I would agree with Greg, it certainly sounds like Green Sunfish to me. If you want some info, do a Google search for Green Sunfish, there are several sites that have pictures posted. However, your description of a "Bluegill" with a big mouth probably seals it. A pond that size would be difficult if not impossible to fish out, so draining is likely to be the best answer. Since the LMB population has been wrecked, I think you can be philosophical about killing everything in the pond and starting over. Great results are possible in 2 years if you follow the advice of the many talented people on this site.
Good Luck
Jim
