As of noon, the dolphin would swim in one direction for a couple of
minutes, then turn around and swim back, remaining in the area roughly
bounded by the East 90s of Manhattan, Randalls Island and the end of
Astoria Boulevard in Queens.

The Riverhead Foundation noted on its Facebook page:

“We
have documented four occasions of reports of cetaceans in or near the
East River since 2010. Three of those events were in February and March
timeframe. In all four events, intervention was not deemed necessary and
the animals did leave the area without further sighting reports.”

Dolphins
usually travel in groups, though, and Ms. Durham added, “a bottlenose
dolphin alone is definitely something that gives me a little bit of
concern.”

It is unusual for dolphins to swim in the waters around Manhattan, but it is not unheard of. 

In 2010 there were four separate sightings.

Dolphins are used to a cleaner environment than the East River, and
the 30-billion gallons of sewage and water that the river contains may
not be good for the dolphin’s health.

“They come from very pristine waters, so it’s conceivable their
immune system may be unprotected from these pulses of bacteria,” said
Riverkeeper John lipscomb.

The mammal was spotted in the water again Thursday morning.

As of this writing, we have not heard any further updates.