Hundreds of Atlantic bottlenose dolphins are spending their summers in Chesapeake Bay waters. And now, with the help of crowd-sourced sightings reported each of the last four years, researchers are beginning to understand when and where these mesmeric marine mammals are likely to emerge.
While dolphins have had a presence in the Bay since the late 1800s, researchers think they are arriving in larger numbers than before. That could be in part because warmer waters are pushing the ranges of both fish and the mammals that follow them farther north. It could also reflect rebounding populations after disease outbreaks impacted the region’s dolphin numbers between 2013 and 2015.
The bottlenose dolphins (Tursiops truncatus) that travel to the Chesapeake are a migratory species that spend the rest of the year at other locations along the Atlantic Coast. While dolphins have been studied in captivity, research on their behavior in the wild — where hunting strategies can vary widely from one region to another — is still relatively new, particularly in the Bay.
After witnessing the first dolphin birth in the Potomac River in 2019, scientists now think the Bay could be providing a safe haven from predators for the dolphins to mate and, about a year later, give birth to their calves.
“We are not sure if the population is growing, because it’s been monitored for a short time,” said Lauren Rodriguez, a graduate research assistant at the University of Maryland who authored the latest study as part of her work with Chesapeake DolphinWatch.
But, she said, “if they are having babies, then we assume that the population has the potential to grow here.”
Since 2017, Chesapeake DolphinWatch has been tracking hundreds of dolphin sightings reported to its website and app to develop a picture of their presence in the Bay and its rivers. Many of the reports include photos or videos of fins slicing through the waves by the dozen. In May, scientists used these observations to publish the first study detailing when and where dolphins are spending time in the region.
The study shows “that dolphins are all over the place, even past Annapolis, especially in the summer months,” Rodriguez said.
Nearly 1,000 registered users of Chesapeake DolphinWatch reported 2,907 sightings between June 2017 and October 2019, enough for the researchers to begin seeing trends. By contacting users individually, they were able to confirm nearly 70% of those sightings.
Beyond the time period covered in the study, researchers have confirmed more than 2,600 sightings from June of 2017 through the end of 2020. And users were already busy posting sightings this June.
The study concludes that dolphins are often concentrated around Bay shorelines, but they are spotted in the mainstem as well. Dolphins have been sighted at the mouths of multiple tributaries, primarily near and in the Potomac, Rappahannock and York rivers. The highest frequency of sightings (nearly 136) was logged at the mouth of the Rappahannock, but these were all made by a single observer.
“We were unable to test if the Rappahannock River was truly the most significant location for dolphin sightings, or if this user’s diligent efforts created spatial bias,” the report states.
Information contributed by hundreds of people scattered across the watershed makes this type of spatial research possible and affordable. Though the sightings are not conducted by scientists, a lack of dolphin lookalikes in the Bay (like seals, for example, which don’t venture that far into Bay waters) makes them easy to identify.
But it can be hard to confirm whether the sightings increase at certain times of year because there are more dolphins or because more people are on the water.
“The main week we get sightings is July 4th week,” Rodriguez said. “We’re not sure if that’s because of human activity — a lot of people out on their boats — or if it’s the dolphin activity.”
The DolphinWatch researchers also use underwater microphones, called hydrophones, to listen for dolphin chatter and confirm the trends that emerge from sighting reports. The study found a significant correlation between the frequency of weekly acoustic detections and weekly sightings from 2017 to 2019.
But the July 4th peak also matches the rest of the data, with sightings rising farther north in the Bay with warmer midsummer temperatures and ebbing as they cool off in the fall. Since tracking began here, the first dolphin sightings tend to be reported in early April in the lower Bay. From June to August, dolphins can be spotted in the Lower, Middle and Upper Bay. And, at the end of October, they are seen primarily in the southern portion of the Middle Bay and the Lower Bay.
That bell curve of dolphin activity mirrors some trends in fish spawning behavior as well, which could be one of the factors driving dolphins farther north as waters warm. In 2018, DolphinWatch users reported 714 sightings in June, July and August alone.
Overall, Rodriguez said, “There are way more than anyone thought.”
The study describing when and where dolphins occur in the Bay could help guide human activities, such as bridge construction or military trainings, that might impact dolphins.
Rodriguez said her team provided comments to the U.S. Navy, for example, recommending that an Environmental Impact Statement underway for the Patuxent River Complex should incorporate the latest research on bottlenose dolphins, which are included in the Marine Mammal Protection Act.
Helen Bailey, director of Chesapeake DolphinWatch, said the acoustic data scientists collect not only confirms where dolphins are but also provides clues about their behaviors.
Bailey said a mother dolphin nursing her calf will over the first year develop a specific whistle for the baby that becomes its name. Other whistle types indicate other behaviors. When dolphins feed, for example, their clicking noises bounce back and forth so rapidly it sounds like a door creaking. The researchers are even beginning to recognize the sound a dolphin mother makes when she is scolding her baby.
“Other members of my team are looking at these calls and trying to see what the dolphins are saying and how many dolphins there are with signature whistles,” Bailey said.
Next, Rodriguez plans to use eDNA technology to better understand what the dolphins might be eating during their summer forays. The work would involve running water samples through filters and analysis to determine which fish species’ DNA is present alongside the dolphins’. Combined with what they can see and hear about the dolphins, this could help researchers understand what’s drawing them farther into the Bay.
“Bottlenose dolphins are a protected species, so we don’t want to harm them or bother them,” Rodriguez said. “But we do want to study them.”