West Indian Manatee

Trichechus manatus

Description

The West Indian manatee (Trichechus manatus ), a massive aquatic mammal, attains a length in excess of 12 ft (3.7 m) and may weigh up to 3,500 lb (1,587.6 kg). Its clumsy appearance belies its agility in the water. It has no hindlimbs but features flippered forelimbs and a spatula-shaped tail that forms a rounded rudder with a boneless medial ridge. Its skin is nearly hairless; its muzzle bristles with stiff whiskers. Front teeth are lacking. The upper lip is divided and can be used for grasping food. Sexes are distinguished by the position of the genital openings and presence or absence of mammary glands.

Also known as the Florida manatee or the Florida sea cow, the West Indian manatee belongs to an order of mammals known as Sirenia that also includes the dugong. A third member of the order—Steller's sea cow—was hunted to extinction in the eighteenth century.

Behavior

The manatee may live as long as 50 years. It feeds on a wide variety of floating and submerged aquatic plants, varying its diet according to plant availability. It will sometimes also feed on shoreline vegetation and occasionally eats small fish. Manatees may spend about five hours a day feeding and may consume 4-9% of their body weight a day.

Without establishing territories, manatees browse slowly along a river or coastline and may cover 150 mi (241.4 km) in a summer. Some manatees migrate south to warmer waters in winter, returning to the same sites year after year.

The female manatee is fertile for two to four weeks, during which time she mates with several males. After a gestation period of 150-180 days, she usually bears a single calf. Births occur during all months of the year with a slight drop during winter months. Though single births are the norm, about 1.5% of births are twins. Calves reach sexual maturity at four (for females) to seven (for males) years of age. Mature females may give birth every two to five years. The only long-term, stable bond between manatees is that between a cow and her calf. Weaning generally occurs between nine and 24 months of age, although a cow and calf may continue to associate with each other for several more years. There is little information on the lifetime reproductive output of females.

Though mothers and calves are strongly bonded, other social bonds are more casual. In summer, loosely associated herds gather for feeding but may disperse after a few days or weeks. During winter, larger groups form in refuge waters. As many as 350 animals have been observed in these winter herds. Incapable of aggression when threatened, the manatee invariably retreats.

Habitat

The manatee is found in freshwater rivers or in salty and brackish coastal waters in shallow inlets, river estuaries, and bays. Individuals rarely venture far out into the open ocean. Between October and April, Florida manatees concentrate in areas of warmer water. When water temperatures drop below about 69.8-71.6°F (21-22°C), they migrate to south Florida or form large aggregations in natural springs and industrial outfalls. Severe cold fronts have been known to kill manatees when the animals did not have access to warm-water refuges. During warmer months they appear to choose areas based on an adequate food supply, water depth, and proximity to fresh water. Manatees may not need fresh water but they are frequently observed drinking fresh water from hoses, sewage outfalls, and culverts.

Distribution

The manatee was once numerous and widespread in rivers and along coasts throughout the subtropical United States, the Caribbean, and northeastern South America. It was hunted extensively in the eighteenth and nineteenth century and probably fell to an all-time low in Florida waters during the 1940s.

By the beginning of the twenty-first century, fewer than 3,000 manatees were swimming the waters of the southeastern United States, primarily in the coastal areas of Florida. The coastal rivers and ocean waters of the southeastern United States are the northern limit of the West Indian manatee's range. Although the population is more abundant to the south, both Caribbean and South American populations have been reduced by hunting. Numbers in U. S. waters increased to slightly more than 1,000 in the 1970s and have been climbing slowly.

In the summer, manatees are found along the southeastern Atlantic Coast, occasionally as far north as the Chesapeake Bay. Florida waters support year-round populations along both coasts and provide refuges for migrating animals in winter.

Between 1976 and 1985 over a dozen aerial surveys were conducted to determine the distribution of the Puerto Rican manatee population. Counts from these surveys ranged from 44-62 animals per flight, with most animals concentrated along the southern shore, at the eastern end of the island, and around Vieques Island. No manatees were observed along the northwestern coast. These surveys were designed primarily to locate manatee concentrations; counts do not reflect the actual number of animals.

