FORT LAUDERDALE, Fla. – Manatees have swarmed into South Florida in huge numbers in the past few weeks, driven south by cold weather and traveling routes that increase their risk of fatal collisions with boats.

Since Thanksgiving, boats have killed two manatees in Palm Beach County, Fla., and one in Broward County, Fla., according to preliminary reports. Increased marine patrols began in both counties last month, and biologists are urging boaters to keep a close watch to avoid harming the large sea mammals.

Manatees always travel to South Florida in the winter. When local temperatures drop, they seek refuge in the warm-water discharges of power plants such as those in Riviera Beach, Port Everglades and Fort Lauderdale.

Two things are different this year: Their increased numbers and more frequent movement, possibly due to fluctuating temperatures, as they leave warm-water refuges in search of food and return when the temperature drops.

“What’s unusual is you have a lot of manatees in the traveling zones, traveling back and forth on the Intracoastal Waterway between Broward and Palm Beach counties,” said Christy Hudak, a research associate with the Florida Fish and Wildlife Conservation Commission. “That does raise the risk.”

An aerial survey last week counted a record 490 manatees in Broward’s waterways. Hovering in a helicopter over the usual manatee areas, county biologists counted 205 at one time in the cooling lakes of the Fort Lauderdale power plant, located east of State Road 7 and south of Interstate 595, and they photographed manatees swimming next to boats.

Hoping to minimize their risk, Pat Quinn, the biologist in charge of the county’s manatee program, sent e-mails describing the unusual concentrations to law enforcement agencies, boating groups, tug boat operators and Port Everglades.

“It’s surprised me we’ve had this number of manatees this early in the season,” he said. “We really need to let the boaters know they’re here and to be vigilant. Even a slow-moving boat can do a lot of damage to a manatee.”

Both counties have put more law enforcement on the water, as part of state-required plans intended to allow the construction of additional marinas without endangering manatees.

Broward County has also paid for a full-time sheriff’s deputy to patrol what’s called the Manatee Loops, a circuit of waterways from the New River to southern Broward County, favored by manatees because they link the two power plants.

Carcasses of manatees suspected of being killed by boats were recently found in Spanish River Park in Boca Raton, Highland Park and south of the Port Everglades entrance channel.