Dolphin pod

DolphinWatch app user Rhiana Scholz captured this image of a group of dolphins in the mid-Chesapeake Bay on July 3, 2018.

Dolphins that visit the Chesapeake Bay  in the summer have been listened to, photographed, identified by their dorsal fins and documented in a crowd-sourced app for going on five years. But there is much more that scientists want to learn.

“Have they been coming sooner and staying later? It’s hard to say,” said Ann-Marie Jacoby, associate director of the Potomac-Chesapeake Dolphin Project and a doctoral student at Duke University.

More people have been spotting Atlantic bottlenose dolphins during the animals’ summer visits to the Bay in recent years, but that could be for a host of reasons.

The Chesapeake DolphinWatch phone app, developed by the University of Maryland Center for Environmental Science, is heading into its fifth season as a way for citizen dolphin spotters to log their findings. The app has nearly 10,000 users and has made many boaters more aware of the dolphins’ presence. Last summer, they logged more than 1,000 sightings, with researchers confirming 70% of them, said project coordinator Jamie Testa.

“That’s a big sighting year for us,” she said.

Lauren Rodruigez, a graduate research assistant with the DolphinWatch program, used the data from three years of spotting as the foundation of a May 2021 report on the trends of dolphin presence in the Bay. The paper informs environmental impact assessments at military facilities in the region, where dolphins may come near ships or shoreline assets more frequently than previously thought.

“Before, the data showed that dolphins only used the Lower Bay. But this data shows they use the whole Chesapeake and [we] need to take it into consideration,” Rodriguez said.

In 2021, dolphins regularly appeared in the Upper Bay off Rock Hall, MD. They traveled well up the Chester River, too, Rodriguez said, “probably chasing prey and fishing boats, or just exploring.”

Dolphins on the Patuxent

This pair of dolphins was photographed in September 2019 in Maryland's Patuxtent River by a user on the DolphinWatch app registered as Glen.

Potomac River researchers have documented dolphins as far upstream as the Gov. Harry W. Nice Memorial Bridge, where U.S. Route 301 crosses the Potomac just south of Popes Creek, MD. That’s nearly a 50-mile trip upriver, almost halfway to Washington, DC.

According to historical accounts, dolphins were spotted in 1884 as far up the Potomac as the Aqueduct Bridge, just south of Georgetown University in DC. Still, people consider spotting dolphins to be a relatively new phenomenon, especially in the Bay’s upper reaches.

“We hear from people, anecdotally, that say they’ve lived here 25 years and have never seen a dolphin until now,” Testa said.

She’s talked to others who’ve been on the water for 40 years and say they see far more of the marine mammals now than they used to.

“Some people are still blown away,” she said.

Data from DolphinWatch app can help predict when the mammals will arrive to summer in the Chesapeake. In the most recent years, they have begun showing up in April and were mostly gone by October. The numbers appear to peak in July. Patterns show that dolphins only visit the Bay’s upper reaches in midsummer.

But their population dynamics and travel patterns are incredibly complex.

Researchers view dolphins in Potomac River

Researchers Ann-Marie Jacoby (far left) and Janet Mann (far right) observe and photograph dolphins near the mouth of the Potomac River. (Photograph taken under NMFS Permit No. 19403 by Madison Miketa)

The dolphins that visit the Chesapeake come from waters up and down the mid-Atlantic, from Florida to New York. Some travel farther than others. Distinct groups that reside along the coasts of various states generally stay together during their visits, but they also overlap in ways that make it tricky to track travel patterns across the system.

“The marine environment doesn’t have the same barriers as terrestrial populations, so there can be a lot of mixing between groups,” Jacoby said.

That’s one of the reasons Potomac researchers wanted to study dolphins here. The Chesapeake is hotbed of dolphin feeding and social behaviors and a great place to study both. Researchers who have been identifying, counting and following them for several years say they have now laid the foundation needed for additional work.

Melissa Collier, a doctoral student at Georgetown and field researcher with the Chesapeake-Potomac Dolphin Project, is studying disease transmission among dolphins. In 2013, in what scientists call an unusual mortality event, nearly 1,600 dolphins washed up along the East Coast, almost all killed by a respiratory disease.

