It has been a bad year for Florida manatees. Very bad. As of Dec. 13, a total of 803 manatees had died in the state's waters this year, representing 16 percent of the states' estimated population of 5,000 manatees, according to news reports. It's the first time more than 800 manatees have died, at least since records began in the 1970s.

The deaths had two major causes, one of which remains unclear. At least 273 manatees were killed by an outbreak of red tide, a toxic algae, off southwestern Florida, the Miami Herald reported. Many were also killed under mysterious circumstances in the Indian River Lagoon on the state's east coast. Some scientists have suggested that a pollutant or a type of toxic brown algae may be the culprit in those cases.

Last year 392 manatees died, which is closer to average, the paper reported. The previous record for deaths, 766, was set in 2010. But hundreds of those manatees were killed by a long cold snap, which tends to kill young manatees. In comparison, many of the manatees that died this year were older, and 173 were females of breeding age, according to the Herald.

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