Changes in Ocean Currents and Temperatures Leave Cold-Stunned Turtles Stranded on Cape Cod

Last fall, a slew of articles from prominent publications[1] relayed a disturbing new trend: an increase in sea turtle strandings on Cape Cod, Massachusetts. The articles, accompanied by heart-wrenching photos of turtles in critical condition, noted that the majority of turtles found stranded on the Cape’s shores are Kemp’s Ridley–the smallest and most critically endangered of the seven sea turtle species. While strandings peaked in 2014 at 1,241, the number of strandings continues to remain orders of magnitude higher than what historical records from the region dating back to the late 1970’s show.

Trapped by a Shifting Current

A research study (Griffin, et al. 2019) published last year confirmed what volunteer rescuers had anecdotally reported: higher rates of cold-stunning (and consequently, more sea turtle strandings) occur when the Gulf of Maine has warmer sea surface temperatures in late autumn, namely late October through early November. Normal migratory patterns of the turtles involve following the Gulf Stream north from their hatching sites along the Gulf of Mexico. However, as water warms farther north, the turtles venture farther into the Gulf of Maine and linger later in the season, placing them in a perilous position when temperatures drop precipitously in the fall.

Turtles are ectotherms (“cold-blooded”), meaning that their internal temperature varies with the ambient environment. When water temperatures plummet, sea turtles are susceptible to cold-stunning—a condition similar to hypothermia resulting from a rapid drop in body temperature. A lower body temperature slows turtles’ heart rate and circulation, lowering blood sugar and oxygen levels and leading to severe lethargy. If left untreated, cold-stunning can progress to shock, pneumonia, frostbite, and even death, as turtles become incapable of eating or swimming back to the safety of warmer water.

The geography of Cape Cod in relation to the Gulf of Maine makes it even harder for turtles to escape to lower latitudes once temperatures drop; swimming south from the Gulf of Maine brings them into Cape Cod Bay, which is bordered to the south by a hooked peninsula. To leave, turtles would have to swim back in the direction from which they came (north) and then east around the hook’s tip to warmer seas, but encountering the cold northerly waters confuses them. As one expert commented, “their instinct tells them to retreat back into the shallow warmer water of the bay and wait it out,” which ultimately leaves them trapped and in dire need of rescue and rehabilitation.

To better predict when and where strandings might occur, researchers have combined models and observations and found that cold-stunning is most likely when water drops below 10.5°C (about 50°F) and strong winds persist (Liu, et al. 2019). To pinpoint where exactly along the coast turtles are likely to end up, however, more research is needed to identify where in the water column the majority of cold-stunned turtles are found; this knowledge will allow for more accurate models that incorporate the effects of surface wind and waves.

In the past decade, the Gulf of Maine has warmed 99% faster than the rest of the ocean. A recent study linked the warming to a northward shift in the Gulf Stream as well as changes in the Atlantic Multidecadal Oscillation and Pacific Decadal Oscillation (Pershing, et al. 2015). The authors explained that as climate change melts Arctic ice around Greenland, the water becomes fresher and less dense, weakening the (colder) descending Labrador Current and allowing the (warmer) Gulf Stream current to push farther north into the Gulf of Maine. The gulf’s C-shape, shallow depth, and topography exacerbate the effects of these changes in ocean circulation by holding warmer waters in place longer than would a more free-flowing system. Continued changes in the physical oceanographic conditions that drive biological patterns of behavior, such as migrations, suggest that the plight of sea turtles stranded on Cape Cod may increasingly worsen in the years ahead.

Volunteer Efforts Are Making A Difference

Nationally, YPCCC data shows that 69% of adults think global warming will harm plants and animals. Yet in Massachusetts, that number is 75%, and in Barnstable County (which includes Cape Cod), 72% of adults believe that global warming will harm wildlife. Moreover, 68% of adults in Massachusetts believe that citizens should do more to address global warming. This value, higher than the national average by 4%, may help to explain the robust roster of volunteers involved in rescuing turtles stranded on Cape Cod.

Volunteers — including families, retirees, and entire classes— typically head out once water temperatures in the low 50s (Fahrenheit) and at least a day or two of sustained winds are likely to have blown the turtles ashore. Coordinated by the Mass Audubon Wellfleet Bay Wildlife Sanctuary, they spread out to cover more ground and search for turtles in trouble. Cold-stunned turtles are first brought to the Sanctuary, where they are placed in a room at 55°F (13°C). It’s important not to expose them to warm temperatures or loud noises too quickly, because that send them into shock. After being weighed, measured, and scanned for microchip tags, the turtles are placed in banana boxes — an ideal, low-budget tool for transporting the turtles safely and limiting their view, which further helps keep them calm in transition.

From Wellfleet, the turtles are taken to the New England Aquarium’s Animal Care Center in Quincy, just outside of Boston. After being marked by a painted number on their shell, the rescued turtles are placed in water heated to 55°F. A team of staff and volunteers assists turtles not yet able to swim on their own to improve their circulation. Over the course of a few days, the turtles are gradually moved to warmer and warmer pools, incrementally increasing the ambient water temperature from 55°F to 75°F. As part of the standard rehabilitation process, every turtle is given fluids and a preventative antibiotic. Sadly, about half also have to be intubated and put on breathing machines due to pneumonia-like complications.

If, after tolerating exposure to the 75°F water, the turtles remain stable, they are sent to aquariums along the Atlantic seaboard because the Animal Care Center in Quincy has limited capacity. The turtles are often transported by private plane, courtesy of a network of volunteer pilots and a nonprofit group called “Turtles Fly Too.” Once fully rehabilitated, they are taken to a beach with suitable ocean temperatures and released back into the wild.

Though these turtles face an uncertain future due to a rapidly changing climate system, continued monitoring and volunteer efforts will be essential to preventing their extinction.



[1] See articles in the New York Times, Smithsonian Magazine, and Atlas Obscura.