MIAMI (Reuters) - Toxic algae blooms known as a red tide have been found off beaches near Miami, officials in Miami-Dade County said on Thursday, a rare east coast sighting of a phenomenon more often seen in the Gulf of Mexico that can kill fish and irritate humans.

County Mayor Carlos Gimenez said public beaches would be closed north of Haulover Inlet, an area marked by secluded green spaces, a boat marina and a nude beach on a barrier island about 15 miles (25 km) north of downtown Miami.

The algae can exude toxins that irritate swimmers’ skin and get carried ashore by winds, causing people to cough and wheeze. The toxins can kill shoals of fish that later wash up on the beach. Eating improperly harvested shellfish affected by a red tide can make people sick.

A months-long bloom off Florida’s Gulf coast this year, the worst in more than a decade, has killed scores of manatees, stranded hundreds of sea turtles and left officials from Tampa to Naples with piles of rotting fish to clear from beaches.

Red tides naturally occur in the Gulf off Florida’s west coast when microscopic algae multiply into dense concentrations, but scientists have documented them on the east coast at least eight times since the early 1950s, according to the Florida Fish and Wildlife Conservation Commission.

“It tends to happen every 10 or 15 years and will show up on the beaches for a while and is flushed away,” said Larry Brand, a marine biology professor at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.

Floating near the state’s biggest city and its main tourism engine, the single-celled organisms have put local leaders on high alert.

Sugar cane and dairy farms in the center of the state have hampered the natural flow of water through Florida’s wetlands, causing vast stretches of stagnant water that are filled with nutrients on which the algae thrive, Brand said.

State scientists also found red tide algae further up the east coast in Palm Beach County at the end of September.