Taking swift action after two sightings of a potential great white shark just yards off the sands of Mission Beach last week, lifeguards twice shut down a two-mile stretch of beach last week between the South Jetty and Santa Clara Place. The first closure took place shortly after noon on Aug. 25 when a lifeguard reported seeing an 18-inch dorsal fin — the size and dimension of which led lifeguards and marine experts to guess the animal might have been a great white shark measuring at least 10 feet long. Lifeguards urged surfers, swimmers and bathers to get out of the water and launched personal watercrafts and boats to pinpoint the shark’s location while a fire helicopter scoured the ocean from above. The beach remained closed the rest of that day and was briefly reopened the morning of Aug. 26 until a surfer reported spotting a 14-inch dorsal fin in the same general area around 8:15 a.m., triggering a second closure that remained in effect for 24 more hours. There were no reports of injuries or further sightings. There have been a handful of closures around San Diego County beaches over the last two years, mostly in the La Jolla area.