I agree with bucket stocking , the 3lber spawned and you added a lot of forage at the right time for max growth for the new fingerlings. Now you are seeing them running out ou forage for the number that were in the pond, that's why they appear to be thinning now.. Food requirment as they grow overwhelmed your spawn of CNBG or eating a lot of your brood stock depending on what size you put in this spring. That's my thinking at this point from what I am reading.
