American submarines active against Japanese shipping in the Bonin Islands area according to the U.S. Joint Intelligence Committee. Daily summary dated 30 March 1942

An item reported that the Japanese considered the activities of the American submarines in the Bonin Island area as a large threat and all of their ships entering this area on their way to the Mandates towards Japan or reverse were escorted.

Source

Map Room Papers (Roosevelt Administration), 1942 - 1945. U.S. Joint Intelligence Committee. Daily summary No. 110 dated 30 March 1942.