His name is "Sutro Sam" and he's becoming very popular at the ruins, where he lives in a freshwater pond feeding on goldfish that have been released there.
Experts studying river otters are at a loss to explain where Sutro Sam came from, but their best guess is that he swam across the Golden Gate from Marin County where there is a growing population.
He is a young river otter of the age when they leave their parents and he's not afraid of people.
Sutro Sam was first spotted by bird watchers in September. Two weeks earlier there were reports of a river otter at Baker Beach in Lobos Creek.