For the first time in decades, a river otter has made San Francisco its home, taking up residence in the ruins of a 19th Century seaside bath near the Golden Gate Bridge.

The otter has mystified and delighted conservationists since, who are piecing together clues to figure out how he got there. The whiskery creature was first spotted by birdwatchers in September.

River otters once thrived in the San Francisco Bay area. But development and the fur trade in the 19th and early 20th centuries nearly wiped them out.

A group called the River Otter Ecology Project studies otter populations further north and in the bay. It says until now it had no evidence the creatures had returned to San Francisco.

The otter is nicknamed "Sutro Sam" after the old baths, which were named after former San Francisco Mayor Adolph Sutro, who built the building which at the time was an engineering marvel.

The facility opened in 1896 on a cliff facing the Pacific Ocean, its baths fed by the salty ocean tides and a freshwater seep. They were torn down and burned in a fire in 1966, and the building's carcass has long been a tourist draw on the city's rugged, western shoreline.

The marine mammal seems to have found the mix of the environment he needs to make a home, to the delight of tourists and local nature lovers.

"They do need freshwater to drink and keep their fur clean," Megan Isadore, director of outreach and education for the river otter group. "They are also happy in salt and brackish water -- wherever there is food -- and he is getting freshwater from seeps behind the baths."

River otters can be found in other regions of the San Francisco Bay area. To the north of the Golden Gate, the researchers are tracking a group in Marin County. They have also found river otters in shore-side San Francisco Bay area communities of Alameda, Richmond and Martinez.

While there is no certain reason for Sam's appearance in San Francisco, Isadore and biologists working to unlock more clues have some leads to go on.

He could have swam across the bay's mouth from Marin County, and scat collected from Sam will be analyzed to see if there's a genetic link to that population.

"We're just trying to piece things together in a logical way," Isadore said. "River otters sometimes even stow away on boats, we just don't know."