• Olivia Bourque

Mass Audubon Wellfleet Bay Wildlife Sanctuary Drifter Project: Motivated by Sea Turtle Strandings

Every fall, the sea turtles that enter the Gulf of Maine to feed in nutrient-rich waters during the summer, begin to migrate south to warmer waters. Tragically, some of those sea turtles become trapped by Cape Cod’s hook-shaped geography. As winter approaches and Cape Cod Bay’s water temperature gradually drops below 55ºF, the remaining sea turtles become too cold to swim and their important bodily functions slow down. We refer to this hypothermic state as “cold-stunned.” All of the cold-stunned turtles eventually get pushed or blown ashore, but they do not all survive the journey; those that do survive are in need of immediate intervention. That is why Mass Audubon’s Wellfleet Bay Wildlife Sanctuary (WBWS) established a Sea Turtle Rescue Team on the Outer Cape.

A small team of WBWS research staff works together with help from over 175 trained volunteers to retrieve the cold-stunned sea turtles that strand on bayside beaches throughout the winter. Live turtles are rushed to the New England Aquarium’s rehabilitation facility in Quincy, MA, for necessary medical attention. In recent years, between 400 and 1200 turtles have stranded in a 3-month time span, with 60-70% found alive. But the colder the water gets and the longer a turtle is cold-stunned before washing ashore, the worse its chances of survival becomes.

Figure 1. Number of cold-stunned sea turtle strandings by species (bar color) and year from 1979 – 2017.

We believe that a combination of the bay’s tides, winds, and currents determine when and where cold-stunned sea turtles strand. Strandings typically occur around high tide after a strong wind event, and we can roughly predict where turtles will end up based on which direction those strong winds are blowing. However, not as much is known about local offshore currents within Cape Cod Bay, which also impact the passive movement of cold-stunned sea turtles. The Cape Cod Bay drifter project was implemented to aid in our understanding of the processes behind this troubling annual event.

The Cape Cod Bay drifter project is an ongoing effort that continues to grow with help from a number of collaborative organizations and partners, namely the National Oceanic Atmospheric Administration (NOAA), the Gulf of Maine Lobster Foundation, Mass Audubon, and a handful of local schools. Mass Audubon’s WBWS research staff became interested in the drifter project because of the drifters’ ability to provide valuable information about the bay’s currents and their potential influence on where cold-stunned sea turtles wash ashore. NOAA oceanographer James Manning and his team of researchers set out to gather and analyze the data collected. For years, they have developed, tested, and altered drifter prototypes according to what works best for this study.

Figure 2. A drifter ready to be deployed in Cape Cod Bay.

One of the first ocean drifter designs was cleverly shaped like a turtle and built to float. The current models look much different and for good reason. Modern drifters are better designed to replicate the passive movement of cold-stunned sea turtles drifting within the Bay. Surface drifters are built mainly out of lightweight aluminum and canvas with sails that extend approximately 5 feet underwater, catching shallow subsurface currents and waves. NOAA also developed a drogued drifter that is designed to catch deeper water currents using a tether that extends 10 meters deep. Since we do not know exactly where in the water column cold-stunned sea turtles spend most of their time, this range in drifter sizes and designs is intended to be all-inclusive. Ideally, every surface drifter would be deployed simultaneously with a drogued drifter to illustrate how their movements within Cape Cod Bay differ. Both types are equipped with