The japanese put hydrophones on many merchants. My guess is that at some point a large % of IJN comandeered merchants got them. Many also got DCs by late in the war, but they were not really "escorts." They'd have a few ready DCs on deck, and if a ship was attacked, they'd dump a few over the side in an attempt to drive the sub down. Not aimed. Ship goes "boom" and merchants drop DCs whereever they happen to be.
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