They left Saipan on 27 February and headed for their assigned patrol area which began in the region of the East China Sea southwest of Kyūshū and extended north into the Yellow Sea. On the night of 6 March and 7 March, "Tench" passed through the Colnett Strait south of Yakushima and into the East China Sea. The four submarines rotated patrol, weather-reporting, photographic-reconnaissance, and lifeguard duties. On 18 March, "Tench" received orders to take up lifeguard station off the western coast of Kyūshū during Fifth Fleet carrier air raids on Nagasaki. Just before the noon watch came on duty, the submarine got word of a dye marker sighting (presumably evidence of a downed American aviator) sighted by friendly aircraft in a bay on the coast of Kyūshū near the town of Akune.