December 1939, Sturgeon departed Blyth for a patrol are off the British east coast. However, she was recalled two days later. On 17 December, she again departed Blyth for another war patrol, her sixth. She was ordered to patrol the area west of Denmark. On 20 December, Sturgeon was ordered to patrol the Skagerrak strait instead, but she was ordered to return to her original patrol area the next day. Sturgeon returned to Blyth on 30 December. After an uneventful seventh war patrol in the North Sea, Sturgeon underwent a refit at Wallsend until 14 April 1940. After shifting back to Blyth, she conducted an uneventful eighth patrol southwest of Stavanger, Norway from 30 April to 11 May 1940. From 22 May to 5 June, Sturgeon conducted another uneventful patrol northwest of Terschelling, Netherlands. On 9 June, she was ordered to patrol close to the British east coast because of fears that German warships would conduct a raid there. She was recalled the next day. Starting on 26 June, Sturgeon patrolled off Texel, Netherlands. On 30 June, she hit the deep bottom, sustaining minor damage. She ended her patrol on 11 July. On 27 July 1940, Sturgeon departed Blyth for her twelfth war patrol, off Texel, Netherlands. On 4 August, she sighted the German patrol boat , which was being towed by a tug after hitting a mine. Six torpedoes were fired, but all missed their target. V-811s escorts, the German auxiliary anti-aircraft ships FL 21 and FL 24, attacked Sturgeon for 30 minutes, dropping six depth charges. Sturgeon returned to Blyth on 9 August. Sturgeon departed Blyth again on 27 August, on her thirteenth patrol, in the Skagerrak strait. On 2 September, she torpedoed and sank the German troop transport north of Skagen, Denmark in position . On 10 September, Sturgeon sighted the German U-boat in position and fired six torpedoes at it; the torpedoes missed their target although Sturgeon reported a hit. She returned to Blyth on 13 September at the end of her patrol. After an uneventful patrol in the western entrance of the Skagerrak strait from 29 September to 14 October, Sturgeon left Blyth for her fifteenth patrol, again close to the Skagerrak strait, on 26 October. On 3 November, she sank the Danish merchant in position with torpedoes. The next day, Sturgeon attempted to attack the Norwegian vessel at , but both torpedoes missed their