The YP have LMB and the NP preying on them. I have already noticed a decline in YP numbers even though I stocked 25 more last spring. I will stock another 25 this spring but if they dont start making lots of babies soon they wont be around long.
I have stocked more YP than anything but I have yet to get a spawn. I have stumps, Christmas trees, and always put branches in for the egg strands to hang on but have not had luck yet.
I still say you have more in the pond than you think, and I doubt if the northern will wipe them out. Yes the largemouths and northern will keep their numbers down but that may increase the average size for you.