The population of manatees in Florida has been estimated to be 2,000-2,500 individuals. There are about 60-100 manatees in Puerto Rico. The captive population has gradually increased due to the growing number of injuries, orphaned manatees, and captive births. Sea World of Florida (Orlando), Miami Seaquarium, Lowry Park Zoo (Tampa), Homosassa Springs State Wildlife Park, and Living Seas at EPCOT Center (Lake Buena Vista) were caring for more than 50 manatees by late 1995.

Since the mid-1980s yearly mortality in Florida has averaged nearly 150 animals a year, double that of the preceding decade. In 1994 at least 193 manatees died from all causes, marking one of the worst years on record for the beleaguered population. Starting in 1996 a dramatic increase was noted in manatee deaths, possibly caused by a disease or contaminant, and this became a cause for concern. The average proportion of first-year calves in the population was 10% with a range of 5-15%.

Threats

In the past, manatees were hunted for sport, for oil, or for their skins. At the beginning of the twenty-first century there was no commercial trade in manatees other than supplying animals for zoos, although they were hunted for food in some parts of the Caribbean and in South America. Poaching remains the greatest threat to the Puerto Rican manatee population. The recovery effort in Puerto Rico is focused on determining abundance and distribution, documenting causes of mortality, and enforcing existing laws against poaching.

Residential development along Florida's rivers and waterways has added considerable stress to the manatee's habitat. Perhaps more immediately life-threatening to the animal than development, however, is the tremendous increase in the number of recreational powerboats. More than 450,000 boats were registered in Florida in 1988, and the number of reported manatee deaths through collision doubled in the following years. Most captured and tagged animals are marked with the scars of encounters with speedboat propellers. The state of Florida long ago imposed fines and jail sentences for hunting manatees, but there seems little the state can do to stem deaths caused by powerboats outside of state refuges. The first refuge for manatees was established in the Everglades in 1948.

A new deadly threat became apparent in 1996. In a six-month period that year, more than 260 manatees—about 10% of the known U. S. manatee population—was found dead. Even more alarming was the fact that about 160 of those deaths occurred in southwest Florida between March and May. Researchers launched a major study to identify the cause of this deadly scourge; analyses of blood and tissue samples had yet to determine a cause for the deaths by June 1996, but scientists believed the likely cause was either a biological toxin (ala red tide), a disease caused by an unknown virus or bacterium, or a contaminant substance such as a pesticide. The deaths had slacked off by late May 1996, leading scientists to suspect the danger was only short-term. Studies are ongoing to determine the cause of the mass die-off, which was devastating to the species.

Conservation and Recovery

Considerable research has focused on the manatee population that winters at Crystal River on the Gulf Coast of Florida. Animals have been followed with aerial photography and radio telemetry, allowing biologists to define more closely their summer and winter ranges. This data, in turn, has allowed more effective protection of the manatee's habitat. Because of these efforts, the Crystal River population has increased from a low of about 50 in the late 1960s to a current size of more than 200. Survey and telemetry techniques developed at Crystal River are now being applied to manatee populations in the Caloosahatchee River.

In 1988 researchers from the U. S. Fish and Wildlife Service (FWS) National Ecology Research Center, the state Marine Research Laboratory, and the Beaufort Laboratory of the National Marine Fisheries Services began a study of manatee populations on Florida's Atlantic Coast. A coordinated data collection project was begun using radio telemetry, aerial surveys, food habits analysis, and necropsy (autopsy). Findings from this effort are preliminary but suggest that many accidental deaths can be prevented by slowing boat speeds and by establishing sanctuaries where the manatees naturally congregate.

Research has also been initiated in Georgia on the spring and summer manatee population found in Cumberland Sound. Using radio-tagged animals, researchers hope to determine the amount of time spent in the region, the areas of greatest use, feeding behavior, and subsequent migration patterns.

Based on revised 1989 recovery plan recommendations, the primary objective in the recovery of the Florida population of the West Indian manatee is to reestablish and maintain optimum sustainable populations in natural habitats throughout the manatee's historic range by 1) minimizing human-caused injuries and fatalities; 2) minimizing habitat destruction and over development; 3) minimizing harassment of manatees from boat and barge traffic, fishing, diving, and swimming; 4) determining and monitoring their population status, life history, and ecology; and 5) coordinating recovery activity implementation.

