As you can see from the IJN ASW link, they had no real doctrine at all, frankly. Not in relation to merchant shipping in particular.
It's important to remember that DCs are, um, explosives, and they sit on the deck, unprotected. For a ship that saw its role as using them as offensive weapons against their submarine foe, they are critically important, and ready for quick use. For a navy whose doctrine was that DDs were offensive platforms with capital ships as the foe, not submarines, DCs on the deck are unarmored bombs waiting to be detonated by just about anything heading their way.
For a feeling of IJN ASW doctrine, I really suggest buying The Japanese Merchant Marine in World War II by Mark Parillo. Unfortunately it is out of print. He goes into great detail regarding the timeline of IJN ASW activities.
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