I'm 100% positive that pen raised HSB when dumped into a pond with decent size LMB turn out to be nothing but an expensive snack. I don't think they have any real skill set when first put in the pond, either for hunting or evading, and LMB can fit a pretty dang good size one in their mouths....hence the name.
I made the mistake of stocking 4-6" in mine several years ago. I stocked 250 and never caught one. Saw several get eaten by bass I'd have sworn weren't big enough. I then stocked 8-10" a year later.
Do yourself a favor if you want HSB in an established BOW and ladder stock them big enough that nothing has a CHANCE of eating them. It'll save you $$ and heart ache in the end.
AMHIK