Each winter, a host of harbor seals make themselves at home along the mouth of the Chesapeake Bay.
They lounge on Eastern Shore beaches and around the Chesapeake Bay Bridge-Tunnel, and make the occasional foray into the bay or down to the Oceanfront.
Harbor seals are mostly missing from the historical record in coastal Virginia, but started coming here consistently a few decades ago.
Their numbers have grown dramatically in recent years — and scientists don’t fully understand why.
“This is a new phenomenon,” said Rick Sherwin, biology professor at Christopher Newport University. “They're really well adapted to wintering in cold, clear water that has lots of prey in it. But these ones, for whatever reason, come south and return here pretty predictably.”
His team is working to unravel the marine mystery.
The CNU research project began a few years ago with Michelle Bordeaux, then earning a master’s degree in environmental science. She interned with the Navy’s Marine Species Monitoring program.
The Navy was interested in collecting data about the expanding harbor seal population in Hampton Roads, given the animals’ proximity to major installations and training activities.
Officials tagged some seals, deployed infrared cameras at seal hotspots on the Eastern Shore and CBBT and flew over the sites in a Cessna airplane to conduct aerial surveys.
While helping with fieldwork, Bordeaux said she grew curious about the seals’ behavior, such as what they were doing at night.
“There's all sorts of questions that are unknowns about what the seals are doing here,” she said.
Atlantic harbor seals have thick layers of blubber to survive in near-freezing waters. They breed in Canada and Maine and migrate south for winter, usually around New England.
Harbor seals are “opportunistic carnivores,” eating all types of fish and shellfish.
The species was hunted nearly to local extinction in the early to mid-20th century, largely because of state-financed bounties that encouraged killing seals to help the fishing industry.
The population rebounded after the hunting program ended in 1960 and the federal Marine Mammal Protection Act was passed about a decade later.
During her research, Bordeaux searched historical records in Virginia to look for mentions of harbor seals and found very little besides a few newspaper clippings mentioning individual sightings.
Reports picked up steam in the 1990s and early 2000s. By the early 2010s, anglers started noticing a lot more seals in the area, especially around the Bay Bridge-Tunnel.
“It’s kind of a big unknown whether back in the early 1900s they were here in as large of numbers as they are now, or if it's a new expansion,” Bordeaux said.
Through camera surveys between 2019 and 2025, officials found up to 80 seals hauled out per season, which stretches from late fall to early spring. Researchers observed seals on about 75% of the days they looked for them.
Scientists have several theories about what’s behind the increase.
For example, harbor seals might be driven south by competition with gray seals, which are rapidly growing in the North Atlantic and known to eat pups.
Climate change is likely a big factor, but it’s unclear exactly how. It could be changing ocean currents or air and water temperatures.
Theoretically, warming waters would push cool-loving seals north. But Sherwin said the creatures could be testing – and expanding – their limits.
“When things change a lot and change quickly, it does provide new opportunities as well as challenges,” he said. “So this may be related to climates changing but allowing animals, a small number, to do something that's different from what they did historically.”
Bordeaux suspects higher tides could also play a role by swamping some of the shoreline that seals use to rest.
“Because of sea level rise, they might be losing sites, so they're having to spread out a little bit more,” she said. “They could potentially be losing sites farther up north.”
She said the best way to find out is to keep observing patterns over long periods of time.
That’s where Lindsey Johnson comes in. After Bordeaux finished her thesis and graduated last year, current master’s student Johnson is taking over the research.
On a blustery afternoon in March, Johnson and Sherwin carefully walked around the concrete islands of the CBBT, searching for seals.
They peered down to rocky outcrops beneath the bridge, pummeled by strong waves.
“They can get hidden through the cracks kind of, so it’s hard to see them from certain angles,” Johnson said.
More than an hour went by with no luck. Then they saw a silvery flash in the water as one seal dove down. Rounding the corner, a dark speck appeared on the horizon, then a few more.
Several seals calmly bounced in the bay like buoys, alternating between going underwater and poking their noses up at the surface.
“They kind of bob around like bowling pins,” Sherwin said.
Seals like to “haul out” at their favored spots to rest and digest, including these rocks by the bridge and beaches on the Eastern Shore.
Johnson travels to haul-out sites whenever possible to make observations about their numbers and behavior.
But she’s also expanding the research to include more citizen science, collecting anecdotes from people frequently out on the water, such as anglers and boat tour operators.
“That will help get a better idea of where the seals are in the water, where they're going when they're not resting on land,” Johnson said.
The Navy is retiring its monitoring cameras, making crowdsourced data even more important.
Johnson said she’s already learning a lot. Out on the bridge-tunnel, for example, she and Sherwin thought the bobbing seals were hunting. But after additional research, they realized they were likely sleeping.
On a recent trip to the Shore, Johnson saw eight harbor seals hauled out on a sandbank across from where one gray seal was resting.
“This separation may indicate some level of interspecific competition, with harbor seals potentially avoiding gray seals due to their more dominant and occasionally aggressive behavior,” she said.
Regardless of the reason, it’s a good thing to see harbor seals. They like clean water with plenty of food, and they seem to love it here.
People can report local harbor seal sightings to CNU researchers through this Google form.
