That's the highest number of deaths from a red tide of the gentle, slow-moving creatures -- sometimes known as sea cows -- in a calendar year, state wildlife officials told CNN Monday.
Red tides are an almost annual occurrence in Florida, usually lasting just days or weeks, but this year's event has lingered and settled in an area of warm water frequented by manatees, they said.
"It's kind of filled in an area where they've congregated and are feeding on sea grass where the toxins settle on," Kevin Baxter, a spokesman for the Florida Fish and Wildlife Conservation Commission spokesman Kevin Baxter said.
Those toxins can affect the central nervous systems, causing the animals to die, experts said.
Manatees are listed under the Endangered Species Act of 1973 but conservation efforts have resulted in an increase in the manatee population, leading the U.S. Fish and Wildlife Service to consider reclassifying them from endangered to threatened.