New water ways,
It can be a sign of stress but I have also seen sunfish do this that are not stressed. My smaller sunfish do it all the time in summer. Some other reasons they do this is, because the water is warmer near the surface, more D.O near the surface or even a more confortable lesser amount due to supersaturation, they are waiting for terrestrials to blow in, or they feel safer near the surface.
Are they healthy otherwise and are they feeding? If so, I wouldn't worry about it. If they are gulping near the surface (called piping in aquaculture circles) that is indicative of an oxygen problem.
