In rare sighting, harbor porpoise wanders into Petaluma River

A harbor porpoise seen swimming Tuesday morning in the Petaluma River marked the first time one was spotted in a local waterway, according to an area expert.

Stephanie Disney, a clinical psychologist in downtown Petaluma, said she was taking a walk at about 10:15 a.m. Tuesday along the river, across from the Grand Central cafe, when she saw a fin protruding from the water. Customers at the cafe also gathered outside to figure out what it could have been.

“I thought maybe it was a fish or a shark and then it kept coming up and I could see the arch of its back,” Disney said Wednesday afternoon phone call, adding that she realized the animal wasn’t a shark because it was moving in an up-and-down pattern instead of side to side. “I was kind of in shock that that was what I was seeing.”

The group on onlookers then believed it to be a dolphin, one that didn’t appear in distress.

Bill Keener, a cetacean expert with the Marine Mammal Center in Sausalito, said the center received multiple photos from residents reporting the animal, which helped him determine it was harbor porpoise, similar in appearance to a dolphin but with different fin features.

This is a first for a porpoise to show up in Petaluma, Keener said.

“I’ve never seen that before (in the Petaluma River), so that was new,” Keener said Thursday.

The porpoise likely wandered up river from the San Francisco Bay, he said.

The first report of a porpoise entering a North Bay river was in 2007 in the Napa River. Another was spotted in the same river in 2015 and 2016, and in 2021 in the Sacramento River.

Keener said no rescues of the animal took place, and it’s possible the porpoise just returned to the bay.

“It’s not unheard of for porpoises to explore a river system looking for fish,” he said.

Keener said harbor porpoises largely thrived in the pre-World War II era, but then diminished due to polluted waters and a struggling fish population. Porpoises made a return in 2007, he said, around the same time they were first spotted in the Napa River.

Tuesday’s sighting could indicate a potential for more in the future in Petaluma, according to Keener.

“It was really special,” Disney recalled of seeing the animal, which was last seen in the area around 11 a.m. Tuesday.

Keener said any resident who thinks they may have spotted a porpoise can record their sighting at the Marine Mammal Center website.

Amelia Parreira is a staff writer for the Argus-Courier. She can be reached at amelia.parreira@arguscourier.com or 707-521-5208.