Because manatees generally travel at about three to five miles per hour and can travel up to only 20 mph in short bursts, environmentalists support slow speed zones in areas where manatees are common. More than 80 percent of living manatees have scars on their bodies left by boat propellers. Manatees can also get pulled in and chopped up by large boat propellers. On large vessels such as tug boats and freighters, propeller guards could decrease the likelihood of manatee mortality because these types of boats move slowly. 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.