Virginia beaches were the epicenter of that outbreak, with more than 400 dolphins stranded, most of them fatally. Necropsies revealed that the fatalities were largely from cetacean morbillivirus, a virus in the same family as measles. Collier and other researchers want to better understand how animals that spend most of their time underwater share a virus that is transmitted when they breathe, similar to the way COVID-19 is spread among humans.

“The thought process is that an epidemic occurs and natural immunity spreads to the population,” Collier said. “So then it dies out and no individuals can get infected.”

That is, until new generations are born without immunity, she said. The previous outbreak took place in 1987, causing researchers to speculate that, if the quarter-century cycle holds up, another could occur in the late 2030s. Meanwhile, researchers wonder if human disturbances, such as water pollution, could reduce dolphin immunity over time, making them more susceptible to diseases.

To study dolphin behaviors, including those that might spread disease, researchers do a “focal follow” on a particular dolphin or pod. They write down whether the animals appear to be feeding, mating and surfacing at the same time.

While tracking the dolphins this way, Collier, Jacoby and another researcher were on a team that witnessed the first dolphin birth in the Potomac River in 2019. Bottlenose dolphins are among the most studied species in the world, but a wild birth has only been documented in scientific literature once, in 2013 off the coast of Georgia.

The Potomac birth lends support to the hypothesis that dolphins come to the Bay in summer because it is relatively free of predators, compared to the open ocean, and therefore a safer place for newborns. Dolphins carry their young for 12 months, so it’s possible that any born here were also conceived here. Predator avoidance could also explain why they seem to be swimming farther up the Bay than they used to.

And there are likely other factors — more food and less competition, for starters. Or it could be simple wanderlust, Collier said. “[Maybe] they just want to explore more habitat.”

(2) comments

Jim Cummins

Nice article! It does miss an important aspect, however. Dolphins are major predators of American shad, which was once the eastern US's most abundant migratory fish and thereby food for a host of predators. Dolphins are likely benefitting greatly from the American shad recovery in the Potomac River. Perhaps the best illustration of the predator/prey relationship is that American shad are quite unique fish in that they have sound reception skills which key in on the very high frequency sound waves, echolocation, emitted by dolphins (see Dennis T. T.Plachta and Arthur N. Popper, 2002. Evasive responses of American shad (Alosa sapidissima) to ultrasonic stimuli. Acoustic Research Society of America, Acoustic Research Online (ARLO 4(2), April 2003)). The shad’s avoidance of these dolphin sounds is interpreted as evolutionary development due to their predator-prey interactions. Indeed, the use of dolphin sound is being employed at fishways constructed on western US dams to repel shad, a non-native species to that coast, to improve salmon and western trout fish passage. I led the effort to restore shad in the Potomac, and I am pretty confident the dolphin's resurgence is also related to the shad's recovery. I noted the preceding information in my 2008-2016 annual reports on the Potomac's American shad restoration. I also noted that when we restore the American shad, we are also helping to restore other species like the bottlenose dolphin, the American eagle, and cod, all of which are major predators of shad. Shad return to the Bay and its rivers as adults to spawn, but many of those adults remain in the Potomac for months, as they transition back to a saltwater fish. The young-of-year shad which move downstrean and out into the Bay as they head to the ocean in the fall would also be good food for the young dolphin. Oh, and yes, I did include cod, of course they occur off of New England coastline and their dependence on the American shad and other herrings was reported in the 1880s by Spencer Baird, head of the then new US Fish Commission (tasks now under the purview of the USFWS and NMFS). We have largely forgotten about the importance of the American shad because they have been in such low numbers since the 1970s. Their improvement in the Potomac is a good start to their full recovery, which will not happen until more rivers again have good shad runs. That will also help dolphin along the entire east coast.


Last Sunday (July 24th) around 1:30 we came across about 8-10 young (or small because of the size) dolphins just south of Deale in the Chesapeake Bay. We have a video but no still photos of the event. It was very exciting.

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