The Viz volume has some notes on the subject at the end of the volume, and the translator has a good enough track record that I think their sources are valid.
Through my research, I did find 1 instance of
live decoy ducks being mentioned. The article also calls them 'call ducks'.
The part I'm not able to substantiate is that specifically the 'aigamo' hybrid breed is also referred to as a 'decoy'. Far as I can tell, that's not what Japan breeds the aigamo for.