A key factor driving the rise in manatee deaths is red tide, which accounted for a third of all the deaths, far more than in any year on record, the Public Employees for Environmental Responsibility said in a release.
The organization said December's statistics from the Florida Fish and Wildlife Conservation Commission haven't been tabulated.
The 2013 death toll eliminated nearly 17 percent of the manatee population, including 126 calves, PEER said.
Manatees are an endangered species under federal and Florida laws.
A key factor driving the rise in manatee deaths was toxic red tide events caused by algal blooms, the Public Employees for Environmental Responsibility said. There were 276 red tide-related manatee deaths in 2013, nearly as many as for the previous eight years combined.
"This hike in manatee mortality seems to be the product of systemic environmental irresponsibility," stated Florida PEER Director Jerry Phillips, a former water quality enforcement attorney for the state Department of Environmental Protection. "Basic permit regulation for wastewater discharges and enforcement against water pollution violations have completely broken down in the state of Florida."
PEER said at least 115 manatees died in 2013 from an as-yet undiagnosed illness in Indian River Lagoon, a major manatee spot, while the number of boating-related manatee deaths fell to 72 from 82 in 2012.