The dolphin — which officials identified as a common dolphin — was bleeding from its dorsal fin as it bobbed around the shallow waters of the canal, occasionally spraying water in the air as it breathed.
The Riverhead Foundation for Marine Research and Preservation was on scene to look into rescuing the dolphin, but a Riverhead spokeswoman confirmed shortly after 6 p.m. that the animal had died. The cause of death was not immediately known.
The presence of the wild creature in one of the country's most contaminated waterways shocked and horrified some onlookers. Dozens of people lined the banks of the canal to snap photos while three news helicopters hovered overhead.
Worried nature lovers called authorities like the Coney Island Aquarium for help, while others searched for someone with a canoe who might be able to rescue the dolphin, which seemed disoriented and was surrounded by murky water covered in an oily rainbow sheen.
Crews from the NYPD Emergency Service Unit responded, along with a team from the Long Island-based Riverhead Foundation.
Officials with the foundation said they planned to monitor the dolphin first before deciding whether to rescue it from the frigid canal, parts of which had frozen earlier in the week, locals said. The dolphin was considered to be "free swimming," meaning that it wasn't trapped, stuck or beached, officials said, so they did not immediately fish it from the canal.
"If the animal comes on a beach or something we can definitely respond to it," said Rob DiGiovanni, executive director and senior biologist for the organization. "But in this case here, when the animal is free swimming, the procedure is to monitor it for another tide cycle or two and see if moves back out again."
Local resident Kat Egan said she was surprised officials wouldn't rescue the dolphin as soon as possible.
"This guy doesn't look happy or healthy," Egan said before the dolphin died. "I can't believe there's not a better solution. He seems like he's struggling. It's sad to watch."
The canal was at low tide when the dolphin was spotted, and police said they planned to wait until after high tide at 7:10 p.m. to see if the dolphin is able to swim away, but the animal died before then.
DiGiovanni said sea animals like dolphins frequently make their way up the canal, but return to the harbor without incident.
Julika Wocial, a marine biologist with Riverhead who was at the canal on Friday, said the dolphin appeared to be an adult, about 6 to 7 feet long and 200 pounds.
Wocial added that it's unusual to see a dolphin alone in the canal, since they tend to swim in the open ocean and in groups.
"They are very social animals," Wocial said.
Wocial spotted a barnacle growing on the dolphin's dorsal fin, which she said could indicate that the dolphin is sick, or at the very least slow-moving.
“It’s definitely concerning," she said earlier in the afternoon. "At this point, we don’t know what the outcome is going to be. We’re trying to monitor the animal."
The Gowanus Canal, which was officially declared a Superfund site in 2010, smelled of sulfur Friday.
"It’s weird seeing a dolphin in the Gowanus Canal, in Brooklyn, in 19-degree weather," said Jose Padillo, 32, who also lives nearby.
"It's sad," said Joe Burchfield, 68, as he watched the dolphin. "Whether it's disoriented or wounded, or how it got up here, who knows? We've all seen dolphins on TV playing in the water. This one didn't look well."
Rebecca Rogers-Hawson stood surrounded by dozens of concerned Brooklynites at the Union Street bridge Friday about 12:30 p.m. watching the dolphin swim around inside the canal.
"It was probably injured or sick — I don't know why it would want to come in here," said Rogers-Hawson, who lives in the area. "It's disgusting. Hopefully we can get it out before it gets worse."
The Twitter reports about the dolphin started about 12:30 p.m., indicating that the animal was spotted in the waterway near the Union Street bridge.
"There's a dolphin in the Gowanus Canal," Twitter user Wei Tchou wrote at about 1 p.m.