2thDoc, do your mussels look like this?
If so, they are not mussels but rather Asiatic clams... They can overpopulate a pond at times. However, these are usually yellowish in color but are sometimes black and rarely grow much bigger than 1.5" allowing them to stay in a size range where RES and catfish can more easily prey on them so I doubt they are it.
They are most likely Eastern Elliptios. They are common species in the coastal and piedmont areas of the south eastern states. They survive well in ponds and can sometime get out of control. Do they look like this?
If so, that is what you got... Eastern Elliptios
