(CNN) -- Florida's endangered manatees have long suffered from human activity, but this year they face an especially deadly threat hidden in the waters where they swim.
An algae bloom off southwest Florida, called Florida red tide, has killed 174 manatees since January, the highest number to die from red tide in a calendar year, state wildlife officials said Monday.
A red tide is a higher than normal concentration of a microscopic algae that appears in the Gulf of Mexico. At high enough concentrations, the algae can turn the water red or brown, hence the name.
Red tides happen almost every year in southwest Florida and sometimes last just a few weeks, but this year the red tide has lingered and settled in an area of warm water where the manatees have migrated.
"It's kind of filled in an area where they've congregated and are feeding on sea grass where the toxins settle on," said Kevin Baxter, a spokesman for the Florida Fish and Wildlife Conservation Commission.
Those toxins can affect the central nervous systems of fish and other vertebrates, causing the animals to die.
Wildlife officials and their partners have this year rescued 12 manatees suffering from the effects of red tide. They asked the public to alert them to other ailing manatees who may be showing a lack of coordination and stability in the water, muscle twitches or seizures, and difficulty lifting their heads to breathe.
Unlike other algae blooms, red tides are not caused by pollution, the wildlife service said.
"Red tides occurred in Florida long before human settlement, and severe red tides were observed in the mid-1900s before the state's coastlines were heavily developed," the commission said.
The blooms usually develop 10-40 miles offshore, away from man-made nutrient resources, it added.
Red tides were documented in the southern Gulf as far back as the 1700s and along Florida's Gulf coast in the 1840s, the commission said. "Fish kills near Tampa Bay were even mentioned in the records of Spanish explorers."
Manatees are listed under the Endangered Species Act of 1973. Conservation efforts have led to an increase in the manatee population, the commission said, and the U.S. Fish and Wildlife Service is working on a rule that would reclassify the manatee from endangered to threatened.
Most manatees die from collisions with watercraft or from "cold stress" in chilly waters, Baxter said.