The Florida manatee (Trichechus manatus larirostris) is a sub-species of the West Indian manatee (Trichechus manatus), one of the three different species of manatees. Manatees are in teh order Sirenia along with their marine relatives the dugong and the now extince Stellar's sea cow. THe name Sirenia arose because manatees were mistaken for mermaids and the word siren was used to describe mythical mermaids(Bossart 1999).
Manatees have large gray bodies with a paddle-like tail that propels them at two to seven miles per hour and two flippers for steering which allow them to be agile and perform activities such as barrel rolling and body surfing. Full-grown manatees aberage nine to ten feet in length and weigh between 900 and 1200 pounds (Bossart 1999). Manatees are herbivores and spend most of their day eating or resting. In order to deal with the abrasive plants and sand that are part of their diet, manatees have adapted continouusly replacing molars which move from the back to the front of the mouth (SMC Handbook). Another adaptation to a diet of approximately 100 pounds of vegetation per day is a large digestive tract (Bossart 1999). Manatees eat plants that are above, in, or under the water. Seagrass beds are especially important to manatees and in some cases exotic plant species in Florida are the only food sources for manatees (SMC Handbook).
Manatees emit many sounds for communication, not echolocation, to express emotions including sexual arousal and fear. The sounds are used in communication to maintain contact betwen manatees, often a mother and her calf, during feeding or traveling (SMC Handbook). The hairs and bristles on a mantee have innervation for tactile experiences and feeding. This may be an adaptation for locating food and other objects because manatees lack echolocation (Reep and Marshall).
When manatees are ready to breed, they form mating groups. One female is followed by a group of approximately a dozen males, but no permenent bonds are formed. Manatees have one calf every three to five years, which is a low reproductive rate. This is partially due to the fact that a mother may nurse her calf for up to two years (SMC Handbook). Manatees can live for more than fifty years, and a long life is essential to maintain a population with such a low reproductive rate (Bossart 1999).
Manatees have a low metabolic rate, which prevents them fro maintaining their body temperature in colder water. Thus, manatee habitats are restricted to warm water. THe warmth may come from natural springs, power plants, or other industries, and manatees will reavel long distances to warm water in teh winter. Different species of manatees live in the Amazon River, the coastal areas of West Africa, and the waterways of Central and South America. In the United States, manatees live primarily in Florida (SMC Handbook). A characteristic unique to Florida manatees is their ability to live in freshwater, brackish, or marine habitats. They can live in clear, muddy, or semi-polluted water because of their strong immune systems (Bossart 1999).
The size of the wild manatee popluation is determined by an aerial survey, and the last count, in March ot 1999, found 2,353 manatees, so the population is endangered (SMC). Manatees are protected by the U.S. Endangered Species Act of 1973, the U.S. Marine Mammal Protection Act of 1972, the Florida Endangered and Threatened Species Act of 1977, and the Florida Manatee Sanctuary Act of 1978 (Marmontel et al. 1997). Even with this protection, manatees continue to die of natural and human-related causes.
According to teh Save the Manatee Club, 69% of manatee deaths occur form natural of undetermined causes. Natural mortality includes perinatal deaths and deaths caused by red tide, diseases, and cold weather. Red tide describes blooms of a type of algae called dinoflagellates. The algae produce a neurotoxin which affects the brain and central nervous system. These toxins accumulate in organisms that are ingested by manatees, so their diet makes it difficult to avoid red tide (SMC Handbook). In 1996, a mysterious respiratory syndrome killed many manatees. The pneumonia-like disease caused their lunge to become inflamed and bleed, and the cause fo the disease was unknown (Luoma 1996). Because of manatees' low metabolic rate, cold weather can be fatal. When the water temperature remains cold (lower than 68 degrees Farenheit) for long periods of time, the manatees cannot tolerate the temperature. Thus, manatee mortality increases during unusually cold Florida winters (SMC Handbook).
Natural mortality is detrimental to manatee populations, but Florida Manatees are endangered because of human-related mortality, which accounts of 31% of mortality according to the Save the Manatee Club. Water craft collisions, loss of habitat, pollution, litter, and flood control structures are all sources of human-related mortality.
