Manatees die from natural circumstances as well, such as cold stress or disease. In the winter of 1989-90, at least 45 manatee deaths occurred from cold stress because of a prolonged cold front in December. During that time, temperatures in Florida's coastal waters dropped to between 40 and 50 degrees Fahrenheit. Because their reproductive rate is low, manatee populations recover very slowly when they are depleted. Scientists believe female manatees don't become sexually mature until around five years of age.
Males are mature at approximately nine years.
Female manatees usually bear one calf every two to five years, and twins are rare. The Figure for 1996 is unusually high due to an outbreak of toxic algae that killed a large portion of the population. Data in the above graph comes from the Florida Fish and Wildlife Conservation Commission and the Florida Marine Research Institute.