バラバク海峡
"Trout" was next ordered to plant naval mines in Api Passage, off the northwest coast of Borneo. She got underway on 22 March 1943 and on 4 April, while en route from Balabac Strait to Miri, fired a spread of three torpedoes at a naval auxiliary. One hit the target amidships, raising a plume of water into the air, but the warhead did not explode. "Trout" fired a fourth torpedo but the ship saw its wake, turned, and dodged it. The next day, "Trout" launched three torpedoes at another ship with no results. She planted 23 naval mines in Api Passage on 7 and 8 April and then began patrolling the Singapore trade route. On 19 April, she launched four torpedoes at a freighter but scored no hits. Later in the day she fired a spread of three torpedoes at a tanker and missed. "Trout" sighted two trawlers on 23 April and battle surfaced. Her deck guns soon stopped the first ship dead in the water and set it on fire; they then turned the second one into a burning wreck. Since there was only one torpedo remaining, the submarine headed for Fremantle, where she arrived on 3 May 1943.