Endangered sea turtles are washing up on the shores of Cape Cod later this winter than ever before, reflecting another sign of the sharp rise in the temperature of the waters off New England, officials from local conservation groups say.
Over the past eight weeks, 481 Kemp's ridley turtles, a relatively small, olive-colored species with flippers for front limbs, have washed up on beaches from Sandwich to Truro, including more than 50 over the past few days. Another 38 green sea turtles, which until recently were almost never seen so far north, and six loggerhead turtles were found on Cape shores.
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The cold-blooded turtles feed on crabs and shellfish in Cape Cod Bay during the summer and frequently become trapped by the Cape's hooked arm, leaving them unable to swim south as colder weather arrives in the fall. Many become hypothermic and wash ashore.
Their numbers have spiked in recent years — last year, a record 1,200 turtles were stranded on Cape beaches — and now they're washing up at times when much of the bay would typically be frozen.
"The Gulf of Maine is warming up, and this is just another species that is being disrupted," said Bob Prescott, director of Mass Audubon's Wellfleet Bay Wildlife Sanctuary, who has been rescuing sea turtles on Cape beaches since the 1970s.
A recent study by scientists at the Gulf of Maine Research Institute found that the mean surface temperature of the Gulf of Maine rose 4 degrees between 2004 and 2013 — a greater rate of increase than nearly any other large body of saltwater in the world. They attributed the rapid warming to a northward shift in the Gulf Stream and changes to other major currents in the Atlantic and Pacific oceans.
Following Boston's warmest December on record, Cape Cod Bay is now roughly 47 degrees, about six degrees warmer than typical for this time of year, Prescott said.
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Turtle stranding in the region usually ends by Christmas, he said. But more than 21 turtles have already been found so far in 2016.
Turtles recovered this late in the season have a much lower chance of survival. Of the 55 that were found in the past week, only nine have survived.
The rescued turtles are being taken to the New England Aquarium's Animal Care Center in Quincy for treatment. If they survive, they're eventually transported back south and released, allowing them to return to their nesting grounds along the Gulf of Mexico.
So far this season, the aquarium has received 306 living turtles, 90 of which died shortly after they arrived. More than 150 have been transferred to rehabilitation centers for treatment in other states.
Tony LaCasse, a spokesman for the aquarium, said this year's 71 percent survivability rate is down from a typical rate of somewhere in the 80s.
"A later stranding turtle is directly correlated with a sicker turtle," he said. "Those turtles are more dehydrated, emaciated, and immune-compromised."
He said most of the Kemp's ridleys that have arrived since the third week of December haven't survived.
Another sign of the warming waters, he said, was how far north some of the turtles are now being found. In November, two Kemp's ridleys were found in Maine, while one was found in Nova Scotia — farther than ever recorded.
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"That's mind-blowing," he said, noting that Cape Cod has long been the northern edge of their habitat. "This reflects the extension of their summer range."
But the stranding of more turtles on local shores may not be all bad news. Some say it indicates that efforts to protect their nesting grounds in Rancho Nuevo, Mexico, and the Padre Island National Seashore in Texas appear to be working.
"We think it's a positive sign that more turtles are making it up here," said Michael Sprague, turtle rescue coordinator for Mass Audubon.
David Abel can be reached at dabel@globe.com. Follow him on Twitter @davabel.