The Florida Fish and Wildlife Research Institute in St. Petersburg says it's the highest number of deaths for the slow-moving, vegetation-eating mammals since the institute began keeping records in the 1970s, the Tampa Bay Times reported Friday.
The numbers of manatees found dead so far this year represents about 16 percent of the state's estimated population of 5,000.
Of the dead, 173 were females of breeding age, said the institute's Martine DeWit.
In 2012, 392 manatees died in Florida waters, a number that is considered relatively average.
The high number of deaths this year was blamed on two factors: a massive bloom of Red Tide algae along the state's southwestern coast and a mysterious illness in the Indian River Lagoon on the east coast.
Scores of dolphins and pelicans also died in the lagoon. No one is sure of the cause of the deaths, but scientists speculate they may be a result of algae blooms caused by pollution that wiped out 47,000 acres of sea grass in the 156-mile-long lagoon.
Scarlett Johansson steps out with fiance after pregnancy reveal
Dennis Rodman pledges to end trips to North Korea