When the weather turns cold and days get short, Annemarie O’Connell heads to the beach. Late on a December afternoon in 2025, she bundled up and drove to the shore in Truro, Mass. It’s near the tip of Cape Cod, a peninsula that sticks out into the Atlantic Ocean.

It was dark and chilly as O’Connell scanned the beach with a flashlight. She was looking for stranded sea turtles — ones too cold to swim.

Like other reptiles, most sea turtles don’t generate their own body heat. Instead, they seek out water with the right temp to stay comfortable. Ideally, that’s between about 17° to 29° Celsius (62° to 85° Fahrenheit).

As the ocean warms in summer, many turtles swim north, toward the Arctic, to stay in that just-right temp zone. Later, as the water cools in fall and winter, they’ll head back south, toward the tropics. (This is true for sea turtles in the northern hemisphere. Below the equator, turtles swim toward cooler Antarctic waters in the Southern Hemisphere summer.)

But if the water drops below 10 °C (50 °F) before they move on, these turtles will get too cold. They’ll weaken and slow down. They’ll have less energy to dive and feed. It’s also harder for their bodies to fight off infections. Sometimes they’ll stop swimming and just drift at the surface, exposed to cold winds.

This condition, called cold stunning, leaves the turtles dazed and numb.

Every winter, hundreds of cold-stunned turtles wash up on Massachusetts beaches. Many had been swimming south but couldn’t get beyond Cape Cod. Cold stunning is a problem along Gulf and Pacific coasts, too. Coastal shallows along Florida and Texas can chill quickly during cold snaps. Thousands of turtles that winter in this area can end up stunned.

Government workers and conservation groups patrol beaches in late fall and winter looking for cold-stunned turtles. Without help, these animals may die. But if quickly moved to special hospitals, many will survive.

Sea turtle rehab

That’s what had brought O’Connell, a volunteer patroller, to the beach that chilly December afternoon. She visited around high tide, when turtles are most likely to wash ashore. That day, she found a large loggerhead, about one meter (three feet) long. “It was huge,” O’Connell remembers, like “a rock that had plopped out of the sky.”

The turtle’s eyes were swollen shut. But when O’Connell approached, it moved its head. This turtle was alive, but stunned. A journalist who was with O’Connell helped slide the turtle onto a flat sled and get it to her car.

They drove to a wildlife refuge run by Mass Audubon, a conservation group. There, staffers carefully loaded the turtle into a van to take it to a special hospital. Their goal was to get it into care as fast as possible. “ ’Move, move, move’ is the mantra,” says O’Connell.

The New England Aquarium runs a sea-turtle hospital in Quincy, Mass. Other centers in New York, New Jersey, North Carolina and Florida also treat stunned turtles.

The Quincy hospital fills part of a huge building that used to be a ship factory. Turtles here swim in rows of open-topped tanks that sit on a raised platform. Big water pumps thrum loudly as they move water through the tanks. Around the platform are exam rooms and labs filled with medical equipment.

When a new patient arrives, like the big loggerhead, the first task is to warm it.

Caretakers do this slowly, moving the turtle between tanks with increasingly warmer water. They aim to raise the animal’s temperature by about 2.8 degrees C (5 degrees F) per day. Faster warming could shock the reptile’s system.

Next, veterinarians diagnose the turtle’s health. Many cold-stunned turtles are very sick. They may have pneumonia from inhaling seawater, notes veterinarian Kathryn Tuxbury. Their lungs may be infected with bacteria or fungi. The turtles can have infected or broken bones, disease in their digestive tracts or eye problems.

Cold also damages turtles’ shells and limbs, similar to frostbite in people. (That’s when cold freezes your skin and the tissues beneath it.) Bad cases can cause lifelong harm.

Sea turtles “are good at hiding illness,” says Tuxbury. That makes them tricky to care for. Their shells also make it hard to reach their central organs during surgery. “Everything is accessed from the area in front of their hind limbs,” Tuxbury explains.

Technicians take blood samples. They’re looking to see if the turtles are dehydrated or anemic. Anemic ones have low levels of red blood cells, the type that carry oxygen around their bodies. Low blood oxygen makes them sluggish and reduces their appetite.

