It's not just temperature but dissolved oxygen as well.
Brown trout can handle temps over 80 degrees if dissolved oxygen is at or very near full saturation. However, you are rarely if ever going to have DO levels that high in a pond situation. Some studies I have read concerning survival rates of brown trout say they can handle temps pushing 85 degrees for short periods of time and temps around 78 indefinitely if the DO level is at saturation. Rainbow trout can tolerate temperatures fairly close to this, perhaps a a couple degrees less. However, anything above 74 is going to stress them with temps over 78 heavily stressing them. For browns, shoot for 70 and under, for rainbows 68 and under and for brookies 66 and under. That is assuming DO levels are at least around 6 ppm or higher. Different strains of trout in the same species can even have different tolerances. Age of the trout also is a factor, with YOY fish and larger adult fish being the least tolerant to higher temps. Trout that are in good condition will have better survival rates in sub par temps as they can handle stress and subsequent body weight loss compared to those fish which aren't in as good of a condition. Most literature seems to suggest that rainbow trout grow best at around 66-67 degrees and cease to grow around 74.
In the creek behind our hunting cabin, water temps in the summer will climb into the upper 70's and sometimes over 80 near the surface. DO levels are usually close to saturation though. Depending on conditions for each year, we generally see most brown trout survive 3 out of 4 years and rainbow trout survive 1 out of 3 years.
