SAN DIEGO -- A young gray whale that wandered into San Diego Bay and became a popular tourist attraction passed the three-week mark in the harbor Tuesday.

A U.S. Coast Guard lieutenant saw the 30-foot cetacean surfacing about 150 yards offshore from the maritime agency's local headquarters about 10 this morning, USCG Petty Officer Jetta Disco said.

"Diego" -- as he's been dubbed by fans -- was first spotted in the bay on March 10, cruising near Shelter Island.

The federal maritime agency has been asking boaters to stay at least 100 yards from the whale, and personnel with the U.S. National Marine Fisheries Service have been keeping tabs on its movements, hoping it will make its way back to the open ocean sooner rather than later.

The leviathan, believed to be 1 or 2 years old, may have been migrating by itself for the first time when it took a detour into the harbor, NMFS biologist Joe Cordaro said.

Gray whales, which can reach more than 50 feet in maturity, travel some 10,000 miles on their annual migration from the lagoons of Baja California, where they calf and mate in wintertime, to Alaskan waters, where they spend summers.

Around the end of February, southbound stragglers mix with others already heading north, Cordaro said.

It is not altogether uncommon for whales to stray from their migration routes. In 1992, a roughly 35-foot gray spent about two weeks in San Diego Bay before being found dead with a gash to its head, apparently having been struck by a boat.