A recovery plan developed specifically for the manatee population in Puerto Rico indicates three primary objectives for recovery: 1) to identify, assess, and reduce human-related mortality, especially that related to gill net entanglement; 2) to identify and minimize alteration, degradation, and destruction of habitats important to the survival and recovery of the Puerto Rico manatee population; and 3) to develop the criteria and biological information necessary to determine whether and/or when to delist or downlist (reclassify to threatened) the Puerto Rican manatee population.

Among the innovative measures being used to track the movements of manatees is the use of satellite-monitored tracking devices. Chessie, a West Indian manatee fitted with satellite tracking gear, made the national news as interest arose about his travels from Florida to New England in the summers of 1995 and 1996.

The Department of the Interior's Sirenia Project has been conducting research on manatees in and around Florida since 1974. In 1992 the Sirenia Project teamed up with the Roosevelt Roads Naval Station, FWS Caribbean Field Office, Puerto Rico Department of Natural and Environmental Resources, and Caribbean Stranding Network (CSN) to radio-tag and monitor individual manatees using satellite-based techniques. With the addition of funds from the Department of Defense's Legacy Resource Management Program, the effort was the first radio-tagging of West Indian manatees outside the continental United States. The tracking efforts are yielding a great deal of data useful to biologists studying manatee range and use of habitat.

Another part of the manatee recovery effort involves a statewide partnership to rescue, rehabilitate, and—whenever possible—release manatees back into the wild. Private citizens, nonprofit organizations, businesses, and government agencies at all levels contribute to the rehabilitation effort. Manatee rescues in the Florida area are coordinated by the Florida Department of Environmental Protection, in cooperation with the Florida Marine Patrol and 11 organizations. Authorized participants in the rescue program respond to hundreds of reports of manatees in distress annually, and 20-30 animals are rescued for treatment each year.

The captive population has gradually increased due to the growing number of injuries, orphaned manatees, and captive births. Although many manatees have been released successfully, an acute crowding situation has developed at some facilities. In response, participants in the recovery program have developed an ambitious "soft-release" approach to introduce rehabilitated manatees to a seminatural environment, providing an intermediate phase between the captive facility and a truly wild habitat. The results of this approach may show whether long-term captives, orphaned, and possibly captive-born manatees can be integrated into the wild population. When possible, however, injured manatees judged suitable for direct release are still returned to the general vicinity of their rescue as soon as they are fit.

The FWS developed the first soft-release site in 1994 at Merritt Island National Wildlife Refuge near Cape Canaveral. In cooperation with the Kennedy Space Center, and with financial support from the 38,000-member Save the Manatee Club and the Florida Department of Environmental Protection (with money raised from sales of specialty license plates), three fenced enclosures covering 4.5 acres (1.8 hectares) were constructed in a seagrass bed. Manatees with minimal wild experience now can be introduced to a seminatural habitat and diet in a setting where managers can keep a close eye on their progress. This method shows great promise. Observations of feeding behavior, interaction with other manatees, and general activity levels are used to assess each manatee's acclimation to its new surroundings and its suitability for eventual release.

Evaluating the success of the soft-release approach will take time and a sustained effort by all parties involved. Many creative techniques are being employed on a case-by-case basis, and every trial provides new lessons. With the spirit of cooperation and determination shown by partners in the recovery effort, the long-term prospects for released manatees in the "real world" of Florida's busy waterways are looking brighter.

Contact

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
(404) 679-4000
http://southeast.fws.gov/

References

Ferrara, J. 1984. "Digging In." National Wildlife 22(2): 22-28.

Hartman, D. S. 1979. "Ecology and Behavior of the Manatee in Florida." American Society of Mammalogists : Special Publication Number 5.

U. S. Fish and Wildlife Service. 1980. "West Indian Manatee Recovery Plan." U. S. Fish and Wildlife Service, Atlanta.

U. S. Fish and Wildlife Service. 1985. "Recovery Plan for the Puerto Rico Population of the West Indian Manatee." U. S. Fish and Wildlife Service, Atlanta.