In August 1915 the Germans detached a large portion of the High Seas Fleet for operations in the Gulf of Riga in support of the advance of German troops. It was planned to enter the Gulf via the Irben Strait, defeating any Russian naval forces and mining the entrance to Moon Sound. The deployed forces included the 5th Torpedo Boat flotilla. On 10 August, while the battlecruiser and cruiser bombarded Utö island, "G11" reported spotting the periscope of a submarine, one of several reported submarine sightings that caused the day's bombardment to be abandoned.