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  • 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 a GPS tracking unit that transmits a satellite signal every hour.

In August 2016, James Manning organized a Kickoff Meeting at St John Paul II High School in Hyannis, MA. NOAA staff, WBWS research and education staff, graduate students, and teachers from multiple Cape Cod high schools (Cape Cod Tech, Sturgis, Saint John Paul II) met to go over project details. The new drogued drifter prototype was also introduced. At this meeting, it became clear that both the Cape Cod Bay drifter project and Mass Audubon’s role in it were expanding. In prior years, drifters were incorporated into a few local schools’ curriculums through Mass Audubon sea turtle programs. But it wasn’t until 2016 that the drifter project was also incorporated into Mass Audubon’s Sea Turtle Rescue Program.

Figure 3. Students and volunteers build new drifters.

In the early fall of 2016, WBWS staff began to refurbish, assemble, and deploy surface drifters within the Bay. (Note: In both 2016 and 2017, WBWS only deployed surface drifters because we had leftover supplies from previous years and the number of drogued drifters was limited.) Then, at our annual Cold-Stunned Sea Turtle Volunteer Training, Mass Audubon volunteers were recruited to help retrieve stranded drifters on Cape Cod Bay beaches. James Manning, Bob Prescott, and I monitored the drifters’ satellite tracks within the bay and formulated a recovery plan when it looked like a drifter was about to wash ashore.

Cape Cod Bay drifter deployments typically take place between October and December, during the very beginning of the cold-stunned sea turtle stranding season. We charter boats that often leave from Dennis, MA and head towards the middle of the bay. Deployment locations within the bay are largely dependent on the boat’s speed and how much time we have available. Ideally, multiple drifters are released in various locations far away from the coast. The farther out the better as that increases the chances of the drifter spending more time in the bay before stranding, allowing for more data to be collected. In general, we follow deployment guidelines provided by James Manning (Fig. 4).

Figure 4. Nautical chart of Cape Cod Bay, showing preferred location for drifter deployment as determined by James Manning..

In 2016, Nauset High School, Monomoy High School, Cape Cod Academy, Middlesex Community College, and the WBWS Sea Turtle Field School all participated in WBWS-led surface drifter deployments. There were also several staff deployments that coincided with efforts to catch floating cold-stunned sea turtles in the bay. In 2017, both Nauset and Monomoy high school groups and the WBWS field school deployed drifters with us. Unfortunately, no additional deployments were conducted in 2017.

Figure 5. Nauset High School students deploying a drifter.

One of our goals throughout this whole process has been to connect young Cape Cod residents to their surrounding environment and provide engaging opportunities for experiential learning. Prior to the drifter deployments, participating classes received in-class lessons about the massive cold-stunning event affecting sea turtles each winter and how the drifters play an important role in our study of this phenomenon. Then, the students got to assemble and decorate their own drifters before launching them into the Bay. They could even monitor their drifters’ movements online and conduct minor data analyses. This link shows all tracks from WBWS-led drifter releases in 2017:

At a glance, it appears that the surface drifters we deploy are at least partially affected by the same environmental variables causing sea turtles to strand at certain times and locations. For example, there were two different occasions in 2016 where a surface drifter washed up on a beach no more than 10ft away from a stranded sea turtle. It is also interesting to see unusual drifter tracks such as the red one in Figure 6, which appears to have oscillated a number of times due to the ebb and flow of the tides in Wellfleet Bay in November 2017. This same drifter also took the longest to strand (26 days), reminding us that the luckiest turtles are those who get pushed ashore quickly and rescued in a timely manner.

Figure 6. Map showing tracks (colored lines) and stranding locations (red dots) for 6 drifters in Cape Cod Bay.

These drifter data are important for updating local surface current models. Coupled with the sea turtle stranding data we collect every year, they have the potential to teach us a lot about the bay’s oceanography and its impact on cold-stunned sea turtles. Right now, one of the graduate students working closely with James Manning is in the process of publishing a manuscript that looks at sea turtle stranding locations, water temperature, wind speed and direction, and drifter tracks. He uses numerical models to determine the origin of those sea turtles within the bay days earlier. Although these findings are not yet widely available, I believe they will eventually allow us to better predict sea turtle strandings in the future.

The Cape Cod Bay drifter project has developed into a multifaceted tool used for conservation, education, and research. If we can begin to connect points and see patterns in the data, then we will better understand the mechanical elements of offshore waters within the bay and have greater success at improving our protection of the impacted species. This type of science-based inquiry is also beneficial for local students who are just beginning to study environmental science and become stewards of the bay. The methods employed for this sea turtle study could even be loosely applied to other drifting organisms or objects (i.e. trash) in Cape Cod Bay and beyond. If given the chance to continue developing a few more years, I foresee this drifter project further improving prediction processes for sea turtle strandings and inspiring similar projects in other coastal areas.


Olivia Bourque received a B.S. in Environmental and Conservation Biology in 2015 and a M.S. in Environmental Science and Policy in 2016. For the past two years, Olivia worked at Mass Audubon’s Wellfleet Bay Wildlife Sanctuary (WBWS) on Cape Cod, MA. During her first summer in Wellfleet, she assisted with stranded sea turtle necropsies and the Diamondback Terrapin Conservation and Nest Protection Program. She took over as Diamondback Terrapin Program Coordinator in 2017, continuing nest protection efforts and launching a pilot tagging effort. Both falls and winters, Olivia coordinated the Cape Cod Drifter Project while working full-time as part of the WBWS Sea Turtle Rescue Team. When Olivia isn’t looking for turtles on cold and windy beaches, she enjoys travelling, hiking and snorkeling. Olivia is passionate about wildlife and habitat conservation, and she plans to continue pursuing field research opportunities in coastal and marine environments.

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