It is species dependent, Rainbow Trout are able to survive in temperatures up to and exceeding 77°F (24°C), they stop growing at 73°F (23° C). A trout, that is oxygen stressed while in the water uses minimum energy, will be dramatically more stressed after being hooked. In many cases, a fish improperly handled and released might not to not survive. For brook trout, these limits are generally accepted to be a few degrees lower (some sources suggest as low as 65°). For many fishermen including myself, 70°F (21°C) has become a "don't fish" limit.