SAN FRANCISCO (AP) — Earlier this year, a river otter named Sutro Sam became the first of the whiskered critters to be seen in San Francisco for decades.

The juvenile male otter drew crowds to a brackish pool on a seaside cliff where he swam and ate for a few days, thrilling onlookers before disappearing quietly.

Then, in October, a rower on Oakland’s Lake Merritt photographed a river otter hoisting itself onto a dock and munching a fish. It was hailed as a success for cleanup efforts at the trash-strewn waterway.

Earlier this month, a man taking his regular walk along Richmond Marina was surprised to see another otter.

In all, researchers have received 600 reported sightings throughout the San Francisco Bay region over the past two years in the first population study of the weasel-like creatures ever done here. Most of the sightings have been confirmed through photos and video taken by bystanders in an area where the species was nearly wiped out after decades of of hunting, development and pollution.

“We’re getting more and more sightings from all over the bay area every day,” said Megan Isodore, a researcher with the nonprofit River Otter Ecology Project. “The otters are clearly thriving more than they were.”

River otters are mammals that, unlike sea otters, can survive on land and in water. For decades, they were prized for their amazing pelts, which are warm and water resistant. The critters, which can weigh up to 30 pounds, live in burrows near waterways, and eat fish and other prey they can get their paws on.

And while scientists say it’s too early to claim victory for the river otters’ recovery here, many are encouraged by the apparent bounce back.

The evidence of a recovery is promising, but still largely anecdotal, said Isodore, because there is no historic population study to compare against current sightings.

In addition to the individual otters that made news in the heavily urban environments of San Francisco, Oakland and Richmond, researchers and volunteers throughout the region are reporting otter families and groupings at more isolated locations in the north and east bay.

The operating theory is that bay waters and surrounding watersheds have become better river otter habitat due to decades of clean water laws that were passed after otters declined in the mid-20th Century. Also, the state passed a ban on hunting the creatures for their fur in 1962.

Darren Fong, an aquatic ecologist with the U.S. Fish and Wildlife Service, said there has been an uptick in otter sightings in the Golden Gate National Recreation Area in Marin County.

After noticing more otter sightings, Fong and staff decided to check natural history logs dating back to the late 1980s to see if otters were being mentioned by volunteer observers. “There were very few mentions in the 80s and early 90s,” Fong said. “Then in the 2000s it became more frequent.”

Still, it is not clear whether there is a boom in river otter populations as many believe, or if the ecology project’s study is influencing more people to pay attention.

Either way, experts say, San Francisco Bay otters still face long-term challenges from mercury and other industrial pollutants in the water that can cause long term reproductive harm.

Still, environmental protections and the creation of GGNRA in San Francisco and Marin in the 1970s seems to have given otters the privacy and space they need to re-establish themselves.

Mia Monroe, site supervisor of the Muir Woods National Monument, said families of otters are now regularly spotted in areas that include a former parking lot that’s been restored to wetlands.

She said the otters, which need clean water and lots of fish and other food to survive, are a living testament to restoration work and environmental improvements.

“We know that if you have otters, it’s this important clue that other things are getting better,” Monroe said.