SAN FRANCISCO, April 25 (UPI) - A gray whale migrating north up the Pacific coast took a detour into San Francisco Bay, to the delight of tourists, the Coast Guard said.The whale, which appeared to be a juvenile, was originally spotted by passengers on a tour boat Thursday morning off Crissy Field, an airfield converted into a public park near the Golden Gate Bridge, The San Francisco Examiner reported.The whale, about a quarter-mile from shore, quickly became a magnet for camera-toting tourists on the boat and in the park.The whale reappeared in the afternoon.Whales have been seen in the same area since early in the week, officials said.Jim Oswald of the Marine Mammal Center in Sausalito, Calif., said the whales are moving up the coast from their winter breeding ground in Mexico to spend the summer in Alaskan waters, The Mercury News reported."It's normal to see a whale or two come into the bay," "But we don't want anybody to get close to them or disturb them, so they'll go on their way north." Oswald said.The Coast Guard put out an alert warning crews of ships in the bay to proceed carefully.The main danger to whales are ship strikes from the many large vessels in the bay, officials said.