations filed a legal petition with the USFWS requesting a revision of the current critical habitat designation to reflect current regulations and guidelines regarding procedures for critical habitat designation, as well as much new information acquired during the past several decades about manatee distribution, habitat use, and habitat needs, such as the use of natural warm-water sites and power plant discharges. On January 12, 2010, the USFWS released its formal finding agreeing that a revision of the current critical habitat designation was warranted, but stating that such a revision could not be completed in the foreseeable future due to a lack of resources (Federal Register, 75 FR 1574-1579).

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Management

The U.S. Fish and Wildlife Service (USFWS) developed the initial recovery plan for the West Indian manatee in 1980. This initial plan focused primarily on manatees in Florida, but included Antillean manatees in Puerto Rico and the U.S. Virgin Islands as well. In 1986, USFWS adopted a separate recovery plan for manatees in Puerto Rico. To reflect new information and planning needs for manatees in Florida, USFWS revised the original plan in 1989 and focused exclusively on the Florida manatee. This first revision covered a 5-year planning period ending in 1994. USFWS revised and updated the plan again in 1996, covering a 5-year planning period ending in 2000. In 1999, USFWS initiated the process to revise the plan for a third time. An 18-member recovery team (consisting of representatives of the public, agencies, and groups that have an interest in manatee recovery and/or could be affected by proposed recovery actions) was established to draft this revision (USFWS 2001).

Outside the United States, remaining populations of the West Indian manatee are believed to be much smaller than the U.S. population and are subject to poaching for food, incidental take in gill nets, and habitat loss. Manatee protection programs in many countries are not well organized or supported and, in this context, protection of the Florida population takes on international significance (USFWS 2001).

Because watercraft operators cannot reliably detect and avoid hitting manatees, federal and state managers have worked to limit watercraft speed in areas where manatees are most likely to occur to give both manatees and boaters time to avoid collisions. Two types of manatee protection areas also have been developed by USFWS: (1) manatee sanctuaries; and (2) manatee refuges. Manatee sanctuaries are areas in which all waterborne activities are prohibited, while manatee refuges are areas where certain waterborne activities are restricted or prohibited. Efforts have been made to target enforcement in an attempt to increase boater compliance with speed zones and ultimately reduce manatee injuries and death. Along with habitat management and other efforts, public awareness waterway signs and other boater education efforts are an essential component of manatee population management (USFWS 2001).

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