A sure sign of winter in New England is the start of the sea turtle stranding season. On Wednesday, the first live, hypothermic sea turtles of the season washed up in Dennis on Cape Cod.

The two green sea turtles were collected by Mass Audubon’s Wellfleet Bay Wildlife Sanctuary and transported to the New England Aquarium’s sea turtle hospital in Quincy. There, the sea turtles – with body temperatures around 47 degrees – were slowly rewarmed over several days. Normal temperatures are around 70 degrees for sea turtles. 

They also are being treated for other medical conditions, such as pneumonia and severe dehydration, which are the product of slowly becoming hypothermic over the past two months as ocean temperatures gradually dropped.

These sea turtles are mostly juveniles of three different species that visit southern New England waters each summer to feed on crabs. Many of those that get on the north side of Cape Cod are unable to figure out how to navigate out of Cape Cod Bay in the autumn. As water temperatures drop, they slowly become hypothermic and very inactive. If they are lucky, they get washed ashore.

All of these species are either threatened or endangered due to the impacts of human activities. Rescuing, rehabilitating, and eventually releasing these critically important marine animals has been a joint conservation effort of the aquarium and Mass Audubon for more than 25 years.

The first live sea turtles arriving on Nov. 15 is a late start to this season. On average, the first stranded turtles arrive during the first week of November. However, this fall’s record-breaking warmth until early November kept local ocean temperatures above normal. The cold weather snap of the past week has resulted in a drop of sea temperatures into the low 50s, which is the critical threshold at which strandings begin. Wind volume and direction are also important. Northwest or northerly winds of 10 mph or more help create enough wave activity to wash the floating, inert turtles to shore.

Sea turtles will continue to strand Massachusetts beaches until mid- to late December, depending on the weather. Prior to 2011, an average year yielded about 90 live sea turtles being rescued and treated, but over the last several years, that average has moved to more than 300.