Why Japanese submarines lacked an Atlantic-style trade target

Should Japan have used a more aggressive submarine doctrine against merchant shipping?

Only with difficulty: Pacific trade routes were distant and diffuse, and Japan lacked Germany's proximity to a concentrated enemy supply line.

The decisive question is which dense, accessible trade route Japanese submarines could attack from sustainable bases. American-South American and American-Australian routes were distant, dispersed, and expensive for Japan to reach. Japan itself had been America's largest trans-Pacific trading partner, so war removed much of the obvious traffic crossing directly toward Japan. German submarines operated much closer to a concentrated British supply line and could exploit Atlantic geography in a way Japan could not copy. The answer therefore treats commerce doctrine as a problem of basing, distance, and target density rather than aggression alone.