U-156's ammunition supply helps explain the likely choice of gunfire against slow coastal traffic. She carried 18 torpedoes, alongside 1,672 rounds for her 5.9-inch guns and 764 rounds for her 88 mm guns. Torpedoes were a much smaller resource to expend. Gunfire therefore offered a way to attack the tug and its tows while preserving torpedoes, although the explanation is an inference about the commander's choice.
Notable Point · 1:28:29-1:30:08
Why U-156 would conserve its torpedoes
U-156 carried far more gun ammunition than torpedoes, making gunfire a plausible choice against a slow tug and its tows.