From the Patrol Report of Lieutenant-Commander Edward Bickford, on HM Submarine Salmon:
Salmon was on the surface at 0745 in position 57 degrees 2′ North 5 degrees 52′ East, when forced to dive by a Heinkel 70. At the time it was thought curious that enemy aircraft should be so far from home at the break of dawn. Salmon, who was regaining patrol position, decided to proceed deep at speed before starting a normal periscope watch.
At 0930 Asdic office reported hydrophone effect 200 revolutions. Salmon came to periscope depth to investigate and found Bremen crossing her stern at high speed, range 2,000 yards. The submarine turned to a firing course at full speed while I searched Bremen through high-power [magnification] for signs of offensive armament. None could be seen and she looked much the same, except for funnels which had been painted light grey, as when I had crossed to America and back on her in 1933. I decided to surface on a firing course and stop her with my gun with the intention of firing torpedoes if she opened fire on me, or gunning her only if she refused to stop but did not open fire.
0940. Surfaced and made ‘K International’ by Aldis Lamp five times over the space of a minute. There was no reply and I ordered a round to be fired ahead of her. Just as the gun layer was about to fire, a Dornier Do 18 appeared and I was forced to dive.
By now Bremen was nearing the limit of effective torpedo range, being on a 140 degree track, steaming at high speed, range 5,000 yards. Went deep as I still considered I was unjustified in firing torpedoes at her… Shortly afterwards I intercepted signal from Admiralty telling [HMS] Ursula that Bremen was not a target and felt much relieved… I decided that this area would most likely become unhealthy in the near future and so decided to go beyond my patrol line to the Great Fisher bank in the hopes of stalking a U-boat whose periscope had been reported the previous day by aircraft. .. from 2100 to 2100 I investigated three merchant ships…
See TNA ADM199/1839