Janet Mann, a biology professor at Georgetown University, is director of the Shark Bay Dolphin Research Project and the director of the Potomac-Chesapeake Dolphin Project. Her latest book, “Deep Thinkers,” was published in 2017.

A sleek gray dolphin emerges, blows and glides beneath the surface, then another and another. Seventeen bottlenose dolphins, including several newborn calves and their mothers, swim closely together in the lower Potomac River. Although I have studied wild dolphins for more than 30 years, I didn’t really know they were so close to home until my husband and I bought a small cottage on Virginia’s Northern Neck, one of the most beautiful places I’ve seen — a mere two hours from the District.

In 2015, with a little help from Georgetown University and some foundation funds, we bought Ahoya, a small used skiff, and a camera and began systematic surveys of the lower Potomac waters with a team of undergraduate and graduate students — looking for dolphins.

Our first year, we had only a few survey trips but we identified about 200 dolphins based on their dorsal fin shapes and marks. In 2016, more than 400 individual dolphins graced the Potomac. Now, we are well over 500 and counting. We have had some groups with more than 100 individuals.

Scientists aren’t the only ones who get very excited about dolphins. A story in the Chesapeake Bay Journal about our work sparked many watermen, boaters, local government officials and other groups to contact us to tell us what they have seen or just to ask questions.

Bottlenose dolphins are apex predators, relying on fish and squid, and are what biologists call “bioindicators.” That means they tell us whether the environment is healthy. If there are a lot of fish, there will be dolphins. Commercial fisheries will also benefit. So while some fishers might be disappointed that the dolphins spirited away a few fish, dolphins have been a sign of good luck to watermen for centuries.

As apex predators, dolphins accumulate whatever toxins are in the fish and store them in their blubber. If there is chemical pollution, it winds up in fish and ends up in dolphins. (Humans store toxins in their fat tissue, too.) Unfortunately, the only way to offload these chemical cocktails is in their milk supply. In Florida, scientists found that firstborn offspring, who receive all the pollutants that their mother has accumulated, are much more likely to die than later-born offspring. As a female dolphin has offspring, she has fewer pollutants in her system. Males are not as lucky and accumulate these toxins indefinitely.

It is striking to see so many newborn dolphins here in the Potomac. This means they come here each year — they are here from April to October — not only to feed but also to mate and give birth. Pregnancy is 12 months long, so the Potomac is both a breeding and feeding ground for the dolphins.

While there are reports of dolphins near the District back in the late 1800s, and up to Indian Head, Md., in the early 1800s, we do not know how long dolphins have been here or how far up the Potomac they go. We know that many dolphins are repeat visitors, returning three years in a row. We intend to find out how many keep coming back and where they winter. These dolphins are part of a very large network of dolphins that extends far into the Chesapeake. Northern Neck locals have seen dolphins over the years but often remark that the numbers seem to be increasing. Have the Potomac and Chesapeake cleanup efforts helped? Obviously, they haven’t hurt. With the water getting cleaner, dolphins are a good sign.

We name every dolphin. Being that it is the Potomac, we started with Founding Fathers, presidents, vice presidents and their immediate family members. We have a Richard Nixon. And Barbara Bush apparently loves to come back to the Potomac each year. We have started naming them after Supreme Court justices. House speakers will have their turn, assuming the dolphins keep coming to the cleaner and productive waters of the Potomac and Chesapeake.

Read more about this issue:

The Post’s View: The Potomac River is getting cleaner. Don’t stop funding now.

Hedrick Belin and Ed Merrifield: Recipe for a healthy Potomac

The Post’s View: The Trump administration’s attack on the Chesapeake Bay