A group of teenagers was learning to sail on the Patuxent River last summer when one of them spotted it in the distance: A dolphin's fin bouncing above the water. They steered closer, and were quickly surrounded by friendly marine mammals.
"All of a sudden there were like 50 of them around us," said Patuxent High School student Carolyn Wilson, an instructor for the Southern Maryland Sailing Association's summer camp. She pulled out her iPhone and snapped some pictures.
Those photos helped researchers confirm one of hundreds of dolphin sightings reported around the Chesapeake Bay last year. An effort to better track movement of dolphins through the bay and its tributaries began last June, and the response has been overwhelming, said Helen Bailey, a research professor at the University of Maryland Center for Environmental Science.
"We were only expecting maybe 25 to 30 [dolphin sightings] a year," Bailey said. "We had over 900 reported last year, and we were able to verify nearly 450 of those."
Scientists think dolphins used to visit the bay more frequently. Published reports of sightings date back into the 1800s. But as pollution degraded water quality in the Chesapeake through the 20th century, they became more rare.
Now researchers are exploring whether more dolphins are swimming up the bay, possibly invited by clearer waters, abundant submerged grasses and rebounding fisheries. Through a website they set up to collect sighting reports — and a smartphone app to launch soon — the researchers are learning that the beloved creatures venture miles upstream in rivers such as the York and Potomac, and as far north as Annapolis and the Bay Bridge.
"It's very likely they're following fish into the bay. Hopefully that's a good sign," Bailey said. "It doesn't look like it's just amusement from the coast into the bay."
Bottlenose dolphins, popular for their perceived humanlike intelligence and personalities, are common throughout the world's oceans and in many estuaries. An estimated 11,500 migrate along the Atlantic coast from the Carolinas to Long Island. Other distinct populations migrate along the Southeast coast and in Central and Southern Florida.
As in the Chesapeake, there are populations of unknown size, maybe a few hundred dolphins, in such waterways as Biscayne Bay in Florida and Charleston Harbor in South Carolina.
Janet Mann, a professor of biology and psychology at Georgetown University who collaborates with Bailey, says the dolphins are important to track because they serve as a visible indicator of ecological health.
Because they often come to the surface, unlike most other aquatic life, it's easy to notice when they are present, or conspicuously absent. And any lesions or other signs of disease are also visible. Because they are at the top of the food chain, Mann said, any toxins in the environment build up in their bodies.
Advocates for the environment can use excitement about dolphins to promote broader ecological health, Mann said.
"They're charismatic animals, so they're a really good to target for public interest and protection of the animals," she said. "If you are interested in protecting dolphins, you have to protect all the animals in the area they use."
Introduced in the summer, when dolphins in the bay were at their most plentiful, the site quickly became a hit.
"What became clear is, we were right in the thick of it," Bailey said.
Hundreds of entries came in, reporting dolphins in groups of 10 or even a few dozen. But there were also reports of dolphin pods 100 strong. Eight people estimated they saw swarms of more than 100 dolphins.
Then, defying the researchers' expectations, sightings continued into the fall, in decent numbers into November. A few were sighted over the winter. And reports have picked up, even as the weather was slow to warm this spring.
The website has 1,500 active users.
One morning last month, someone spotted a few dolphins off Solomons Island in Calvert County. An hour later, there was a report of five or 10 in Annapolis harbor. And then there were dozens — maybe more than 100 — seen at the mouth of the Severn River nearby.
The scientists say it's hard to draw conclusions yet. They noted that the data is biased, because more sightings occur in the places people gather most frequently for recreation or commerce.
Researchers are surveying in some parts of the bay using underwater microphones designed to detect the clicks and squeals dolphins use to communicate with each other.
But they said it's safe to guess there are probably thousands of dolphins in the Chesapeake during the warmest months. Using photos and video of dolphin fins, Mann and colleagues have analyzed scarring and other markings to identify 500 individual dolphins. Using that data, and perhaps eventually the sound records, researchers hope to get a better estimate of the population, and a sense of how often individual dolphins are venturing into the bay and how long they stay.
Any input from the public helps, they said, as long as people are observing the dolphins responsibly. They ask people to watch from a safe distance and not to touch, feed or swim with the dolphins. They tell boaters to maintain their speed and direction if the curious creatures approach, as they often do.
The dolphins have built up a reputation among those who frequent the bay and its rivers. Pasadena waterman C.J. Canby said friends have told him about pods of as many as 50 dolphins swimming around the Bay Bridge or in the lower bay off Calvert County.
Canby was disappointed that in 16 years of crabbing, he had never seen one himself — until last July, near the mouth of the Patapsco River and Bodkin Creek.
"It had to be at least six or eight of them. They went right past us as we were working," he said. "To see them this far north is pretty crazy."