I think there were be more occurrences if there were no FHM in the mix. IMO having smaller fish in the tank lets them take aggressive behavior out on the smaller fish rather than each other.
Or the fatheads are a temporary distraction from their territorial behavior? I know giving trout a current to swim against in an aquaculture setting among other benefits can reduce territorial behavior. In my trout pond I feed all around the pond so some trout do not keep others from feeding. Trout are very territorial hence the fin nipping in high density situations. Contrary to popular belief it's not the concrete raceways that cause the horrible fin stubs but it's the nipping.
As I'm sure you know unfortunately territorial behavior is a given among the sunfish species, and is stated on the NANFA website where the members put native fish in aquariums.
I've even heard of male bluegills killing a perspective female mate just because she was a new arrival regardless if she was a female and he was guarding a nest. I've had some females mysteriously float up dead in my hatchery pond just after I added the males and females to the pond. I wonder if that was it?
You could see this behavior pick up as the photoperiod increases? Or they could establish a hierarchy and be done with it?
Travis (CJBS2003) could chime and perhaps give us some more insight?
As I'm sure you're also aware territorial behavior is interesting as it varies among species, and with some species it goes up with increased density and others it goes down as there is no territory to defend.
Did you know that even songbirds are territorial and birds fight over landing spots such as tree branches etc?
