Belive it or not but alot of very large bluegill can acutally be a sign of overpopulated bass. If bass populations were low than there would be a huge abundance of 3-5 inch blugill (wich is what many pond owners shoot for). Since it can be hard to find a hook that fits into a 3" blugills mouth, you could see if they are there by walking the shores at night with a big flashlight. Did you check the weights of the bass that you caught and if so did you check on the relitive weight charts? If not, do so. You can learn a whole lot about the health of your pond by checking the relitive weights of your bass.
The low numbers of bass that you saw could have been a result of thier acivity level. Sometimes I could get my bass to srike a cigar butt and sometimes I wonder if they are there at all. The bluegill, on the other hand, almost always seem to be hungry. What was the water temperature, what kind of cover were you fishing, and how deep?
If the relitive weights on the bass you caught were low I would guess that there is actulally an overpopulation of BASS rather than bluegill and that you didn't see many because they just weren't active. If they are high (sugesting a low population of bass) than I would guess that there had previously been an overpopulation of bass resulting in the large amount of big bluegill and that there was somthing like an oxygen depletion over the summer that killed mostly bass due to thier higher oxygen demand (particularly the larger ones).
note: I'm not a biologist so that was only my best guess. All I can tell you for sure is that generaly alot of large bluegill is a sign of too many bass, alot of small bluegill is a sign of a heathfully low population of bass, and that you can learn almost as much by running some relitive weights as you could with a shocking survey.
-Scott
