I think RES can handle the cold relatively well. The problem is when they get squeezed out by other sunfishes when forage base does not offer something RES can go after when competition with other species gets seasonally intense like in the summer. You do not have to starve to be wiped out, rather you have reduced fitness that causes population to decline over time. Way up north you have Pumpkinssed that overlap quite a bit and likely do have a leg up when it comes to thermal adaptions.
When I go sampling streams attached to reservoirs supporting strong populations of RES, the RES tend to be most abundant in YOY as part of cohorts produced earlier in the breeding season like in late April through May. The June and into early July are poorly represented. I have not seen literature that parses out whether nutritional stressed adults breeding or reduced competitiveness of early life-stage RES in face of BG is the cause for this. Either way, most of the later RES spawning effort up here seems to be futile. Some years late season RES spawning effort does pay off.
