The whale spotted in the Hudson River Thursday in Friday was likely led there by food, the head of a local whale research group said.
Photos of the humpback whale showed it "lunge feeding," said Paul Sieswerda, president of Gotham Whale.
"That's when they come up and break the surface and consume these bait balls of menhaden," he said.
Predators drive menhaden, sometimes known as bunker fish, into tight balls. This allows whales to open their mouths and eat them all at once.
The humpback whale seems to have been chasing menhaden up the river, Sieswerda said. Sometimes when whales end up in unexpected places it means they're disoriented or sick, but the feeding photos would seem to rule that out.
The whale wasn't seen Saturday or Sunday morning, Sieswerda said. Members of Gotham Watch are examining photos of four whales seen in Raritan Bay Saturday to see if the Hudson whale matches any of them.
The number of humpback whale sightings in the area has increased every year since 2011, Sieswerda said. That's because the number of menhaden has also gone up due to better water quality and a reduction in fishing.
If this keeps more, humpback whale sightings in the Hudson River might not be so rare, Sieswerda said.
"They're coming back more and more and it's most likely going to be a more common occurrence," he said.
Sieswerda asked anyone who sees a whale to send a photo, location, time and, if possible, species to paul@gothamwhale.org.
Myles Ma may be reached at mma@njadvancemedia.com. Follow him on Twitter @MylesMaNJ. Find NJ.com on Facebook.