It's importnat to note that merchant shipping wasn't convoyed much until '44 at all. Before that, convoys were military shipping only. The IJN and IJA comandeered ~ 1/2 of japanese shipping between them. The IJN stuff would have been used to bring supplies to the IJN bases at the front. Truk, Palau, Tawi Tawi, Rabaul, Singapore, etc. Such shipping was convoyed and escorted. IJA shipping and plain old civilian merchant shipping was simply SOL until '44 (and after that sometimes as well).
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