Is there any inlet or outlet? Are you planing on stocking the pond? Can you control what fish are harvested? Are there any rough fish? Sounds like an interesting situation. It sounds like you need to improve the predator population. Large mouth bass, walleye and muskelunge are all possible additions.
20- 12" muskelunge will eat alot of small bass and Bg. Walleye can also prey heavily on BG and smb.