The IJN did run a few anti-merchant campaigns in 1941-42, nost notably against shipping in the NEI and later in the Indian Ocean, but they were limited affairs. One gets the impression that Allied naval vessels were the intended targets and merchants attacked "only because there is nothing else". Even against the shipping in the NEI which was, largely, unescorted and unarmed due to lack of escorts and guns to arm merchants, their lack of success was spectacular; at the same time, a few U-Boote were wreaking havoc on the US East Coast. In all, IJN subs sank only some 180 Allied merchants during WW2. Certainly they could have done much more damage in the first half of 1942 at the very least; that they did not is something to be thankful for.
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