6:40 p.m. w/videoThe cetacean visitor pictured swimming in the Napa River has been identified as a harbor porpoise, a spokesman for The Marine Mammal Center in Sausalito said Thursday.

Fishermen and others have reported watching one or more porpoises swimming in the Napa River between Copia and the Napa Valley Yacht Club since Monday.

The Marine Mammal Center — a nonprofit organization that rescues stranded marine mammals — will continue to monitor the situation for another 24-48 hours before sending a rescue team because the animals appear to be swimming freely, spokesman Jim Oswald said Thursday.

“We’re going to keep an eye on them and see what they do,” he said, adding they hope the animals will find their way out on their own.

In the meantime, people should stay at least 100 yards from the animals, he stressed. It is also illegal to feed them according to the 1972 Marine Mammal Protection Act which protects marine mammals, Oswald said.

Doreen Gurrola, the center’s assistant director of education, was the marine biologist who identified the harbor porpoise from a Register photo, Oswald said.

David Casper, a veterinarian at the UC Santa Cruz Long Marine Lab, said he also believes the picture of the animal in the Napa River is one of a harbor porpoise.

Harbor porpoises are often found in harbors, Casper wrote in an e-mail Thursday. “It doesn’t take much of a leap to conclude they could swim into a river mouth.”

Oswald said the center is also in contact with a wildlife biologist from the National Marine Fisheries Services in Long Beach, an agency under the National Oceanic and Atmospheric Administration.

In Napa, Chad Edwards, a fisheries biologist with the Napa County Resource Conservation District, has never seen a dolphin or porpoise in the Napa River. He could not see one earlier this week, he said. 

He intends to try again.

On Thursday, one mammal was seen swimming leisurely in the river under the First Street bridge near Copia.

“Pretty cool,” said Lindsey Douglas, 23, of Napa, as she and two friends watched from their fishing boat a mammal’s fin pop in and out of the water.