Experts in California said they were shocked by the presence of the first river otter to be seen in San Francisco in at least five decades.

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Megan Isadore, the co-founder and director of outreach and education for the River Otter Ecology Project, said the lone otter was dubbed Sutro Sam after he was seen swimming in the ruins of Sutro Baths, the San Francisco Chronicle reported Thursday.

"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," Isadore said. "He's a beautiful animal, well fed. He appears to be perfectly happy and not afraid of people."

Isadore and other experts said the otter must have crossed into the area from Marin County where a large number of the animals have recently been seen in several areas.