Because manatees live nearshore and must breath air they spend most of their time in shallow water near the surface. Consequently, they are particularly susceptible to being struck by boats, injured in locks and flood control structures, entrapped in fishing nets and lines, or through ingesting foreign objects such as fish hooks. Iillegal intentional killing, red tides and cold stress are also factors associated with mortality. Parasitism and disease also pose a low level but definite threat. Habitat modification including increased disturbance, human-induced declines in aquatic vegetation, particularly associated with weed control, and blocked access to areas of potential use are also regarded as long-term threats.
Two subspecies have been recognized but this is disputed by some workers. Trichechus manatus latirostris is said to inhabit the southeastern US. Trichechus manatus manatus occurs throughout Cuba, Hispanola, and the Carribean, and historically in Texas and Louisiana.
No precise estimates of population size exist but it is clear from historical records that the species is confined to only a small portion of its original range. Populations are believed to be roughly equally divided between Atlantic and Gulf coast populations. It's estimated that as few as 1,900 Florida manatees remain in Florida waters. Total populations during Winter, have been estimated at 1,200 in 1985. An aerial survey conducted in 1991 resulted in a count of 1,465 (Florida Department of Natural Resources,1992). Certain protected areas, such as Crystal River and Blue Spring, Florida saw an increase in 1992, whereas others in Dade and Broward counties have seen recent decreases due to toxicity associated with red tides. The maximum potential rate of population increase is estimated at about 2 to 7 percent, although such estimates based on natural populations do not eliminate the possiblity of a decline in population numbers. About ten percent die each year. If they continue to die at this rate, this Florida manatees could become extinct in the next few decades.
In 1999 several reports of manatee harassment from Volusia and Brevard Counties in Florida, USA. In Volusia County, the majority of the reports centered on a County Park near New Smyrna Beach. In one instance, a concerned citizen reported approximately 20 people in boats and rafts who were feeding manatees and swimming with them. Harassment at this site is reported to be highest on holidays and weekends. In Brevard County, reports were received concerning manatee harassment at a public boat ramp on Turkey Creek in Palm Bay. At this site, the harassment is centered at the docks around the boat ramp and has been seen at all times of the day on both weekdays and weekends. Manatees are learning to approach people for food and are being lured into an area that is dangerous for them. Manatees fed at sites like this are more likely to associate boats with feeding and approach boats or boat facilities looking for food."