Their lifespan is said to be around 60 years or more. That said, you might assume that many manatees existed today. It has to do with boat propellors and boat owners who travel two fast. It has to do with their habitat disappearing and the overall encroachment of man. All of these reasons and more have contributed to the rapid disappearance of these cute creatures.
Manatees swim slowly and on the surface in shallower areas.
Unweary boaters with propellors travel fast and don't see the manatees, while manatees can't move fast enough to get out of the way. As the number of watercraft motorists increases, so does the death rate for manatees. The encroachment of man in general is on the rise as well, and so this also has a profound affect on the rapid rate of manatees disappearing. The next largest human-related cause of manatee mortality is entrapment in flood gates and navigation locks. There are, of course, natural causes of death for the manatee.