Just a thought from a newbie, do you aerate? If not, the depth may not matter as much as the fish might only be able to stay in the top part of the water column. That takes your really deep pond and really makes it alot smaller. If its a clay pit is there anywhere for the BG to hide out? Any structure? 12" lmb could really put a hurting on some BG with no where to hide and nothing else for the lmb to eat. I know its only 6, but maybe the BG are weary and do not want to venture out like they used to. Any idea what the sizes were of the BG? Maybe alot of one particular size that was perfect for a 12" lmb to eat? These are just thoughts from things I have read on here. I'm sure the experts will chime in soon.