Florida’s Gentle Giant: Poor Water Quality and Powerboats Continue to Threaten the ManateeJul 14, 2021 12:12PM ● By WILLIAM R. COX
The Florida manatee (Trichechus manatus latirostris) has been an important member of Florida’s ecology for millions of years. This subspecies of the West Indian manatee (T. manatus) is a large walrus-shaped marine mammal with a flat, rounded tail, a broad head that is undifferentiated from the body, flipper-like front limbs and a brown or gray body. It has small eyes, no external ears and a deeply cleft upper lip bearing stiff bristles. Inner membranes cover the eyes for protection underwater, and the snout has valves that close when submerged.
The manatee’s brain is relatively small for such a large mammal. Adults average 9 to 10 feet and weigh 1,000 pounds. They can reach 3,000 pounds and lengths of up to 13 feet. Males are smaller than females. Calves are darker than adults at birth but lighten in 30 to 40 days.
The manatee is a herbivore and feeds six to eight hours a day on seagrass and plants found along the shore or in freshwater. It eats 60 to 100 pounds of plants a day. Its teeth are continually replaced as worn teeth move forward and eventually fall out. This is an adaptation from eating plants that are covered with sand. A manatee’s intestines are 130 feet long. Bacterial digestion of cellulose happens primarily in the large intestine and processes 80 percent of food that is high in fiber and low in protein.
A large amount of gas is produced during digestion, and the manatee can use this gas to control its buoyancy with no muscular movement. Its lungs are three feet in length, and a manatee can replace 90 percent of the air in its lungs with each breath. (A human can replace only 10 percent.) A manatee at rest can stay submerged for up to 20 minutes, though smaller and active manatees need to resurface every three minutes. Coming to the surface to get air however makes it susceptible to death or injury caused by watercraft collisions.
The manatee can tolerate a variety of marine, brackish and freshwater habitats. It prefers three to seven feet of water and currents at or below three miles per hour. At high tide it can reach shoreline vegetation and feeding grounds that are inaccessible at low tide.
During the summer months the manatee is wide ranging and can be found along both of Florida’s coasts, in estuaries, outlets to sea, bays, lakes and rivers. During the winter it is restricted to springs and warm-water areas (above 46 degrees F). Hundreds of manatees also rely on warm-water sources in Lee County at sites such as Florida Power and Light, Port of the Islands and Matlacha Isles. Major wintering sites include Homosassa River, Crystal River, Tampa Bay, Fort Myers, Boca Grande, Riviera Beach (near Titusville), Port Everglades and St. Johns River near Blue Spring. My wife and I observed a large concentration of manatees in Blue Spring State Park in Volusia County, Florida, in March of this year, where the water temperature was 72 degrees F.
During mating, which can occur anytime from spring through fall, manatees develop temporary breeding herds lasting a week to a month and consisting of several males and one female. The manatee mates and gives birth underwater to one young any time of the year every two to five years. Calves at birth weigh 66 pounds and are 4 to 4.5 feet. Females breed successfully at 6 to 10 years of age with a gestation period of 12 to 13 months.
A high adult survival rate is a must because the species reproduces so slowly.
The Florida manatee is endangered and its future is threatened as a result of habitat loss caused by poor water quality (turbidity, herbicides, industrial pollutants, eutrophication, red tide, etc.). Cold-water temperatures and increased human-related sources also threaten the manatee.
The largest single cause of mortality is collision with watercraft (blunt trauma and propeller scars), especially with large boats of more than 24 feet with inboard motors and propellers more than 15 inches in diameter. Smaller boats can also cause injury and death to manatees. Many manatees carry wounds and scars from propellers.
The Florida manatee is protected by federal and state laws that limit entry to manatee wintering sites and enforce boat speed zones. Public awareness programs and annual population counts and research also bring attention to the plight of this gentle giant in Florida waters.
William R. Cox has been a professional nature photographer and ecologist for more than 35 years. Visit him online at williamrcoxphotography.com.