The rescue of 10 manatees took three days in the tides of the Cooper River below the Interstate 526 bridge. Strong currents made it difficult to circle the nets, and some of the big animals struggled before being caught.

The manatees' dilemma might be getting worse.

More of the migrating animals don't appear to be automatically heading back to their home in warmer Florida waters from their summer haunts in places like South Carolina once the water here cools.

The downside is severe: When the water gets cold enough, it kills them.

“The concern is huge,” said Jon Petersen, rescue coordinator from the research and theme park group SeaWorld that came here last week from Florida to help rescue the stranded animals.

Sixteen people in five boats netted the animals one by one. Each captured manatee was immediately trucked to Florida to be treated and released.

All the animals but one went back into the ocean in Melbourne, Fla., the same day. The tenth is undergoing treatment for cold exposure and hopefully will be released in the next week or two, Petersen said.

The number of animals — 10, eight males and two females — rescued from one spot is unprecedented in South Carolina. Previously, five rescues total were made in the river in the past two years. Before that, only one or two had to be rescued from South Carolina waters in a decade or so.

In the Cooper River, manatees are drawn to industrial discharges of water warmed as it's used to cool equipment, such as KapStone Paper and Packaging mill where the 10 animals last week were rescued, or the Williams Power Station in Goose Creek. 

The sudden water temperature drops during the past few fall seasons more likely contributed to their stranding.

“When that happens, these animals are going to look for warm water," Peterson said. "They’re going to stop.”

Crews from almost a dozen agencies worked the rescue, forced to net the animals during low tides and under time pressure to finish before temperatures plunge later this week, as forecasts predict.

The manatees "did what manatees do," Peterson said. "They showed some energy in the nets. Once we got them in hand, they calmed down and did quite well."

Manatees are large blubberous seal-like creatures that are a federally protected species. Their whiskery, puppy-like faces and lolling, seemingly serene behavior often delight onlookers. About 50 are thought to migrate from Florida to the Lowcountry each summer — about 1 percent of the population — then return when winter waters begin to cool.

Occasionally, stragglers that lull in warmer pockets of water on the South Carolina coast stay or get trapped.

As their numbers rebound in Florida, wildlife biologists are becoming concerned that manatees, like other subtropical species, could be trying to extend their winter range. The animals can’t survive for very long in water temperatures lower than 68 degrees.

The water temperature in Charleston Harbor was 60 degrees Tuesday.

In Florida, industrial discharges are a winter haven for the species. Hundreds often gather at power plants during cold spells.

The SeaWorld rescue team that handled the Cooper River netting was assisted by U.S. Fish and Wildlife Service and S.C. Department of Natural Resources crews, as well as crews from other federal, Florida and zoo organizations.

The public is asked to report any manatee sightings to the S.C. Department of Natural Resources at dnr.sc.gov/manatee/.

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Reach Bo Petersen Reporter at Facebook, @bopete on Twitter or 1-843-937-5744.

Science and environment reporter. Author of Washing Our Hands in the Clouds.