At 20:48, the American submarine , commanded by Commander Joseph F. Enright, picked up "Shinano" and her escorts on her radar and pursued them on a parallel course. Over an hour and a half earlier, "Shinano" had detected the submarine's radar. Normally, "Shinano" would have been able to outrun "Archerfish", but the zig-zagging movement of the carrier and her escorts—intended to evade any American subs in the area—inadvertently turned the task group back into the sub's path on several occasions. At 22:45, the carrier's lookouts spotted "Archerfish" on the surface and "Isokaze" broke formation, against orders, to investigate. Abe ordered the destroyer to return to the formation without attacking because he believed that the submarine was part of an American wolfpack. He assumed "Archerfish" was being used as a decoy to lure away one of the escorts to allow the rest of the pack a clear shot at "Shinano". He ordered his ships to turn away from the submarine with the expectation of outrunning it, counting on his margin of speed over the submarine. Around 23:22, the carrier was forced to reduce speed to , the same speed as "Archerfish", to prevent damage to the propeller shaft when a bearing overheated.