The Northeastern United States was hit by a bitter “cold snap” over Thanksgiving, and the frigid drop in temperature appears to have imperiled tropical and sub-tropical turtles swimming off the coast of Massachusetts. According to Danny Nett of NPR, more than 200 turtles have washed up on the shores of Cape Cod in recent days, many of them frozen to death.

Of the turtles found over the past week, 173 died and 54 survived. A large number of the beached reptiles were juvenile Kemp’s ridley turtles, one of the most endangered sea turtles in the world. The day before Thanksgiving, volunteers with the Massachusetts Audubon Society's Wellfleet Bay Wildlife Sanctuary also discovered a still-living nearly 300-pound loggerhead, which was whisked off to the New England Aquarium for medical care.

As of Monday morning, the sanctuary has already recovered 584 turtles from Massachusetts beaches this season, NPR's Nett reports; of those numbers, 340 turtles were found alive.

This is hardly the first time that the region has seen mass turtle strandings. According to the Massachusetts Audubon, strandings in the fall and early winter have been on the rise over the past four decades, with an all-time high of 1,200 turtles rescued in 2014. Experts say that if the strandings continue at their current rate, this year could see similarly high numbers.

Robert Prescott, director of the Wellfleet Bay Wildlife Sanctuary, tells Doug Fraser of the Cape Cod Times that he expects 1,000 turtles will have washed ashore by Christmas.

The causes of this unfortunate phenomenon are not entirely clear, but the confusing shape of Cape Cod Bay, coupled with forces of climate change, could be to blame. Young sea turtles typically migrate north to cooler waters in the summer, and some spend the hottest days of the year in the Gulf of Maine. When it starts to get colder, these turtles head south again, but a number of them get trapped in the “hook-shaped geography” of Cape Cod Bay, as Massachusetts Audubon explains in a statement.

Turtles are ectothermic, meaning that they rely on external sources to regulate their body temperature, and thus tend to thrive in warmer environments. So if they start their migration early enough, when the waters are still relatively warm, turtles stuck in Cape Cod Bay may have enough energy to find their way out of the hook. But if they are caught by a sudden temperature drop before they can make it out of the bay, they become “cold-stunned,” or hypothermic. Their systems shut down, they can’t swim and some freeze to death as the winds carry them to shore.

As the world’s oceans warm, sea turtles are expanding their migration range, and this may be why strandings on Massachusetts beaches are on the rise. “It’s been speculated that the region’s typically cold water which probably kept sea turtles south of the Cape is now warm enough for them and there’s a lot of food for them here,” Jenette Kerr, communications coordinator for Wellfleet Bay Wildlife Sanctuary, told Earther's Catie Keck by email over the weekend.

“[W]hen we get these quick swings from warm to cooler, the turtles that haven't made it south definitely get into trouble,” adds Wallace J. Nichols, a research associate at California Academy of Sciences and sea turtle biologist, in an interview with Didi Martinez and Julmary Zambrano of NBC News.

Prescott tells the Cape Cod Times’ Fraser that the turtles he found on Thanksgiving looked “like they were flash-frozen, [with] flippers in all weird positions like they were swimming.”

The scene on the beaches of Cape Cod may be rather grim, but the Wellfleet Bay Wildlife Sanctuary’s Facebook page posted some good news on Sunday: a video of a little Kemp’s ridley turtle, once caught in freezing waters, slowly moving its head and flippers.

“We’ve seen far too many dead sea turtles due to the Thanksgiving cold snap,” the sanctuary wrote. “We’re hoping for more like this in the days ahead!”