British and French submarine operations in the Sea of Marmara were the one significant area of success of the Gallipoli campaign, forcing the Ottomans to abandon the sea as a transport route. Between April and December 1915, nine British and four French submarines carried out 15 patrols, sinking one battleship, one destroyer, five gunboats, 11 troop transports, 44 supply ships and 148 sailing vessels at a cost of eight Allied submarines sunk in the strait or in the Sea of Marmara. During the campaign there was always one British submarine in the Sea of Marmara, sometimes two; in October 1915, there were four Allied submarines in the region. "E2" left the Sea of Marmara on 2 January 1916, the last British submarine in the region. Four E-class and five B-class submarines remained in the Mediterranean Sea following the evacuation of Helles. By this time the Ottoman navy had been all but forced to cease operations in the area, while merchant shipping had also been significantly curtailed. The official German naval historian, Admiral Eberhard von Mantey, later concluded that had the sea-lanes of communication been completely severed the Ottoman 5th Army would likely have faced catastrophe. As it was these operations were a source of significant anxiety, posing a constant threat to shipping and causing heavy losses, effectively dislocating Ottoman attempts to reinforce their forces at Gallipoli and shelling troop concentrations and railways.