There is no exact answer... No way to say what the weather will be like 2 weeks from now, or 6 weeks from now. There are many variables about your individual pond to say, on this exact date your trout are gonna croak.
Temperature and dissolved oxygen both play a role in whether trout can live in a certain body of water. There are many ponds that stay cold enough at the bottom for trout to live, the problem is the dissolved oxygen levels down there are extremely low. Brook trout are the least tolerant of lower DO and higher temps. When surface temps push 70, expect the brook trout to begin to struggle and when surface temps reach 74, expect the rainbows/goldens to struggle. In central VA, this may occur late May to early June in all likelihood. It could occur a little earlier if we have unusually warm temps or it could occur a little later.
Remember, the trout may still be alive but not actively feeding as they are too stressed to continue to feed.
It also depends on the strain of rainbow trout you have. The hatchery I get my rainbows/goldens from has two distinct strains, one which I have found it much more tolerant of warmer waters than the other.