Collisions with boats are the largest cause of human-related mortality. These deaths occur because manatees usually stay in water that is three to seven feet deep, they move slowly, and the must surface for air. Boats kill manatees by sharp cuts from propellers or by striking them with the boat hull (Bossart 1999). If a manatee is surfacing for air and struck by a boat, the manatee can sink and drown (Luoma 1996).
Loss of habitat is a serious threat to manatees. As the population of Florida continues to grow, manatee habitats are destroyed. The economy of Florida relies on development and water recreation industries, which harm and destroy manatee habitats (Bossart 1999). Human activities, such as pollution and littering, harm manatee habitats as well. Pesticides, detergents, and other industrial chemicals pollute the water in which manatees live. Litter in the water, such as fishing line and plastic bags, can get tangled around manatees and kill them (SMC Handbook).
Flood control structures are very harmful to manatees. Many manatees get trapped and crushed in canal locks and flood gates (Marmontel et al. 1997). This is a large problem because there are many water control structures in Florida. The suction created by the opening gates can also cause manatees to drown. If manatees get trapped and not killed they are able to connumicate with other manatees so they will not get separated for good (SMC Handbook).
Both natural manatee mortality and human-related manatee mortality have been anaylzed to determine the health and viability of Florida manatees. The viability of the Florida manatee is determined by the age structure, age-specific mortality, and age-specific reproduction of teh population. In a 1997 study, age-specific data was determined for carcasses of manatees found from 1976-1991 (Marmontel et al. 1997). Adult mortality and fertility were the two most important factors in predicting the population size. The study determined that both a 10% increase in adult mortality and a 10% decrease in reproduction would result in extinction of the manatee population. In mortality increases, it follows that reproduction would decrease because there would be fewer females to produce offspring. Thus, managing the Florida manatees for survival and viability must focus on preventing human-related mortality.
Several things have already been done to reduce manatee mortality. The Florida Department of Environmental Protection has implemented boat speed zones and marina rules in the counties with the greatest manatess mortality (Marmontel et al. 1997). Edmund and Laura Gerstein, from Florida Atlantic University, tested two captive manatees and determined that they had difficulty hearing the low-frequency sounds of boat motors, which could be one reason why manatees get hit by boats so often. The Gersteins plan to develop a device that manatees can hear, which would be attached to boat motors (Eliot 2000). Some manatees are not killed immediately when hit by a boat. In this case manatees can be treated and released back into the wild. Sea World of Florida, Miami Seaquarium, Lowry Park Zoo, and Homosassa Springs State Wildlife Park are all rehabilitation facilities in Florida that are able to capture and treat injured manatees. Without proper treatment, even some minor injuries could be fatal (SMC Handbook).
In addition to the progress already made, many more things need to be done to protect the future of manatees. Many effects of recreation are detrimental to manatees. Boats are the largest killer of manatees, and slow speed limits and other safety regulations should be mandated throughout Florida, not only in the counties with the greatest amount of manatee mortality. When activites expose humans to manatees, the manatees become more inclined to go near people again, which increases their chances of being harmed by boats, pollution, or litter. Any development of marinas or other recreational facilities has the capability of harming manatees. Therefore, water recreation should be regulated and possibly made illegal in areas with large populations of manatees. Some manatee sanctuaries already exist where the manatee population is increasing (Miller et al. 1998). Deferring coastal development from manatee rich areas to manatee poor areas would bring less harm to manatees. Since many manatees rely on the warm water from power plants, the operations of those plants must be monitored so manatees are not stranded without the warm water they need to survive. The state of Florida should shift some economic focus to manatees, and use captive manatees as a tourist attraction to raise money to protect them. One of the most important things to do is educate the public. By implementing school programs and reaching out to the community, citizens will learn more about manatees and be more inclined to protect them.
Manatees are fascinating creatures. The Florida manatee can live in any warm water in and around the state of Florida, yet they are endangered because of an array of human actions. Increasing human awareness of manatees and establishing manatee sanctuaries in warm water and vegetation rich areas, free of human activity, are the best ways to save the manatees. If nothing changes, the future of the Florida manatee is grim, but if changes are made to protect the manatees and reduce mortality, there is hope for the future. Manatees evolved with no natural enemies, so the only thing threatening their survival is humans.
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