Perch survive well in water temps all the way down to freezing, and all the way up to the mid-eighties.
From the information I've gleaned from Cecil, Bill and others is that YP will thrive and grow in water temperatures up to about 77 degrees. Between 77 and 82 might be considered sub-optimal but not highly stressful as long as other water parameters such as dissolved oxygen remain good. Above the low-eighties perch discontinue feeding and go into high stress mode. Some scientific evidence supports survival up to close to 88 degrees, but you can count on high mortality and disease.
The water temps in my pond peaked out last summer at 84-85 degrees with virtually zero mortality, but my dissolved oxygen was always high day and night. Observable feeding stopped in the low eighties.
