While manatees have no natural predators, their numbers are threatened by human activities. Because of their low reproductive rate, it is difficult for the species to rebound from a decline in numbers. Although the population in Florida has historically been hunted by Native Americans and, later, by the European inhabitants, it was never the victim of commercial hunting. In other parts of their range West Indian manatees have been exploited commercially and, in some cases, this continues. Although protection laws exist in countries such as Costa Rica and Venezuela, illegal poaching still occurs.
One of the main causes of manatee mortality is collisions with motorboats. Manatees are also killed in canal locks and found entangled in fishing nets. They are also threatened by the loss of (or damage to) sea beds due to agricultural and industrial runoff. These same pollutants have been shown to accumulate in the tissues of manatees and some could be toxic to the animals.
Manatee conservation efforts were initiated as early as the eighteenth century, when the English established Florida as a marine sanctuary for the species. In 1893 a state law was established to protect manatees. At the start of the twentieth century fines were established for the killing of a manatee. Manatees are now protected by the U.S. Marine Mammal Act (1972), the U.S. Endangered Species Act (1973), and the Florida Manatee Sanctuary Act (1978).
(Marsh 1994, Oshea 1998, Reynolds 1995, FPL 1989)