Blood samples also can reveal infection and show how well a turtle’s kidneys and liver are working. These organs filter and break down wastes and toxic materials in the turtles’ bodies.

Vets X-ray turtles to look for broken bones or fluid in their lungs. They test the fluid for bacteria. They examine turtles’ stomachs and digestive systems with endoscopes. These slender, flexible tubes have lights and tiny cameras. A vet puts a turtle under anesthesia, then passes the scope down its throat to look for problems.

Treating any of these conditions may require surgery, medication, special diets or other therapies. Caretakers work to help the turtles regain weight and get stronger. Their goal is to get them healthy enough to release to the ocean.

Some of the sickest and worst-injured animals die. But in recent years, about three-quarters of the cold-stunned turtles rescued in Massachusetts healed and were released to the wild.

“All of these turtles really should be dead after being out at sea in extreme cold and not eating,” says Adam Kennedy, who directs the Quincy turtle hospital. “The fact that they can survive and be released is incredible. We never count a turtle out.”

The loggerhead turtle that O’Connell found was a female. She was given the number 671 and a name: Pretty Pretty Princess. When Princess arrived at the hospital, technicians had trouble detecting her heartbeat. Her body temperature was a chilly 8.2 °C (46.8 °F) — far below ideal. Her right eye was scabbed shut, and her left eye was so swollen it was hard to tell whether it was still there.

Princess received emergency drugs and was put on a breathing tube for several hours. Once she was stable, they focused on healing her eyes so she could see food dropped into her tank. She needed to eat to regain weight and strength.

A fragile comeback

Cold stunning is not the first threat sea turtles have faced. In the 20th century, they nearly went extinct. People hunted them for meat, and urban development destroyed beaches where turtles laid their eggs.

Today most sea-turtle species are starting to rebound. The hawksbill and Kemp’s ridley are still critically endangered. Three more — the loggerhead, olive ridley and leatherback — remain threatened with extinction. One species, the green turtle, is not considered at risk. Too little is known about the seventh, called flatback, to say how well it’s doing.

Each species has its own preferred range. Some travel thousands of miles during their lives. Others stay in smaller areas.

While not new, cold stunning is becoming more common. Between 1999 and 2003, it left an average of 139 turtles stranded on Cape Cod each winter. By 2019 to 2023, that number had risen to 739.

Climate change is likely one reason, scientists say. As the world’s oceans warm, turtles now swim farther north in summer to find comfortable temperatures. But when the seasons change, many can’t make it south fast enough to avoid the cold.

Biologist Samir Patel works at the Coonamessett Farm Foundation. It’s a nonprofit research center on Cape Cod. “We have turtles that’ll come up into our region, and they’ll inhabit Cape Cod Bay and the Gulf of Maine,” he says. But in November and December, “those conditions change very rapidly, too fast for them to figure out how to escape. And then they cold stun and strand on the beaches.”

Patel is part of a team that has tracked loggerheads with satellites. They recorded water temperature and depth where the turtles spent the most time. Warmer oceans are bringing more turtles farther north along the Atlantic coast, they’ve found. His team published this in 2021.

Another group linked higher numbers of stunned Kemp’s ridleys to warmer water. They focused on the Gulf of Maine, which extends from Cape Cod north to eastern Canada. Between 1982 and 2016, more of these turtles stranded in the years with the warmest water in early fall.

This part of the Atlantic is warming faster than the world’s oceans overall. That’s probably due to changes in the Gulf Stream. This powerful current carries warm water from the Caribbean north along the Atlantic coast. In recent decades, it’s gotten warmer and closer to shore, one 2023 study showed. The Gulf Stream has also strengthened since 2014. Scientists reported that in the August 28 Geophysical Research Letters. Turtle strandings will likely keep rising as this current carries warm water further north.

Pacific problems

Veterinarian Heather Harris is working to learn where and why turtles get stunned on the Pacific Coast. She works with a nonprofit called Project Upwelling and the Marine Mammal Center in Sausalito, Calif.

Between 2009 and 2024, Harris and other scientists documented 68 cold-stunned turtles between Santa Barbara, Calif., and British Columbia, Canada. That’s far fewer than on the East Coast. But for species recovering from near-extinction, every animal matters.

