...I believe they can only spawn with other BGs and GSF, but not themselves, so if you only stocked Hybrids...
This is a common misconception. I think fish suppliers pass this on for some reason.. Just for the record, HBG can reproduce all by themselves. The first cross bred results have a very high male to female ratio [mostly male]. This high ratio to males continues into the next generations, but it is said to equalize as the generations continue. Some even say that future generations take on more of the GSF traits. This high male count is what limits them as a forage producer, hence the lack of food for LMB.
I recently added CC to my hybrid only pond and added straight BG as well to help with forage. The HBG have spawned every year since they were stocked 4 years ago, but not alot. Certainly mot enough to feed CC let alone LMB.
