Setting out from Pearl Harbor on 13 September 1944 for her first war patrol, Trepang prowled the waters south of Honshū, the largest and most important of Japan's home islands.

Despite the fact that the phosphorescent waters would make his submarine stand out starkly in the night, Davenport closed at flank speed and fired a full spread of six torpedoes.

The weather was dark, windy, and rough on 6 December as Trepang's conning tower broke the surface after a day's submerged inshore patrol off Luzon.

Meanwhile, as Segundo and Razorback arrived on the scene, Trepang fired all of her remaining torpedoes at a fourth ship which, she reported, blew up and sank soon thereafter.

Trepang encountered no worthwhile targets during the patrol and had to settle for performing lifeguard duty for aircraft carrier assaults on Tokyo.

While maneuvering to finish off the crippled ship, several antisubmarine vessels appeared on the scene from behind a nearby headland and converged on the fleet boat.

[9] Following her return to Guam in March, Trepang headed for the Yellow Sea, a "hazardous duty" area due to its vast stretches of shallow water.

The sole member of this victim's crew, a Korean, understood little sign language, and looked to be of little value for intelligence purposes, so he was put back on board his barely seaworthy craft, with tools and food, and sent on his way.

[10] Leaving the Yellow Sea, Trepang did a short tour of lifeguarding for B-25 Mitchell strikes on Shanghai, China, and for the continuing series of B-29 Superfortress raids on Tokyo, before she returned to Guam.

Trepang's fifth war patrol was divided into two parts—the first saw the ship operating in a lifeguard capacity while the second gave her a more offensive role off northeastern Honshū and eastern Hokkaidō.

On 22 June 1945, she was on the surface in the Pacific Ocean 20 nautical miles (37 km; 23 mi) southeast of Daiozaki Light on the coast of Honshu, Japan, when a flight of seven United States Army Air Forces B-29 Superfortress bombers jettisoned three bombs which straddled her, one landing 1,000 yards (910 m) off her port bow and two landing 2,000 yards (1,800 m) off her starboard quarter.

[11] Having experienced two previous tours of lifeguarding, Trepang's men otherwise expected a series of long dull days, spent moving in circles, squares, or triangles to break the monotony.

However, shortly before 12:00 on 24 June 1945, her first day of lifeguard duty, lookouts spotted a blossoming parachute overhead and soon saw the splash of a crashed P-51 Mustang fighter damaged while escorting Superfortresses to Tokyo.

Trepang bent on full power and soon picked up the downed aviator, Second Lieutenant Lamar Christian, USAAF, safe and sound.

In the middle of the transfer, the submarines picked up a radio message from a "Boxkite" (rescue search plane) that the surviving crew of Superfortress 44-70109 of the 504th Bomb Group,[citation needed] downed on 26 June 1945, was floating a mere 7 nautical miles (13 km; 8.1 mi) from the Japanese seaport of Nagoya.

She torpedoed and sank the lead ship—Koun Maru Number Two—but the other vessels conducted evasive action and sped away from the scene at full speed.

Given another lifeguarding assignment, Trepang stood on the alert to pick up possible downed airmen from British and American aircraft carrier strikes on the Japanese Home Islands.

In addition, on 14 July 1945, she witnessed a shore bombardment conducted by three battleships and a heavy cruiser against Kamaishi, Japan, during which, reportedly, Trepang sank a 100-gross register ton Lugger with her deck gun.

There, she watched the tumbling succession of staggering headlines—first the atomic bombs on Hiroshima and Nagasaki, the Soviet Union's entry into the Far Eastern War, Japan's tentative acceptance of surrender terms, and finally, on 15 August, peace at last.