On one lake I fish, the Rockbass are a sign that the walleye have moved on and we should call it a night (or move shallower but we never do). As soon as we catch a Rockie we never catch another wally in that spot. I feel they must move in shallow just behind the walleye.
I have noticed the same thing on some of the lakes I fish at night also. its has normally been a good sign to call it a night.