On 4 December 1939, while on patrol in the North Sea, Salmon torpedoed and sank . On 12 December 1939, Salmon sighted the German liner SS Bremen. While challenging Bremen, an escorting Dornier Do 18 seaplane forced Salmon to dive. After diving the Salmon's commander, Lieutenant Commander E. O. Bickford, decided not to torpedo the liner because he believed she was not a legal target. Bickford's decision not to fire on Bremen likely delayed the start of unrestricted submarine warfare in the war. She was lost, probably sunk by a mine, on 9 July 1940.