Eavesdropping on Dolphins in Stellwagen Bank National Marine Sanctuary

By Marissa Garcia

April 2020

Gazing out onto miles of crystalline blue, passengers await the first glimpse of a humpback whale in Stellwagen Bank National Marine Sanctuary. The naturalist aboard announces over the intercom that everyone should scan the nearby ocean for patches of green, indicators of a humpback whale’s white flippers beneath the ocean surface. And then, a passenger spots one. A humpback whale breaks the ocean surface, creating steam as it breathes for air. The naturalist on board snaps a picture and then glances downward, quickly identifying the whale based upon their dorsal fins or black and white tail patterns.

Data like the information collected by this naturalist are key to our understanding of how large whales use Stellwagen Bank: thanks to whale watching trips, sanctuary managers have data going back all the way to the 1970s. In contrast, while dolphins are important ecologically, marine scientists know very little about where dolphins live in the sanctuary and how they use the sanctuary’s expanse of ecological resources. Dr. Tammy Silva dedicated her Ph.D. work, funded through NOAA’s Dr. Nancy Foster Scholarship, toward uncovering these answers. She successfully defended her Ph.D. at the University of Massachusetts, Dartmouth, in 2017 and has since continued her research in a post-doctoral position in collaboration with Dr. David Wiley from Stellwagen Bank National Marine Sanctuary. A new paper Silva and her colleagues published using data from her Ph.D. research sheds light on how dolphins use sanctuary waters – and indicates that they may use Stellwagen Bank more than previously thought.

Stellwagen Bank, which in places is six miles wide, is at the eastern edge of Massachusetts Bay. Its lowermost southern boundary is just north of Cape Cod, and it protects one of the world’s premiere whale watching locations, inspiring approximately one million ecotourists a year from all over the world to visit. Whales and other marine mammals abound here because it’s a rich feeding ground with abundant fish, like the small sand lance. Silva leads the Sand Lance Forage Fish research project. “Sand lance, like other forage fishes, undergo natural ‘boom and bust’ cycles, big fluctuations in abundance and distribution,” explains Silva. Changes in the sand lance population influence the populations of predators, from birds to whales, so understanding these population dynamics ultimately help her research on whales and dolphins.

Since the observational records focus on larger whales, like humpbacks, mystery still surrounds how dolphin populations occupy Stellwagen Bank. Baleen whales visit during the summer to feed upon the booming sand lance population. Whale watching vessels do not operate during the winter, and so as a consequence, summer-dwelling baleen whales are significantly better represented in the scientific record. “Toothed whales [like dolphins] are particularly difficult to study since their highest occupancy occurs during the winter months, when weather conditions drastically limit our time on the water,” says Dr. Dave Wiley, sanctuary research coordinator.

Listening in on Dolphins

Silva’s work relies on a different method to better track dolphin habits. She documents dolphins in the sanctuary using gliders, automated underwater vehicles (AUVs) that are typically in the same color scheme as a school bus but instead take deep dives under the sea. As autonomous robots, gliders continually dive between the seafloor and the surface, profiling the water column. These gliders are used by scientists at NOAA, Stellwagen Bank, and UMass Dartmouth to study population dynamics of cod. Silva and her research team additionally equipped these gliders with passive acoustic recorders so that they could record sound as well as salinity, temperature, currents, and other measurements. The acoustic recorders enable her to track dolphins. She is the first scientist to use this kind of technology to study dolphins in the sanctuary.

For Silva, the gliders serve as mobile underwater microphones. If chatty dolphins are in close proximity, the glider will record their vocalizations and GPS location. The glider can collect data for several weeks. Once the glider is recovered, data are downloaded onto a computer. Silva can then create maps of where dolphins were detected over the glider’s track, documenting the dolphins’ presence around Stellwagen Bank. The measurements collected by gliders have provided a lens into how environmental conditions affect the population distribution of dolphins. Dolphins were recorded more often close to Stellwagen Bank, a hot spot for baleen whale foraging in summer, suggesting that dolphins may be foraging here in winter.

New Approaches to Old Questions

Extremely sociable and often found in pods, dolphins produce whistles to communicate with each other. This is especially helpful during the winter season, when whale watching operators aren’t on the water.

As Silva analyzes the acoustic data collected by the gliders, she is looking for dolphin whistles to indicate that dolphins are present. Silva noticed that some unique whistles were produced over and over again. Each dolphin produces its own distinctive whistle. Each signature whistle functions similarly to a name – dolphins use them to recognize each other. Since each whistle represents a different individual, Silva can use them to track individual dolphins’ presence. Silva notes, “Recording the same signature whistles over multiple weeks and consecutive winter seasons suggests that dolphins spend more time than we thought in the area and may not be transient visitors as previously thought. They may also return each year.” Though this was known about humpback whales, these data had not yet been shown for dolphins in Stellwagen Bank.

From late fall until early winter, Silva detected dolphin whistles approximately 74 percent of the time. During the fall and winter, the water is cold and nutrient-rich, fostering a healthy phytoplankton population which in turn feed dolphins’ prey fish. This lures dolphins into Stellwagen Bank.

Acoustics are an established method for studying animal distributions, especially in Stellwagen Bank, but it had not yet been applied to dolphins in this area. This has revealed many new insights about Stellwagen Bank, says Wiley, such as dolphins being “a much more important component of the winter biota than previously thought.” They actively use Stellwagen Bank as a feeding ground and in turn impact the ecosystem. Silva also determined that dolphins call more frequently during the night, a discovery Wiley attributes exclusively to the adoption of acoustics. Research collaborators include Dr. Mark Baumgartner, Dr. Aran Mooney, and Dr. Laela Sayigh of Woods Hole Oceanographic Institution.

Silva emphasizes that this research affirmed that “passive acoustics and ocean gliders [are] extremely valuable tools for studying dolphins.” The benefits of passive acoustic monitoring are extensive, as it means scientists can collect data in bad weather, day and night, when observers cannot physically be there. She adds, “Using gliders adds additional benefits of lower cost, and you can simultaneously collect environmental data to characterize animal habitats.”

Science and Sanctuaries

Silva’s work, as funded by the Dr. Nancy Foster Scholarship, reminds us that national marine sanctuaries can both support and inspire. Born and raised in New England, Silva grew up aboard whale-watching expeditions, struck curious by the sanctuary’s extensive biodiversity. And now, thanks to NOAA, she can give back to the marine environment that originally inspired her professional goals. Her research is uncovering knowledge about dolphins in this region, which will become increasingly important as we continue to understand fish population dynamics that are critical to supporting community livelihoods.

With the help of collaborators like NOAA Fisheries and Woods Hole Oceanographic Institution, Stellwagen Bank is home to the world’s longest running whale tagging research program ever, established in 2004, and Silva’s work is spearheading an acoustic listening program on dolphins. Her work has interwoven itself within the history of the sanctuary, inspiring a generation of future scientists.

Marissa Garcia is a student at Harvard College and a Virtual Student Federal Service intern for NOAA’s Office of National Marine Sanctuaries.