In the Northeast, beach patrollers find mostly Kemp’s ridleys, loggerheads and green turtles. Greens are the most common victims in Texas and Florida. On the West Coast, Harris finds, cold stunning mainly affects olive ridleys, green turtles and loggerheads.

More cold-stunned turtles on the West Coast die — as many as 70 percent. That may have to do with their age, Harris says.

“The turtles they see on the East Coast are predominantly immature Kemp’s ridleys. They are younger, healthier turtles to begin with,” she explains. “We have older, bigger animals, and they have a lot of underlying disease.” This can include pneumonia, infections and skin damage.

Studies show that ocean heat waves are increasing. They’re also covering larger areas and reaching into deeper waters.

As seas warm, more turtles will likely swim farther north along the Pacific Coast. So Harris expects to rates of cold stunning will increase. While that isn’t good news, it could help West Coast vets improve turtle care. “You get better at care when you get to practice more,” she explains.

Turtles in a changing ocean

As ocean warming changes sea turtles’ movements, more cold stunning isn’t their only threat. Turtles can run into human activities, such as fishing and shipping, at new times of year. So rules to aid them may need to change.

One example is scallop fishing in the mid-Atlantic, from Virginia to New York. These shellfish grow on the seabed. To collect them, fishers drag big nets with metal frames, called dredges, over the ocean floor. Small fish and shellfish fall through holes in the nets. Large scallops get hauled up — along with anything else that’s big.

Sea turtles forage on scallop beds. If caught in dredges, they can be hurt or killed. The Coonamessett Farm Foundation, where Patel works, developed a turtle-safe dredge. It has a special bar that pushes turtles out of the net’s path. Explains Patel, it’s “like a cow catcher at the front of a train.”

Scallop fishers must use this special dredge in the mid-Atlantic from May to the end of October. Years ago, that’s when studies showed sea turtles were present on scallop beds. But if turtles swim north sooner and stay longer, those dates may need to adjust. And if turtles start feeding on scallop beds that aren’t yet regulated, fishers may have to start using special dredges there, too.

To guide such changes, scientists need more data on where sea turtles are. They’re collecting it with tracking tags they attach to turtles’ shells. These may be turtles released after treatment. Some tags send data to satellites. Others transmit high-frequency sound waves picked up by receivers installed in the water or carried by researchers.

But the tags don’t work forever. Depending on the type, they usually last from several months to several years before they fall off or lose power.

“We’d like to have longer data sets,” says Kennedy at the Quincy sea-turtle hospital. It may take 10 to 50 years for turtles to reach sexual maturity. Then, females will lay eggs roughly every two to four years, depending on the species. Most will later return to where they hatched to build their own nests, Kennedy notes. “We’d love to know whether Kemp’s ridleys that we release in Massachusetts migrate back to the Gulf of Mexico, where they were born, to nest.”

For now, the most important step is helping turtles heal and return to the ocean (when it is warm enough). Releases in southern states like Georgia start in spring. In Massachusetts, they usually begin mid-summer. 

On June 23, the New England Aquarium released 12 sea turtles it had treated for cold-stunning. More than 100 people came to watch.

The turtles had tiny tags embedded in their flippers to identify them. These are like the microchips that pet owners get for dogs and cats. Each turtle also had an acoustic tag attached to the back edge of its shell. And some had satellite tags with small antennas glued to their backs.

Princess went first. Her eyes were clean and healthy. She now weighed 42 kilograms (93 pounds) — 10.7 kg (23.6 lbs) more than when she stranded. O’Connell, who had found her six months earlier, came to watch. Princess crawled into the water as people cheered behind rope barriers. “That was really cool to see,” says O’Connell.

More hawksbills, greens and Kemp’s ridleys followed. One Kemp’s ridley, named Marigold, was jokingly called a “repeat offender.” She’d stranded and needed care more than once in the past several years. The scientists knew this from her microchip.

As Marigold moved into the water, Kennedy called out, “Please don’t come back!” Viewers laughed and applauded as Marigold’s antenna disappeared beneath the waves — hopefully, for good this time.