S.F.'s only river otter at Sutro Baths WILDLIFE

A river otter, named Sutro Sam by local biologists, eats a fish he caught Sunday while swimming at the Sutro Baths. The otter may have come from Marin County. A river otter, named Sutro Sam by local biologists, eats a fish he caught Sunday while swimming at the Sutro Baths. The otter may have come from Marin County. Photo: Sean Havey, The Chronicle Photo: Sean Havey, The Chronicle Image 1 of / 10 Caption Close S.F.'s only river otter at Sutro Baths 1 / 10 Back to Gallery

Naturalists and wildlife aficionados are atwitter about the unexplained presence of a river otter at the ruins of Sutro Baths, the first of the furry mammals seen in San Francisco in at least a half century.

The otter, dubbed Sutro Sam, has been hanging out in a large spring-fed pool along the rocky coast, munching on the many overgrown goldfish dumped into the pond by residents over the years.

"This otter is the first otter recorded in decades and decades in San Francisco, and as far as I know he is the only otter in San Francisco," said Megan Isadore, the co-founder and director of outreach and education for the River Otter Ecology Project, which is tracking otter sightings around the Bay Area. "He's a beautiful animal, well fed. He appears to be perfectly happy and not afraid of people."

Nobody knows where the cuddly-looking quadruped came from or why he picked such an unusual spot, but Isadore and others believe he must have swum across the Golden Gate from Marin County.

The creatures have recently been seen in coastal waterways along the Marin Headlands, including Muir Woods, Tennessee Cove and Rodeo Lagoon. The otter influx in Marin is such that an "Otter Crossing" sign has been placed on Lucky Drive in Larkspur.

San Francisco is a long, potentially dangerous jaunt from these suburban enclaves, but the Marin otter population is closer than the otter populations in the East Bay. Isadore said there were reports of an otter in Lobos Creek, at Baker Beach, two weeks before the first sighting of Sam.

"Our best guess is that the otter showed up there and then just quickly swam around the point to Sutro Baths," she said. "This guy could have swum across the Golden Gate. We haven't heard of otters doing it before, but they could."

Plenty to eat

The largest pond at Sutro is perfect otter habitat, Isadore said. It is next to the ocean and is filled with fresh water from an underground seep, she said. That allows the otter to dine on abundant local fare, including birds, mussels and, best of all, the hapless descendants of aquarium fish.

"He can hunt in the ocean and also in the biggest bath, where I hear people have been putting goldfish for years," Isadore said. "I've seen him eating these great big golden fish that look like carp. He eats those like potato chips."

The North American river otter, or Lontra canadensis, once lived in almost every creek and lake in Northern California. The fissipeds - which means pad-footed - are members of the weasel family. They can live in salt water, brackish water or fresh water and are agile on their feet, sometimes even climbing trees.

They are much smaller than their sea otter cousins, reaching an average length of about 2 feet and generally weighing between 17 and 25 pounds.

Russian and other European hunters killed thousands of river otters and their kelp-bed-loving cousins in the sea, all so the ladies of the Victorian age could wear their thick, water-repellent fur coats. Habitat loss from construction of dams and canals also depleted the river otter ranks. Pollution, including mercury from gold mining reduced the population further.

He's a star

Although the exact number in the Bay Area is not known, scientists say the river otter population has been slowly recovering since the 1972 passage of the federal Clean Water Act.

Sutro Sam was first spotted by bird-watchers in late September at the ruins of the old swimming complex near the Cliff House. The playful mammal has since become a major attraction because he regularly hangs out on the rocks near people, some of whom have fed him and engaged in other ill-advised activities, Isadore said.

She said one dog owner let his golden Labrador jump in the water with the otter, which swam a few feet ahead of the slow-moving canine, watching curiously.

"It's a big mistake for people to let their dogs swim with him. Their dogs could get bitten," she said. "We really don't want people to feed him either. If people feed him, then he will come up to people and pretty soon someone will get bitten, so we really discourage that."

Researchers have determined from Sam's scat that he is a young male, about the age where testosterone-fueled otters normally disperse from their parental clans. The Otter Ecology Project has also collected a unique jelly-like excretion from Sam's intestines.

The plan, Isadore said, is to conduct genetic testing on the jelly, which is thought to help otters digest fish and bones, and compare Sam's DNA to that of otters in Marin. But the testing is costly, she said, which is why donations are being collected on her group's website, riverotterecology.org.

"I don't know what's going to happen," Isadore said. "At some point I assume that pond could be fished out, but it isn't yet. I assume he will stay as long as there is food or his hormones tell him to go somewhere else."