Newbie thought here as I have no experience with forage ponds even though my small pond is exactly that as it has ba-gillions of FHM fry that will serve as forage for next years stocking...Could you put a large pump in place to pump water up from the main BOW to the forage pond and allow the water to flow over into the main BOW naturally for, say, days maybe. This would allow the forage pond water to match the main BOW's water conditions and likely carry a lot of the YOY to the main BOW. You might want a grate over the outflow to keep your breeders in the forage pond but allow the YOY to pass. If this does not reduce the biomass enough for your liking then you could seine or drain. Or just maintain the pump running occasionally to maintain water quality.

Just some thoughts, my pond has water flowing through the overflow pipe on a regular basis (I have excessive watershed)and I see plenty of FHM fry in the overflow ditch and creek afterwards.

Fish on!,
Noel