However, this measure would not help to protect manatees from the impact with the hull of the boat. Pollution is also a threat to manatees. Excessive nitrogen from agricultural fertilizers and poultry waste may result in an outbreak of toxic organisms known as Red Tide. A Red Tide in 1996 killed at least 151 manatees, about twelve percent of the entire west coast population. A previous red tide outbreak in 1982 is thought to have killed 37 manatees.
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.