The Florida Fish and Wildlife Research Institute in St. Petersburg reports 769 manatees have been killed so far this year, the Tampa Bay (Fla.) Times reported.
"I'm sure we're going to wind up well over 800," said Pat Rose of the Save the Manatee Club.
The previous record was set in 2010 when 766 manatees died -- mostly due to a drop in ocean temperatures.
Authorities say boaters are not to blame, and that overall, the number of manatee deaths from being hit by boats are down.
This year, the manatee die-off has been linked to a massive Red Tide algae bloom in southwest Florida, which caused at least 276 deaths, said Kevin Baxter of the state's wildlife biology lab.
Another major factor into the manatee deaths this year has been a mysterious ailment that has killed 116 manatees in the Indian River Lagoon on Florida's east coast, Baxter said.
An investigation into the Indian River Lagoon deaths is being conducted, he said.
Fish and Wildlife Service spokesman Tom MacKenzie said the manatee may be elevated to an endangered species, rather than threatened, sometime next year.
"We are concerned about the high mortality," MacKenzie said.