The empty ocean was not automatically watched by civilian shipping. Great-circle routes carried liners and merchant vessels away from the Kido Butai's chosen corridor, and fishing boats were scarce there. Some fishing boats had no radios, while others lacked long-range sets. A merchant vessel's radio could also be a receiver used for weather reports, leaving it unable to transmit a sighting. American sanctions removed much of the US-Japan traffic that might otherwise have crossed the Japanese approach, unintentionally making concealment easier.
Notable Point · 22:00-25:18
Why merchant traffic was unlikely to expose the carriers
Great-circle routes, limited shipboard transmitters, and the collapse of US-Japan trade kept merchant shipping from filling the patrol gap.