BOSTON (AP) — Sea turtle strandings in Cape Cod Bay are so common that the phenomenon has its own annual season and an established network of rescuers trained to find and help the endangered animals.

But the brisk rate at which the cold-stunned turtles are washing ashore this year has packed a local rescue hospital and threatens the regional strandings record.

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As of Saturday, 221 turtles had stranded on the Cape this fall. That exceeds the 50 to 200 range that typically are stranded in the season from October to December. It’s off pace to reach the record of 278 set in 1999, but the stranding season can sometimes last into January.

During the last week, six tanks were filled with recovering turtles at the New England Aquarium’s Animal Care Center in Quincy, as were three more 1,000-gallon tanks aquarium officials trucked in to accommodate the overflow.

The hospital was designed to handle 100 turtles, but they were overcapacity a few times last week, and the crush was so severe that more than two dozen were flown to Florida treatment centers by the U.S. Coast Guard to relieve the crowding.

“We’re pretty similar to a big city ER on a Saturday night,” said aquarium spokesman Tony LaCasse.

The stranded turtles are mostly the Kemp’s ridley species, but a few dozen of the larger loggerhead (33 stranded) and green species (21 stranded) are mixed in — far more than normal, according to Bob Prescott, who directs the Mass Audubon’s turtle rescue efforts in Wellfleet.

Strandings occur in spots along the Atlantic Coast, but no place else gets Cape Cod’s high yearly numbers, LaCasse said. “We think it’s the only place in the world that has a mass stranding due to hypothermia on an annual basis,” he said.

The Cape’s unique geography is the culprit. The peninsula hooks like a flexed arm from the mainland and creates a three-sided trap for marine animals moving south along the coast.

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“The Cape is sort of a quirk of geography and geology,” Prescott said. “The glaciers plowed it up 12,000 years ago, the sea level rose, and from that point on, it has been catching turtles every fall.”

All of the stranded turtles are juvenile or sub-adult (comparable to teenagers) that have migrated from their southern birthplaces via the Gulf Stream to feed on prey such as crabs in shallow-bottomed areas off the Northeast coast. They head south as the temperature drops, sometimes smack into the Cape’s lower arm.

Turtles are cold-blooded animals, so their body temperature matches their environment. As temperatures fall and turtles can’t find their way out of Cape Cod Bay, their mobility and strength drops until they’re at the mercy of the wind and currents.

This year, the water temperature dropped later than normal, so the stranding season didn’t really begin until the last weeks of November. Then, it started in earnest, Prescott said. Rescuers began finding double-digit totals of stranded turtles daily, including 30 in one day in the week after Thanksgiving, he said.

Prescott said an unusual period of sustained November winds from the northwest and northeast this year likely had a lot to do with blowing the cold-stunned turtles onto the shores around Cape Cod Bay all around the same time.

Volunteers comb the beaches during stranding season, and move the turtles above the high tide line where they’re picked up by rescuers. It’s easier with the Kemp’s ridley, which weigh as little as 5 pounds as juveniles, but the young loggerheads often crack 50 pounds and can approach 100. Once they’re in hand, the turtles are transported by Mass Audubon to the aquarium’s animal hospital.

Of the turtles found stranded, more than 50 have died. The survivors treated at the Quincy hospital are gradually warmed in incubators to temperatures in the 70s, then face a long convalescence. The turtles are often so deteriorated that their muscle has been replaced by nothing but folds of skin, LaCasse said.

All sea turtle species are endangered, except loggerheads, which are listed as threatened. They face plenty of problems at sea, including such manmade threats as boat strikes and entanglement in fishing nets, said Carrie Upite, the sea turtle recovery coordinator at the northeast office of the National Oceanic and Atmospheric Administration. Turtle strandings in Massachusetts may be as old and natural as Cape Cod, but there’s an obligation to help the struggling species survive them, Upite said. “We need to do what we can to help them recover,” she said.

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Copyright 2012 The Associated Press.